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+The Project Gutenberg EBook of Geology, by James Geikie
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+Title: Geology
+
+Author: James Geikie
+
+Release Date: February 18, 2011 [EBook #35317]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK GEOLOGY ***
+
+
+
+
+Produced by Heather Clark, Tom Cosmas and the Online
+Distributed Proofreading Team at http://www.pgdp.net (This
+book was produced from scanned images of public domain
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+
+
+    CHAMBERS'S ELEMENTARY SCIENCE MANUALS.
+
+
+
+
+    GEOLOGY
+
+
+    BY
+
+
+    JAMES GEIKIE, LL.D., F.R.S.
+
+    OF H.M. GEOLOGICAL SURVEY; AUTHOR OF
+    'THE GREAT ICE AGE.'
+
+
+    [Logo]
+
+    W. & R. CHAMBERS
+    LONDON AND EDINBURGH
+    1883
+
+
+
+
+    Edinburgh:
+    Printed by W. and R. Chambers.
+
+
+
+
+PREFACE.
+
+
+The vital importance of diffusing some knowledge of the leading
+principles of Science among all classes of society, is becoming daily
+more widely and deeply felt; and to meet and promote this important
+movement, W. & R. CHAMBERS have resolved on issuing the present Series
+of ELEMENTARY SCIENCE MANUALS. The Editors believe that they enjoy
+special facilities for the successful execution of such an undertaking,
+owing to their long experience--now extending over a period of forty
+years--in the work of popular education, as well as to their having the
+co-operation of writers specially qualified to treat the several
+subjects. In particular, they are happy in having the editorial
+assistance of ANDREW FINDLATER, LL.D., to whose labours they were so
+much indebted in the work of editing and preparing _Chamber's
+Encyclopaedia_.
+
+The Manuals of this series are intended to serve two somewhat different
+purposes:
+
+1. They are designed, in the first place, for SELF-INSTRUCTION, and will
+present, in a form suitable for private study, the main subjects
+entering into an enlightened education; so that young persons in earnest
+about self-culture may be able to master them for themselves.
+
+2. The other purpose of the Manuals is, to serve as TEXT-BOOKS IN
+SCHOOLS. The mode of treatment naturally adopted in what is to be
+studied without a teacher, so far from being a drawback in a
+school-manual, will, it is believed, be a positive advantage. Instead of
+a number of abrupt statements being presented, to be taken on trust and
+learned, as has been the usual method in school-teaching; the subject is
+made, as far as possible, to unfold itself gradually, as if the pupil
+were discovering the principles himself, the chief function of the book
+being, to bring the materials before him, and to guide him by the
+shortest road to the discovery. This is now acknowledged to be the only
+profitable method of acquiring knowledge, whether as regards
+self-instruction or learning at school.
+
+For simplification in teaching, the subject has been divided into
+sub-sections or articles, which are numbered continuously; and a series
+of Questions, in corresponding divisions, has been appended. These
+Questions, while they will enable the private student to test for
+himself how far he has mastered the several parts of the subject as he
+proceeds, will serve the teacher of a class as specimens of the more
+detailed and varied examination to which he should subject his pupils.
+
+
+NOTE BY THE AUTHOR.
+
+In the present Manual of GEOLOGY it has been the aim of the author
+rather to indicate the methods of geological inquiry and reasoning, than
+to present the learner with a tedious summary of results. Attention has
+therefore been directed chiefly to the physical branches of the
+science--Palaeontology and Historical Geology, which are very large
+subjects of themselves, having been only lightly touched upon. The
+student who has attained to a fair knowledge of the scope and bearing of
+Physical Geology, should have little difficulty in subsequently tackling
+those manuals in which the results obtained by geological investigation
+are specially treated of.
+
+
+
+
+CONTENTS.
+
+
+                                                                  PAGE
+  INTRODUCTORY                                                       7
+
+  CLASSIFICATION OF ROCKS                                            8
+
+  MINERALOGY                                                        12
+      ROCK-FORMING MINERALS                                         14
+
+  PETROLOGY--
+      MECHANICALLY FORMED ROCKS                                     17
+      CHEMICALLY FORMED ROCKS                                       19
+      ORGANICALLY DERIVED ROCKS                                     20
+      METAMORPHIC ROCKS                                             21
+      IGNEOUS ROCKS                                                 23
+      STRUCTURE AND ARRANGEMENT OF ROCK-MASSES--
+          Stratification, &c.; Mud-cracks and Rain-prints;
+          Succession of Strata; Extent of Beds; Sequence of
+          Beds--Joints; Cleavage; Foliation; Concretions;
+          Inclination of Strata; Contemporaneous Erosion;
+          Unconformability; Overlap; Faults; Mode of
+          Occurrence of Metamorphic and Igneous Rocks;
+          Mineral Veins                                          26-46
+
+  DYNAMICAL GEOLOGY--
+      THE ATMOSPHERE AS A GEOLOGICAL AGENT OF CHANGE                46
+      WATER AS A GEOLOGICAL AGENT OF CHANGE                         48
+      GEOLOGICAL ACTION OF PLANTS AND ANIMALS                       60
+      SUBTERRANEAN FORCES                                           64
+      METAMORPHISM                                                  72
+
+  PHYSIOGRAPHY                                                      74
+
+  PALAEONTOLOGY                                                      77
+
+  HISTORICAL GEOLOGY                                                84
+
+  QUESTIONS                                                         89
+
+
+
+
+  [Illustration: GEOLOGY]
+
+
+INTRODUCTORY.
+
+
+1. _Definition._--Geology is the science of the origin and development
+of the structure of the earth. It treats of the nature and mode of
+formation of the various materials of which the earth's crust is
+composed; it seeks to discover what mutations of land and water, and
+what changes of climate, have supervened during the past; it endeavours
+to trace the history of the multitudinous tribes of plants and animals
+which have successively tenanted our globe. In a word, Geology is the
+Physical Geography of past ages.
+
+2. _Rocks._--Every one knows that the crust of the earth is composed of
+very various substances, some of which are hard and crystalline in
+texture, like granite; others less indurated and non-crystalline, such
+as sandstone, chalk, shale, &c.; while yet others are more or less soft
+and incoherent masses, as gravel, sand, clay, peat, &c. Now, all these
+heterogeneous materials, whether they be hard or soft, compact or loose,
+granular or crystalline, are termed _rocks_. Blowing sand-dunes,
+alluvial silt and sand, and even peat, are, geologically speaking,
+rocks, just as much as basalt or any indurated building-stone. The
+variety of rocks is very great, but we do not study these long before we
+become aware that many kinds which present numerous contrasts in detail,
+yet possess certain characters in common. And this not only groups these
+diverse species together, but serves also to distinguish them from other
+species of rock, which in like manner are characterised by the presence
+of some prevalent generic feature or features.
+
+_Classification of Rocks._--All the rocks that we know of are thus
+capable of being arranged under _five_ classes, as follows:
+
+    I. MECHANICALLY FORMED.
+   II. CHEMICALLY FORMED.
+  III. ORGANICALLY DERIVED.
+   IV. METAMORPHIC.
+    V. IGNEOUS.
+
+3. The MECHANICALLY FORMED class comprises a considerable variety of
+rocks, all of which, however, come under only two subdivisions--namely,
+_Sedimentary_, and _Eolian_ or _Aerial_, the former being by far the
+more important. Of the _Sedimentary_ group, there are three rocks which
+may be taken as typical and representative--namely, _conglomerate_ or
+_puddingstone_, _sandstone_, and _shale_. A short examination of the
+nature of these will sufficiently explain why they come to be grouped
+together under one head. _Conglomerate_ consists of a mass of
+various-sized rounded stones cemented together; each stone has been well
+rubbed, and rolled, and rounded. It is quite obvious that the now solid
+rock must at one time have existed in a loose and unconsolidated state,
+like gravel and shingle. Nor can we resist the conclusion that the
+stones were at one time rolled about by the action of water--that being
+the only mode in which gravel-stones are shaped. Again, when we have an
+opportunity of examining any considerable vertical thickness of
+conglomerate, we shall frequently observe that the stones are arranged
+more or less definitely along certain lines. These, there can be no
+question, are _lines of deposition_--the rounded stones have evidently
+not been formed and accumulated all at once, but piled up gradually,
+layer upon layer. And since there is no force in nature, that we know
+of, save water in motion, that could so round and smooth stones, and
+spread them out in successive layers or beds, we may now amplify our
+definition of conglomerate, and describe it as a _compacted mass of
+stones which have been more or less rounded, and arranged in more or
+less distinct layers or beds, by the action of water_.
+
+4. _Sandstone_ may at the outset be described as a _granular
+non-crystalline rock_. This rock shews every degree of coarseness, from
+a mass in which the constituent grains are nearly as large as
+turnip-seed, down to a stone so fine in the grain that we need a lens to
+discover what the particles are of which it is composed. When these
+latter are examined, they are found to exhibit marks of attrition, just
+like the stones of a conglomerate. Sharp edges have been worn off, and
+the grains rounded and rubbed; and whereas lines of deposition are often
+obscure, and of infrequent occurrence in conglomerate--in sandstone, on
+the contrary, they are usually well marked and often abundant. We can
+hardly doubt, therefore, that sandstone has also had an _aqueous_
+origin, or in other words, that it has been formed and accumulated by
+the force of water in motion. In short, sandstone is merely compacted
+sand.
+
+5. If it be easy to read the origin of conglomerate and sand in the
+external character of their ingredients, and the mode in which these
+have been arranged, we shall find it not less easy to discover the
+origin of _shale_. Shale is, like sandstone, a granular non-crystalline
+rock. The particles of which it is built up are usually too small to be
+distinguished without the aid of a lens, but when put under a sufficient
+magnifying power, they exhibit evident marks of attrition. In structure
+it differs widely from sandstone. In the latter rock the layers of
+deposition, though frequently numerous, are yet separated from each
+other by some considerable distance, it may be by a few inches or by
+many yards. But in shale the layers are so thin that we may split the
+rock into _laminae_ or plates. Now we know that many sedimentary
+materials of recent origin, such as the silt of lakes, rivers, and
+estuaries, although when newly dug into they appear to be more or less
+homogeneous, and shew but few lines of deposition, yet when exposed to
+the action of the atmosphere and dried, they very often split up into
+layers exhibiting division planes as minute as any observable in shale.
+There is no reason to doubt, therefore, that shale is merely compacted
+silt and mud--the sediment deposited by water. It becomes evident,
+therefore, that conglomerate, sandstone, and shale are terms of one
+series. They are all equally sedimentary deposits, and thus, if we
+slightly modify our definition of conglomerate, we shall have a
+definition which will include the three rocks we have been considering.
+For they may all be described as _granular non-crystalline rocks, the
+constituent ingredients of which have been more or less rounded, and
+arranged in more or less distinct layers, by the action of water_.
+
+6. The _Eolian_ or _Aerial_ group of rocks embraces all natural
+accumulations of organic or inorganic materials, which have been formed
+upon the land. The group is typically represented by _debris_, such as
+gathers on hill-slopes and at the base of cliffs, by the _sand-hills_ of
+deserts and maritime districts, and by _soil_. All these accumulations
+owe their origin to atmospheric agencies, as will be more particularly
+described in the sequel. As the _Sedimentary_ and _Eolian_ rocks are the
+results of the _mechanical_ action of water and the atmosphere, they are
+fitly arranged under one great class--the MECHANICALLY FORMED ROCKS.
+
+7. CHEMICALLY FORMED ROCKS constitute another well-marked class, of
+which we may take _rock-salt_ as a typical example. This rock has
+evidently been deposited in water, but not in the manner of a
+sedimentary bed. It is not built up of water-worn particles which have
+been rolled about and accumulated layer upon layer, but has been slowly
+precipitated during the gradual evaporation of water in which it was
+previously held in solution. Its formation is therefore a chemical
+process. Various other rocks come under the same category, as we shall
+afterwards point out.
+
+8. The ORGANICALLY DERIVED class comprises a number of the most
+important and useful rock-masses. _Chalk_ may be selected as a typical
+example. Even a slight examination shews that this rock differs widely
+from any of those mentioned above. Conglomerate, sandstone, shale, &c.
+are built up of pebbles, particles, grains, &c. of various inorganic
+materials. But chalk, when looked at under the microscope, betrays an
+organic origin. It consists, chiefly, of the hard calcareous parts of
+animal organisms, and is more or less abundantly stocked with the
+remains of corals, shells, crustaceans, &c. in every degree of
+preservation; indeed, so abundant are these relics, that they go to form
+a great proportion of the rock. _Coal_ is another familiar example of an
+organically derived rock, since it consists entirely of vegetable
+remains.
+
+9. The METAMORPHIC class, as the name implies, embraces all those rocks
+which have undergone some decided change since the time of their
+formation. This change generally consists in a re-arrangement of their
+constituent elements, and has frequently resulted in giving a
+crystalline texture to the rocks affected. Hence certain sedimentary
+deposits like sandstone and shale have been changed from granular into
+crystalline rocks, and the like has happened to beds of limestone and
+chalk. _Mica-schist, gneiss_, and _saccharoid marble_ are typical of
+this class.
+
+10. The IGNEOUS rocks are those which owe their origin to the action of
+the internal forces of the earth's crust. Most of them have been in a
+state of fusion, and betray their origin by their crystalline and
+sometimes glassy texture, and also, as we shall see in another section,
+by the mode of their occurrence. _Lava_, _basalt_, and _obsidian_ are
+characteristic types of this group of igneous rocks. Another group
+embraces a large variety of igneous rocks which are non-crystalline, and
+vary in texture from fine-grained, almost compact, bedded masses, like
+certain varieties of _tuff_, up to coarse, irregular accumulations of
+angular stones imbedded in a fine-grained or gritty matrix, like
+_volcanic breccia_ and _volcanic agglomerate_.
+
+
+
+
+MINERALOGY.
+
+
+11. Having learned that all the rocks met with at the surface of the
+earth's crust are capable of being arranged under a few classes, we have
+now to investigate the matter more in detail. It will be observed that
+the classification adopted above is based chiefly upon the external
+characters of the constituent ingredients of the rocks, and the mode in
+which these particles have been collected. In some rocks the component
+materials are crystalline, in others they are rounded and worn; in one
+case they have been brought together by precipitation from an aqueous
+solution, or they have crystallised out from a mass of once molten
+matter; in another case their collection and intimate association is due
+to the mechanical action of the atmosphere or of water, or to the agency
+of the organic forces. We have next to inquire what is the nature of
+those crystals and particles which are the ingredients of the rocks?
+The answer to this question properly belongs to the science of
+mineralogy, with which, however, the geologist must necessarily make
+some acquaintance.
+
+12. _Granite--its composition._--It will tend to simplify matters if we
+begin our inquiry by selecting for examination some familiar rock, such
+as _granite_. This rock, as one sees at a glance, is crystalline, nor is
+it difficult to perceive that three separate kinds of ingredients go to
+compose it. One of these we shall observe is a gray, or it may be, clear
+glassy-looking substance, which is hard, and will not scratch with a
+knife; another is of a pink, red, gray, or sometimes even pale green
+colour, and scratches with difficulty; while the third shews a
+glistering metallic lustre, and is generally of a brownish or black
+colour. It scratches easily with the knife, and can be split up into
+flakes of extreme thinness. If the granite be one of the coarse-grained
+varieties, we shall notice that these three ingredients have each more
+or less definite crystalline forms; so that they are not distinguished
+by colour and hardness alone. The metallic-looking substance is _mica_;
+the hard gray, or glassy and unscratchable ingredient is _quartz_; and
+the remaining material is _felspar_. The mineralogist's analysis of
+granite ends here. But there is still much to be learned about quartz,
+felspar, and mica; for, as the chemist will tell us, these are not
+'elementary substances.' Quartz is a compound, consisting of two
+elements, one of which is a non-metallic body (silicon), and the other
+an invisible gas (oxygen). Felspar[A] is a still more complex compound,
+being made up of two metals (potassium, aluminium) and one non-metallic
+body (silicon), each of which is united to an invisible gas (oxygen).
+Mica, again, contains no fewer than four metals (potassium, magnesium,
+iron, calcium) and one non-metallic body (silicon), each of which is in
+like manner chemically united to its share of oxygen. Thus the
+rock-forming substances, quartz, felspar, and mica, have each a definite
+chemical composition.
+
+13. _Minerals._--Now, any inorganic substance which has a definite
+chemical composition, and crystallises in a definite crystalline or
+geometric form, is termed a _mineral_. Having once discovered that
+quartz is composed of silicon and oxygen--that is, silica--and that the
+faces of its crystals are arranged in a certain definite order, we may
+be quite sure that any mineral which has not this composition and form
+cannot be quartz. And so on with mica and felspar, and every other
+mineral. The study of the geometric forms assumed by minerals
+(crystallography) forms a department of the science of mineralogy. But,
+in the great majority of cases, the mineral ingredients of the rocks are
+either so small individually, or so broken, and rounded, and altered,
+that crystallography gives comparatively little aid to the practical
+geologist in the field. He has, therefore, recourse to other tests for
+the determination of the mineral constituents of rocks. Many of these
+tests, however, can only be applied by those who have had long
+experience. The simplest and easiest way for the student to begin is to
+examine the forms and appearance of the more common minerals in some
+collection, and thereafter to accustom his eye to the aspect presented
+by the same minerals when they are associated together in rocks, of
+which illustrative specimens are now to be met with in most museums. The
+microscope is largely employed by geologists for determining the
+mineralogical composition of certain rocks; and, indeed, many rocks can
+hardly be said to be thoroughly known until they have been sliced and
+examined under the microscope, and analysed by the chemist. But with a
+vast number such minute examination is not required, the eye after some
+practice being able to detect all that is needful to be known.
+
+   [A] There are various kinds of felspar; the one referred to above is
+       _orthoclase_, or potash-felspar.
+
+
+ROCK-FORMING MINERALS.
+
+14. Nearly all the minerals we know of contain oxygen as a necessary
+ingredient, there being only a very few minerals in which that gas does
+not occur in chemical union with other elements. Three of these
+minerals, _sulphur_, the _diamond_, and _graphite_, consist of simple
+substances, and are of great commercial importance, but none of them is
+of so frequent occurrence, as a rock constituent, as the minerals
+presently to be described. _Sulphur_ occurs sometimes in thin beds, but
+more frequently in small nests and nodules, &c. in other rocks, or in
+joints, and fissures, and veins. It is frequently found in volcanic
+districts. The _diamond_, which consists of pure _carbon_, is generally
+met with in alluvial deposits, but sometimes, also, in a curious
+flexible sandstone, called _itacolumite_. _Graphite_ is another form of
+carbon. It occurs both in a crystalline and amorphous form, the latter,
+or non-crystalline kind, being extensively used for lead-pencils.
+_Rock-salt_ is a _chloride of sodium_, and appears sometimes in masses
+of a hundred feet and more in thickness. Another mineral which contains
+no oxygen is the well-known _fluor-spar_. It occurs chiefly in veins,
+and is often associated with ores. With these, and a few other
+exceptions, all the minerals hitherto discovered contain oxygen as an
+essential element; and so large is the proportion of this gas which
+enters into union with other elements to constitute the various minerals
+of which the rocks are composed, that it forms at least one-half of all
+the ponderable matter near the earth's surface. When the student learns
+that there are probably no fewer than six or seven hundred different
+minerals, he will understand how impossible it is to do more in a short
+geological treatise than point out a few of the most commonly occurring
+ones. And, indeed, a knowledge of the chief rock-forming minerals, which
+are few in number, is all that is absolutely requisite for geological
+purposes. Some of these we accordingly proceed to name.[B]
+
+   [B] It is needless to describe the minerals minutely here. The
+       student can only learn to distinguish the different species
+       by carefully examining actual specimens.
+
+15. _Quartz._--This mineral has already been partially described. It is
+the most abundant of all the rock-forming minerals, and occurs in three
+forms: (1) _crystallised quartz_ or _rock crystal_; (2) _chalcedony_,
+both of which are composed of silica--that is, silicon and oxygen; and
+(3) _hydrated quartz_--that is, silica with the addition of water.
+
+_Hematite._--This is an oxide of iron. It occurs in mammillary rounded
+masses, with a fibrous structure, and a dull metallic lustre.
+_Magnetite_ or magnetic iron ore, _specular iron_, and _limonite_ are
+also oxides of iron. _Hematite_ shews a red streak when scratched with a
+knife, which distinguishes it from magnetite.
+
+_Iron pyrites._--This is a sulphide of iron of very common occurrence.
+Its crystalline form is cubical. When broken, it emits a sulphurous
+smell. The brass-yellow coloured cubes so often seen in roofing-slates
+are familiar examples of the mode of its occurrence. But it is also
+frequently found in masses having a crystalline surface.
+
+16. SULPHATES.--Only two sulphates may be noticed--namely, _gypsum_,
+which is a sulphate of lime, with its varieties, _selenite_,
+_satin-spar_, and _alabaster_; and _barytes_, a sulphate of baryta.
+_Barytes_ scratches easily with the knife, and from its great specific
+gravity is often called _heavy-spar_. Gypsum is softer than barytes.
+
+CARBONATES.--Two of these only need be mentioned: _calcite_ or
+_calc-spar_, a carbonate of lime, which scratches with the knife, and
+effervesces readily with dilute hydrochloric acid; and _arragonite_,
+also a carbonate of lime, but denser than calcite.
+
+SILICATES.--These are by far the most abundantly occurring minerals. The
+species are also exceedingly numerous, but we may note here only a few
+of the more important. They are composed of silica and various bases,
+such as lime, potash, magnesia, soda, alumina, &c. _Augite_ or
+_pyroxene_ is a black or greenish-black mineral, found, either as
+crystals, which are generally small, or as rounded grains and angular
+fragments, in basaltic and volcanic rocks. It never occurs in granite
+rocks. It is brittle, and has a vitreous or resinous lustre. There are a
+number of varieties or sub-species of augite. _Hornblende_, like augite,
+also includes a great many minerals. When the crystals are small, it is
+often difficult to distinguish hornblende from augite. Common hornblende
+occurs crystallised or massive, and is dark green or black, with a
+vitreous lustre. It is generally sub-translucent. It usually
+crystallises in igneous rocks which contain much quartz or silica; while
+augite, on the other hand, crystallises in igneous rocks which are of a
+more basic character--that is to say, rocks in which silica is not so
+abundantly present. _Felspar_ is a generic term which embraces a number
+of species, such as _orthoclase_ or _potash-felspar_, _albite_ or
+_soda-felspar_, and _anorthite_ or _lime-felspar_. _Orthoclase_ is
+white, red or pink, and gray. It is one of the ordinary constituents of
+granite, and enters into the composition of many rocks. _Albite_ is
+usually white. It often occurs as a constituent of granite, not
+unfrequently being associated in the same rock with pink felspar or
+orthoclase. In syenite and greenstone it occurs more commonly than
+orthoclase. _Anorthite_ occurs in white translucent or transparent
+crystals. It is not so common a constituent of rocks as either of the
+other felspars just referred to. _Mica_: this term includes several
+minerals, which all agree in being highly cleavable into thin elastic
+flakes or laminae, which have a glistening metallic lustre. Mica is one
+of the common constituents of granite. _Talc_ is a silvery white,
+grayish, pale or dark-green coloured mineral, with a pearly lustre. It
+splits readily into thin flakes, which are flexible, but not elastic,
+and may be readily scratched with the nail. It is unctuous and greasy to
+the touch. It occurs in beds (_talc-slate_), and is often met with in
+districts occupied by metamorphic crystalline rocks. _Serpentine_ is
+generally of a green colour, but brown, red, and variously mottled
+varieties occur. It has a dull lustre, and is soft, and easily cut; it
+is tough, however, and takes on a good polish. It forms rock-masses in
+some places. The finer varieties are called _noble serpentine_.
+_Chlorite_ is another soft, easily scratched mineral, generally of a
+dark-green colour. It has a pearly lustre. Sometimes it occurs in beds
+(_chlorite-slate_), and is often found coating the walls of fissures in
+certain rocks. It has a somewhat greasy feel. The three last-mentioned
+minerals--talc, serpentine, and chlorite--are all silicates of magnesia.
+_Zeolites_ is a term which comprises a number of minerals of varying
+chemical composition, all of which tend to form a jelly when treated
+with acids. When heated by the blow-pipe they bubble up, owing to the
+escape of water; hence their name _zeolites_, from _zeo_, I boil, and
+_lithos_, a stone. The zeolites occur very commonly in cavities in
+igneous rocks, and also in mineral veins.
+
+Having now mentioned the chief rock-forming minerals, we proceed to a
+brief description of some of the more typical representatives of the
+five great classes of rocks referred to at page 8.
+
+
+
+
+PETROLOGY.[C]
+
+   [C] _Petros_, a rock, and _logos_, a discourse. Some geologists
+       restrict this term to the study of the _structure_ and
+       _arrangement of rock-masses_, and apply the term _lithology_
+       (_lithos_, a stone, and _logos_, a discourse) to the study of
+       the _mineralogical composition of rocks_.
+
+
+MECHANICALLY FORMED ROCKS.
+
+17. (_A._) SEDIMENTARY CLASS.--Three of the most commonly occurring
+rocks of this class have already been described, but a few details are
+added here.
+
+_Conglomerate._--This is a consolidated mass of more or less water-worn
+and rounded stones. These stones may be of any size. When they are very
+large, the rock is called a _coarse conglomerate_; the finer varieties,
+in which the stones are small, are known as _pebbly conglomerates_. The
+ingredients of a conglomerate may consist of any kind of rock, or of a
+mixture of many different kinds. When they consist entirely of quartz,
+the rock becomes _quartzose_. The finer-grained conglomerates usually
+shew lines of deposition or bedding, but in some of the coarser sorts it
+is often difficult to detect any kind of arrangement. The stones are
+usually imbedded in a matrix of quartzose grit and sand, but sometimes
+this is very scanty. When the nature of the material which binds the
+stones together is very well marked, the rock becomes _ferruginous_,
+_calcareous_, _arenaceous_, or _argillaceous_, according as the binding
+or cementing material is _iron_, _lime_, _sand_, or _clay_. _Breccia_ is
+a rock in which the included fragments are _angular_.
+
+18. _Sandstone_ is, as already remarked, merely consolidated sand. The
+coarser varieties, in which the grains are as large and larger than
+turnip-seeds, are termed _grit_. From these coarse varieties the rock
+passes insensibly, in one direction, into a fine or pebbly conglomerate,
+and in another into a rock, so fine-grained that a lens is needed to
+distinguish the component particles. Quartz is the prevailing
+ingredient--sometimes clear, at other times white. Frequently, however,
+the grains are coated with an oxide of iron, which gives the resulting
+rock a red colour. The other colours assumed by sandstone--such as
+yellow, brown, green, &c.--are also in like manner due to the presence
+of some compound of iron. When mica or felspar occurs plentifully, we
+have, in the one case, _micaceous sandstone_, and in the other
+_felspathic sandstone_. A sandstone in which the grains are cemented by
+carbonate of lime is said to be _calcareous_. _Freestone_ is a sandstone
+which can be worked freely in any direction. In most sandstones, the
+lines of bedding are distinct; when they are so numerous as to render
+the rock fissile, the sandstone is said to be _shaly_.
+
+_Shale_ is a more or less indurated fissile or laminated clay. When the
+rock becomes coarse by the admixture of sand, it gradually passes into a
+_shaly sandstone_. There are many other varieties of clay-rocks--such as
+_fire-clay_, _pipe-clay_, _marl_, _loam_, &c.--which are sufficiently
+familiar.
+
+19. (_B._) EOLIAN or AERIAL CLASS.--_Blown-sand_ is found at many places
+on sea-coasts. It generally forms smooth rounded hummocks, which are
+sometimes arranged in long lines parallel to the trend of the coast, as,
+for example, in the Tents Moor, near St Andrews. The _sand-hills_ of
+deserts also belong to this class.
+
+_Debris_ is the loose angular rubbish which collects at the base of
+cliffs, on hill-tops, and hill-slopes. Immense accumulations of it occur
+in lofty mountainous districts and in arctic regions. In Nova Zembla,
+for example, the solid rock of the country is almost concealed beneath a
+thick covering of debris. But the various kinds of debris will be more
+particularly described further on.
+
+_Soil._--An account of this can hardly be given without entering into
+the theory of its origin, and therefore we reserve its consideration for
+the present.
+
+
+CHEMICALLY FORMED ROCKS.
+
+20. _Stalactites_ and _stalagmites_ are carbonates of lime. They vary
+in colour, being white, or yellow, or brown. Stalactites are usually
+found adhering to the roofs of limestone caverns, &c., or depending from
+limestone rocks; stalagmites, on the other hand, commonly occur on the
+floors of limestone caverns, where they often attain a thickness of many
+feet.
+
+_Siliceous sinter_ is silica with the addition of water--in other words,
+a hydrated quartz. It is not a very abundant rock, and is found chiefly
+in volcanic countries.
+
+_Rock-salt_ has already been described. It occurs either as thin beds,
+or in the form of thick cake-like masses, often reaching ninety or one
+hundred feet in thickness. It is rudely crystalline in texture, and is
+usually discoloured brown and red with various impurities.
+
+
+ORGANICALLY DERIVED ROCKS.
+
+21. _Limestone_ consists of carbonate of lime, but usually contains some
+impurities. The varieties of this rock are numerous; some of them are as
+follows: _Chalk_; _oolite_, a rock built up of little spheroidal
+concretions, whence its name, _egg_ or _roe stone_ (the coarser oolites
+are called _pisolite_, or _pea-stone_); _lacustrine limestone_, &c. When
+much silica is diffused through the rock, we have a _siliceous
+limestone_; the presence of clay and of carbonaceous matter gives us
+_argillaceous_ and _carbonaceous limestones_. _Cornstone_ is a limestone
+containing a large quantity of arenaceous matter or sand. Many
+limestones are distinguished by the different kinds of organic remains
+which they yield. Thus, we have _muschelkalk_ or _shell-limestone_,
+_nummulitic_, _crinoidal_, &c. limestone. The crystalline limestones,
+such as _statuary marble_, are metamorphosed limestones. Not a few
+limestones are chemically formed rocks, and many, also, are partly of
+chemical and partly of organic origin, so that no hard and fast line can
+be drawn between these two classes of rock.
+
+_Dolomite_, or _magnesian limestone_.--This is a compound of carbonate
+of lime and carbonate of magnesia. Its colour is usually yellow, or
+yellowish brown, but gray and black varieties are sometimes met with. It
+is generally fine-grained, with a crystalline texture, and pearly
+lustre. It effervesces less freely with acids than pure limestone. In
+many cases dolomite is merely a metamorphosed limestone.
+
+22. _Coal_ is composed of vegetable matter, but usually contains a
+greater or less percentage of impurities. The varieties of this
+substance are very numerous, and differ from each other principally in
+regard to their bituminous or non-bituminous character. Coal is
+bituminous or non-bituminous according as it is less or more highly
+mineralised. Bitumen results from the decomposition of vegetable matter;
+but, when the mineralising process (to which the formation of coal is
+due) has proceeded far enough, the vegetable matter gradually loses its
+bituminous character, and the result is a non-bituminous coal. Varieties
+of coal are the following: _Lignite_ or _brown coal_; _caking coal_;
+_cannel_, _parrot_, or _gas coal_; _splint coal_; _cherry_ or _soft
+coal_; _anthracite_ or _blind coal_, so called because it burns with no
+flame. _Peat_ may be mentioned as another natural fuel. It is composed
+of vegetable matter. In some kinds it is so far decomposed, or
+mineralised, that the eye does not detect vegetable fibres; when
+thoroughly dried, such peat breaks like a good lignite, and forms an
+excellent fuel.
+
+
+METAMORPHIC ROCKS.
+
+23. _Quartz-rock_, or _quartzite_, is an altered quartzose sandstone or
+grit; it is generally a white or grayish-yellow rock, very hard and
+compact. The original gritty character of the rock is distinct, but the
+granules appear as if they had been fused so far as to become mutually
+adherent. When the altered sandstone has been composed of grains of
+quartz, felspar, or mica, set in a siliceous, felspathic, or
+argillaceous base, we get a rock called _greywacke_, which is usually
+gray or grayish blue in colour.
+
+24. _Clay-slate_ is a grayish blue, or green, fine-grained hard rock,
+which splits into numerous more or less thin laminae, which may or may
+not coincide with the original bedding. Most usually the 'cleavage,' as
+this fissile structure is termed, crosses the bedding at all angles.
+
+25. _Crystalline limestone_ is an altered condition of common limestone.
+_Saccharoid marble_ is one of the fine varieties: it frequently contains
+flakes of mica. _Dolomite_, or magnesian limestone, already described,
+is probably in many cases an altered limestone; the carbonate of lime
+having been partially dissolved out and replaced by carbonate of
+magnesia. _Serpentine_ is also believed by some geologists to be a
+highly metamorphosed magnesian limestone.
+
+26. _Schists_.--Under this term comes a great variety of crystalline
+rocks which all agree in having a foliated texture--that is to say, the
+constituent minerals are arranged in layers which usually, but not
+invariably, coincide with the original bedding. Amongst the schists come
+_mica-schist_ (quartz and mica in alternate layers); _chlorite-schist_
+(chlorite with a little quartz, and sometimes with felspar or mica);
+_talc-schist_ (talc with quartz or felspar); _hornblende schist_
+(hornblende with a variable quantity of felspar, and sometimes a little
+quartz); _gneiss_ (quartz, felspar, and mica).
+
+27. _General Character of Metamorphic Rocks._--All these rocks betray
+their aqueous origin by the presence of more or less distinct lines of
+bedding. They consist of various kinds of arenaceous and argillaceous
+deposits, which, under the influence of certain metamorphic actions, to
+be described in the sequel, have lost their original granular texture,
+and become more or less distinctly crystallised. And not only so, but
+their chemical ingredients have in many cases entered into new
+relations, so as to give rise to minerals which existed either sparingly
+or not at all in the original rocks. Frequently, it is quite impossible
+to say what was the original condition of some metamorphic rocks; often,
+however, this is sufficiently obvious. Thus, highly micaceous
+sandstones, as they are traced into a metamorphic region, are seen to
+pass gradually into mica-schist. When the bedding of gneiss becomes
+entirely obliterated, it is often difficult to distinguish that rock
+from granite, and in many cases it appears to pass into a true granite.
+
+28. _Granite_ is a crystalline compound of quartz, felspar (usually
+potash-felspar), and mica. Some geologists consider it to be invariably
+an igneous rock; but, as just stated, it sometimes passes into gneiss in
+such a way as to lead us to infer its metamorphic origin. There are
+certain areas of sandstone in the south of Scotland which are partially
+metamorphosed, and in these we may trace a gradual passage from highly
+baked felspathic sandstones with a sub-crystalline texture into a more
+crystalline rock which in places graduates into true granite. Granite,
+however, also occurs as an igneous rock.
+
+29. _Syenite_ is a crystalline compound of a potash-felspar and
+hornblende, and quartz is frequently present. _Diorite_ is a crystalline
+aggregate of a soda-felspar and hornblende. Both syenite and diorite
+also occur as igneous rocks.
+
+There are a number of other metamorphic rocks, but those mentioned are
+the most commonly occurring species.
+
+
+IGNEOUS ROCKS.
+
+30. _Subdivisions._--In their chemical and mineralogical composition,
+igneous rocks offer great variety; but they all agree in having
+felspar for their base. They may be roughly divided into two classes,
+distinguished by the relative quantity of silica which they contain.
+Those in which the silica ranges from about 50 to 70 or 80 per cent.
+form what is termed the _acidic_ group; while those in which the
+percentage of silica is less constitute the _basic_ group of igneous
+rocks, so called because they contain a large proportion of the
+heavier bases, such as _magnesia_, _lime_, oxides of iron and
+manganese, &c. Igneous rocks vary in texture from homogeneous,
+compact, and finely crystalline masses up to coarsely crystalline
+aggregates, in which the crystals may be more than an inch in
+diameter. Sometimes they are dull and earthy in texture, at other
+times vesicular. When the vesicles are filled up with some mineral,
+the rock is said to be _amygdaloidal_, from the almond shape assumed
+by the kernels filling the cavities. When single crystals of any
+mineral are scattered through a rock, so as to be readily
+distinguished from the compact or crystalline base, the rock becomes
+_porphyritic_.
+
+
+ACIDIC OR FELSPATHIC GROUP.
+
+31. _Trachyte_ (_trachys_, rough) is a pale or dark-gray rock, harsh
+and rough to the touch, in which felspar is the predominant mineral. It
+is a common product of eruption in modern volcanoes.
+
+_Clinkstone_ or _phonolite_ is a greenish-gray, compact, felspathic
+rock, somewhat slaty or schistose, and weathers with a white crust. It
+gives a clear metallic sound under the hammer. It is a rock not met with
+among the older formations of the earth's crust, being confined to
+Tertiary (see table, p. 85) or still more recent times.
+
+_Obsidian_ or _volcanic glass_ is usually black, brown, or green, and
+usually resembles a coarse bottle-glass. When it becomes vesicular, it
+passes gradually into the highly porous rock called _pumice_. It is
+eminently a geologically modern volcanic rock.
+
+_Felstone_ is a reddish-gray, bluish, greenish, or yellowish, hard,
+compact, flinty-looking rock, composed of potash-felspar and silica. It
+is generally splintery under the hammer. Some varieties are slaty, and
+are frequently mistaken for clinkstone, which they closely resemble.
+When the quartz in felstone is distinctly visible either as grains or
+crystals, the rock passes into a _quartz-porphyry_.
+
+_Granite_ is recognised as an igneous as well as a metamorphic rock.
+Sometimes the veins and dykes which proceed from or occur near a mass of
+granite contain no mica--this kind of rock is called _elvan_ or
+_elvanite_.
+
+_Porphyrite_ or _felspathite_ includes a number of rocks which have a
+felspathic base, through which felspar crystals are scattered more or
+less abundantly. Sometimes hornblende, or augite, or mica is present.
+The colour is usually dark--some shade of blue, green, red, puce,
+purple, or brown--and the texture varies from compact and finely
+crystalline up to coarsely crystalline. Porphyrites are usually
+porphyritic, and frequently amygdaloidal.
+
+
+AUGITIC AND HORNBLENDIC OR BASIC GROUP.
+
+32. _Basalt_ is a dark or almost black compact homogeneous rock,
+composed of felspar and augite with magnetic iron. An olive-green
+mineral called _olivine_ is very frequently present. The coarser-grained
+basalts are called _dolerite_. The columnar structure is not peculiarly
+characteristic of basalt. Many basalts are not columnar, and not a few
+columnar rocks are not basalts.
+
+_Greenstone_ or _diorite_ is usually a dull greenish rock, sometimes
+gray, however, speckled with green. It is composed of soda-felspar and
+hornblende. The fine-grained compact greenstones are called _aphanite_.
+
+_Syenite_, like granite, is recognised as an igneous as well as a
+metamorphic rock. There are several other rocks which come into the
+basic group, but those mentioned are the more common and typical
+species.
+
+33. _Fragmental Igneous Rocks._--All the igneous rocks briefly described
+above are more or less distinctly crystalline in texture. There is a
+class of igneous rocks, however, which do not present this character,
+but when fine-grained are dull and earthy in texture, and frequently
+consist merely of a rude agglomeration of rough angular fragments of
+various rocks. These form the FRAGMENTAL group of igneous rocks. The
+ejectamenta of loose materials which are thrown out during a volcanic
+eruption, consist in chief measure of fragments of lava, &c. of all
+sizes, from mere dust, sand, and grit, up to blocks of more than a ton
+in weight. These materials, as we shall afterwards see, are scattered
+round the orifice of eruption in more or less irregular beds. The terms
+applied to the varieties of ejectamenta found among modern volcanic
+accumulations, will be given and explained when we come to consider the
+nature of geological agencies. In the British Islands, and many other
+non-volcanic regions, we find besides crystalline igneous rocks,
+abundant traces of loose ejectamenta, which clearly prove the former
+presence of volcanoes. These materials are sometimes quite
+amorphous--that is to say, they shew no trace of water action--they have
+not been spread out in layers, but consist of rude tumultuous
+accumulations of angular and subangular fragments of igneous rocks. Such
+masses are termed _trappean agglomerate_ and _trappean breccia_. At
+other times, however, the ejectamenta give evidence of having been
+arranged by the action of water, the materials having been sifted and
+spread out in more or less regular layers. What were formerly rude
+breccias and agglomerates of angular stones now become _trappean
+conglomerates_--the stones having been rounded and water-worn--while the
+fine ingredients, the grit, and sand, and mud, form the rock called
+_trap tuff_. Fragmental rocks are often quite indurated--the matrix
+being as hard as the included stones. But as a rule they are less hard
+than crystalline igneous rocks, and in many cases are loose and
+crumbling. When a fragmental rock is composed chiefly of rocks belonging
+to the acidic group, we say it is _felspathic_. When augitic and
+hornblendic materials predominate, then other terms are used; as, for
+example, _dolerite tuff_, _greenstone tuff_.
+
+
+STRUCTURE AND ARRANGEMENT OF ROCK-MASSES.
+
+34. The student can hardly learn much about the mineralogical
+composition of rocks, without at the same time acquiring some knowledge
+of the manner of their occurrence in nature. We have already briefly
+described certain sedimentary rocks, such as conglomerate, sandstone,
+and shale, and have in some measure touched upon their structure as
+rock-masses. These rocks, as we have seen, are arranged in more or less
+thick layers or _beds_, which are piled one on the top of the other.
+Rocks which are so arranged are said to be _stratified_, and are termed
+_strata_. We may also use the word _stratum_ as an occasional substitute
+for _bed_. The planes of _bedding_ or _stratification_ are sometimes
+very close together, in other cases they are wide apart. When the
+separate beds are very thin, as in the case of shale, it is most usual
+to term them _laminae_, and to speak of the _lamination_ of a shale, as
+distinguished from the _bedding_ of a sandstone. Planes of bedding are
+generally more strongly marked than planes of lamination. The laminae
+frequently cohere, while beds seldom do. In the above figure, which
+represents a vertical cutting or _section_ through horizontal strata,
+the planes of lamination are shewn at _l, l, l_, and those of
+stratification at _s, s, s_. There are hardly any limits to the
+thickness of a bed--it may range from an inch up to many feet or yards,
+while _laminae_ vary in thickness from an inch downwards.
+
+   [Illustration: Fig. 1.--_st_, sandstone, and _sh_, shale: _s_,
+   lines or planes of bedding; _l_, lines or planes of
+   lamination.]
+
+35. Hitherto we have been considering the _laminae_ and _strata_ as lying
+in an approximately horizontal plane. Sometimes, however, the layers of
+deposition in a single stratum are inclined at various angles to
+themselves, as in the following figure. This structure is called _false
+bedding_; the layers or laminae not coinciding with the planes of
+stratification. It owes its origin to shifting currents, such as the ebb
+and flow of the tide, and very often characterises deposits which have
+been formed in shallow water. (Hillocks of drifting sand frequently shew
+a similar structure, but their false bedding is, as a rule, much more
+pronounced.)
+
+   [Illustration: Fig. 2.--False Bedding.]
+
+36. _Mud-cracks and Rain-prints._--The surfaces of some beds
+occasionally exhibit markings closely resembling those seen upon a flat
+sandy beach after the retreat of the tide--hence they are called
+_ripple-marks_ or _current-marks_. They are, of course, due to the
+gentle current action which pushes along the grains of sand, and hence,
+such marks may be formed wherever a current sweeps over the bottom of
+the sea with energy just sufficient for the purpose. But since the
+necessary conditions for the formation of _ripple-mark_ occur most
+abundantly in shallow water, its frequent appearance in a series of
+strata may often be taken as evidence, so far, for the shallow-water
+origin of the beds. Besides ripple-marks we may also detect occasionally
+on the surfaces of certain strata _mud-cracks_ and _rain-prints_. These
+occur most commonly in fine-grained beds, as in flagstones, argillaceous
+sandstones, shales, &c. The _mud-cracks_ resemble those upon a mud-flat
+which are caused by the desiccation and consequent shrinkage of the mud
+when exposed to the sun. The old cracks have been subsequently filled
+up again by a deposition of mud or sand, usually of harder consistency
+than the rock traversed by the cracks. Hence, when the bed that overlies
+the mud-cracks is removed, we find a cast of these projecting from its
+under surface, or frequently the cast remains in its mould, and forms a
+series of curious ridges ramifying over the whole surface of the old
+mud-flat. _Rain-prints_ are the small pits caused by the impact of large
+drops. They are usually deeper at one side than the other, from which we
+can infer the direction of the wind at the time the rain-drops fell.
+Like the mud-cracks, they are most commonly met with in fine-grained
+beds, and have been preserved in a similar manner. Some geologists have
+also been able to detect _wave-marks_, 'faint outlinings of curved form
+on a sandstone layer, like the outline left by a wave along the limit
+where it dies out upon a beach.'
+
+37. _Succession of Strata._--The succession of strata is often very
+diversified. Thus, we may observe in one and the same section numberless
+alternating beds of sandstone and shale from an inch or so up to several
+feet each in thickness, with seams of coal, fireclay, ironstone, and
+limestone interstratified among them. In other cases, again, the
+succession is simpler, and some deep quarries shew only one bed, as is
+the case with certain limestones, fine-grained sandstones (liver-rock),
+and many volcanic rocks. Some limestones, indeed, shew small trace of
+bedding throughout a vertical thickness of hundreds of feet.
+
+38. _Beds, their Extent, &c._--Beds of rock are not only of very
+different thicknesses, but they are also of very variable extent. Some
+may thin gradually away, or 'die out' suddenly, in a few feet or
+yards, while others may extend over many square miles. Beds of
+limestone, for example, can often be traced for leagues in several
+directions; and if this be the case with certain single beds, it is
+still more true of groups of strata. Thus the coal-bearing strata
+belonging to what is called the Carboniferous period cover large areas
+in Wales, England, Scotland, and Ireland, not less, probably, than
+6000 square miles; and strata belonging to the same great period
+spread over considerable tracts on the Continent, and a very extensive
+area in North America. It holds generally true that beds of
+fine-grained materials are not only of more equal thickness
+throughout, but have also a wider extension than coarser-grained
+rocks. Fine sandstones, for example, extend over a wider area, and
+preserve a more equable thickness throughout than conglomerates, while
+limestones and coals are more continuous than either.
+
+39. When a bed is followed for any distance it is frequently found to
+thin away, and give place to another occupying the same plane or
+_horizon_. Thus a shale will be replaced by a sandstone, a sandstone by
+a conglomerate, and _vice versa_. Sometimes also we may find a shale, as
+we trace it in some particular direction, gradually becoming charged
+with calcareous matter, so as by and by to pass, as it were, into
+limestone. Every bed must, of course, end somewhere, either by thus
+gradually passing into another, or by thinning out so as to allow beds
+which immediately overlie and underlie it to come together. Not
+unfrequently, however, a bed will stop abruptly, as in fig. 3.
+
+   [Illustration: Fig. 3.--Sudden ending of Bed at x.]
+
+40. _Sequence of Beds._--It requires little reflection to see that the
+division plane between two beds may represent a very long period of
+time. Let the following diagram represent a section of strata, _s_ being
+beds of grit, and _a_, _b_, _c_, beds of sandstone and shale. It is
+evident that the beds s must have been formed before the strata _b_ were
+deposited above them. At x, the beds _a_ and _b_ come together, and were
+attention to be confined to that part of the section, the observer might
+be led to infer that no great space of time elapsed between the
+deposition of these two beds. Yet we see that an interval sufficient to
+allow of the formation of the beds _s_ must really have intervened. It
+is now well known that in many cases planes of bedding represent 'breaks
+in the succession' of strata--'breaks' which are often the equivalents
+of considerable thicknesses of strata. In one place, for example, we may
+have an apparently complete sequence of beds, as _a_, _b_, _c_, which a
+more extended knowledge of the same beds, as these are developed in some
+other locality, enables us to supplement, as _a_, _s_, _b_, _c_.
+
+   [Illustration: Fig. 4.--Sequence of Beds.]
+
+41. _Joints._--Besides _planes_ or _lines of bedding_, there are certain
+other division planes or _joints_ by which rocks are intersected. The
+former, as we have seen, are congenital; the latter are subsequent.
+Joints cut right across the bedding, and are often variously combined,
+one set of joint planes traversing the rock in one direction, and
+another set or sets intersecting these at various angles. Thus, in many
+cases the rocks are so divided as easily to separate into more or less
+irregular fragments of various sizes. Besides these confused joints
+there are usually other more regular division planes, which intersect
+the strata in some definite directions, and run parallel to each other,
+often over a wide area: these are called _master-joints_. Two sets of
+master-joints may intersect the same strata, and when such is the case,
+the rock may be quarried in cuboidal blocks, the size of which will
+vary, of course, according as the two sets of joints are near or wide
+apart. Joints may either gape or be quite close; so close, indeed, as in
+many cases to be invisible to the naked eye. Certain igneous rocks
+frequently shew division planes which meet each other in such a way as
+to form a series of polygonal prisms. The basalt of Staffa and Giants'
+Causeway are familiar examples of this structure. Jointing is due to the
+gradual consolidation of the strata, and hence, in a series of strata,
+we may find the separate beds, according to their composition, very
+variously affected, some being much more abundantly jointed than
+others. Master-joints which traverse a wide district in some definite
+direction probably owe their origin to tension, the strata having been
+subjected to some strain by the underground forces.
+
+   [Illustration: Fig. 5.--Beds of Limestone (_a_), Sandstone
+   (_b_), and Shale (_c_), divided into cuboidal masses by
+   master-joints.]
+
+   [Illustration: Fig. 6.--Columnar Structure.]
+
+   [Illustration: Fig. 7.--Bedding, Joints, and Cleavage (after
+   Murchison).]
+
+42. _Cleavage._--Fine-grained rocks, more especially those which are
+argillaceous, occasionally shew another kind of structure, which is
+called _cleavage_. Common clay-slate is a type of the structure. This
+rock splits up into innumerable thin laminae or plates, the surface of
+which may either be somewhat rough, or as smooth nearly as glass. The
+cleavage planes, however, need not be parallel with the planes of
+bedding; in most cases, indeed, they cut right across these, and
+continue parallel to each other often over a very wide region. The
+original bedding is sometimes entirely obliterated, and in most cases of
+well-defined cleavage is always more or less obscure.
+
+In the preceding diagram, the general phenomena of _bedding_,
+_jointing_, and _cleavage_ are represented. The lines of bedding are
+shewn at S, S; another set of division-planes (joints) is observed at J,
+J, intersecting the former at right angles--A, B, C being the exposed
+faces of joints. The lines of cleavage are seen at D, D, cutting across
+the planes of bedding and jointing.
+
+43. _Foliation_ is another kind of superinduced structure. In a
+foliated rock the mineral ingredients have been crystallised and
+arranged in layers along either the planes of original bedding or those
+of cleavage. Mica-schist and gneiss are typical examples.
+
+44. _Concretions._--In many rocks a concretionary structure may be
+observed. Some sandstones and shales appear as if made up of spheroidal
+masses, the mineral composition of the spheroids not differing
+apparently from that of the unchanged rock. So in some kinds of
+limestone, as in _dolomite_, the concretionary structure is often highly
+developed, the rock resembling now irregular heaps of turnips with
+finger-and-toe disease, again, piles of cannon-balls, or bunches of
+grapes, and agglomerations of musket-shot. A spheroidal structure is
+occasionally met with amongst some igneous rocks. This is well seen in
+the case of rocks having the basaltic structure, in which the pillars,
+being jointed transversely, decompose along their division planes, so as
+to form irregular globular masses. In many cases, certain mineral matter
+which was originally diffused through a rock has segregated so as to
+form nodules and irregular layers. Examples of this are _chert_ nodules
+in limestone; _flint_ nodules in chalk; _clay-ironstone_ balls in shale,
+&c.
+
+   [Illustration: Fig. 8.--Dip and Strike of Strata.]
+
+45. _Inclination of Strata._--Beds of aqueous strata must have been
+deposited in horizontal or approximately horizontal planes; but we now
+find them most frequently inclined at various angles to the horizon, and
+often even standing on end. They sometimes, however, retain a horizontal
+position over a large tract of country. The angle which the inclined
+strata make with the horizon is called the _dip_, the degree of
+inclination being the _amount_ of the dip; and a line drawn at right
+angles to the dip is called the _strike_ of the beds. Thus, a bed
+dipping south-west will have a north-west and south-east strike. The
+_crop_ or _outcrop_ (sometimes also, but rarely, called the _basset
+edge_) of a bed is the place where the edge of the stratum comes to view
+at the surface. We may look upon inclined beds as being merely parts of
+more or less extensive undulations of strata, the tops of the
+undulations having been removed so as to expose the truncated edges of
+the beds. In the following diagram, for example, the outcrops of
+limestone seen at _l_, _l_, are evidently portions of one and the same
+stratum, the dotted lines indicating its former extent. The
+trough-shaped arrangement of the beds at _s_ is called a _synclinal
+curve_, or simply a syncline; the arched strata at _a_ forming, on the
+contrary, an _anticlinal curve_ or _anticline_.
+
+   [Illustration: Fig. 9.--Anticlines and Synclines.]
+
+   [Illustration: Fig. 10.--Contorted Strata.]
+
+When strata shew many and rapid curves, they are said to be contorted.
+The diagram section (fig. 10) will best explain what is meant by this
+kind of structure.
+
+46. In certain regions, the strata often dip in one and the same
+direction for many miles, at an angle approaching verticality, as in the
+following section. It might be inferred, therefore, that from A to B we
+had a gradually ascending series--that as we paced over the outcrop we
+were stepping constantly from a lower to a higher geological horizon.
+But, in such cases, the dip is deceptive, the same beds being repeated
+again and again in a series of great foldings of the strata. Such is
+the case over wide areas in the upland districts of the south of
+Scotland. The section (fig. 11) shews that the beds are actually
+inverted, the strata at x x being bent back upon strata which really
+overlie them.
+
+   [Illustration: Fig. 11.--Inversion of Strata.]
+
+   [Illustration: Fig. 12.--Contemporaneous Erosion.]
+
+47. _Contemporaneous Erosion._--Occasionally a group of strata gives
+proof that pauses in the deposition of sediment took place, during which
+running water scooped out of the sediment channels of greater or less
+width, which subsequently became filled up with similar or dissimilar
+materials. The diagram (fig. 12) will render this plain. At _a_ we have
+beds of sandstone, which it is evident were at one time throughout as
+thick as they still are at x x. Having been worn away to the extent
+indicated, a deposition of clay (_b_) succeeded; and this, in turn,
+became eroded at _c_, _c_, the hollows being filled up again with coarse
+sand and gravel. In former paragraphs, we found reason to believe that
+lines of bedding indicated certain pauses in the deposition of strata.
+Here, in the present case, we have more ample proof in the same
+direction.
+
+   [Illustration: Fig. 13.--Unconformability.]
+
+48. _Unconformability._--But the most striking evidence of such pauses
+in the deposition of strata is afforded by the phenomenon called
+_unconformability_. When one set of rocks is found resting on the
+upturned edges of a lower set, the former are said to be _unconformable_
+to the latter. In the above section (fig. 13), _a_, _a_, are beds of
+sandstone resting on the upturned edges of beds of limestone, shale, and
+sandstone, _l_, _s_. Figs. 14 and 15 give other examples of the same
+appearance. It is evident that, in the case of fig. 14, the discordant
+bedding chronicles the lapse of a very long period. We have to conceive
+first of the deposition of the underlying strata in horizontal or
+approximately horizontal layers; then we have to think of the time when
+they were crumpled up into great convolutions, and the tops of the
+convolutions (the anticlines) were planed away: all these changes
+intervened, of course, after the lower set was deposited, and before the
+upper series was laid down. In the case represented in fig. 15, we have
+a double unconformability, implying a still more elaborate series of
+changes, and probably, therefore, a still longer lapse of time.
+
+   [Illustration: Fig. 14.--Violent Unconformability.]
+
+   [Illustration: Fig. 15.--Double Unconformability.]
+
+49. _Overlap._--When the upper beds of a conformable group of strata
+spread over a wider area than the lower members of the same series, they
+are said to _overlap_. The accompanying diagram shews this appearance.
+An overlap proves that a gradual submergence of the land was going on at
+the time the strata were being accumulated. As the land disappeared
+below the water, the sediment gradually spread over a wider area, the
+more recently deposited sediment being laid down in places which existed
+as dry land at the time when the earliest accumulations were formed.
+Thus, in the accompanying illustration (fig. 16), the stratum marked 1,
+resting unconformably upon older strata, is overlapped by 2, as that is
+by 3, and so on--all the beds in succession coming to repose upon the
+older strata at higher and higher levels, as the old land subsided.
+
+   [Illustration: Fig. 16.--Overlap.]
+
+   [Illustration: Fig. 17.--Fault.]
+
+50. _Dislocations or Faults._--When strata, once continuous, have been
+broken across, and displaced or shifted along the line of breakage, they
+are said to be _faulted_, the fissure along which the displacement
+occurs being termed a _fault_ or _dislocation_. The simplest form of a
+fault is that shewn in the following diagram, where strata of sandstone
+and shale, with a coal-seam, S, have been shifted along the line _f_.
+The direction in which the _fault_ is inclined[D] is its _hade_, and the
+degree of _vertical displacement_ of the beds is the _amount_ of the
+dislocation. Generally, the beds seem to be pulled _down_ in the
+direction of the _downthrow_, and _drawn up_ on the opposite side of the
+fault, as shewn in the diagram. Sometimes the rocks on each side of a
+fault are smoothed and polished, and covered with long scratches, as if
+the two sides of the fissure had been rubbed together. This is the
+appearance called _slickensides_. Slickensides, however, may occur on
+the walls of a fissure which is not a displacement, but a mere joint or
+crack. A dislocation is spoken of as a downthrow or an upcast, according
+to the direction in which it is approached. Thus, a miner working along
+the coal-seam S, from _a_ to _b_, would describe the fault, _f_, as an
+_upcast_, since he would have to mine to a _higher_ level to catch his
+coal again. But, had he approached the fault from _c_ to _d_, he would
+then have termed it a _downthrow_, because he would see from the hade of
+the fault that his coal-seam must be sought for at a _lower_ level.
+Faults are of all sizes, from a foot or two up to vertical displacements
+of thousands of feet. Powerful dislocations can often be followed for
+many miles across a country, running in a more or less linear direction.
+Thus, one large fault has been traced across the breadth of Scotland,
+from near St Abb's Head, in the east, to the coast of Wigtown, in the
+west. Every large throw is accompanied by a number of smaller
+ones--some of which run parallel to the main fault, while many others
+seem to run out from this at various angles. Faults are of all
+geological ages. Some date back to a most remote antiquity, others are
+of quite recent origin; and no doubt faults are occurring even now. In
+the following diagram, the strata, _a, a_, have been faulted and planed
+away before the strata, _b_, were deposited. Hence, in this case, it is
+evident that if we know the geological age of the beds, _a_ and _b_, we
+can have an approximation to the age of the fault. If the beds, _a_, be
+Carboniferous, and those at _b_ Permian, then we should say the fault
+was _post-Carboniferous_ or _pre-Permian_.
+
+   [D] The degree of inclination is very variable. It may occur at
+       almost any angle up to vertical. But, as a rule, the hade of the
+       more powerful faults is steeper than that of minor displacements.
+
+   [Illustration: Fig. 18.--Ground-plan of Large Main Fault and
+   Minor Displacement Fissures.]
+
+   [Illustration: Fig. 19.--Faulted Strata covered by undisturbed
+   Strata.]
+
+51. _Metamorphic and Igneous Rocks--mode of their occurrence._--In the
+foregoing remarks on the structure and arrangement of rocks we have had
+reference chiefly to the aqueous strata--that is to say, the
+_mechanically_, _chemically_, and _organically_ formed rocks. We were
+necessarily compelled, however, to make some reference to, and to give
+some description of, certain structures and arrangements which are not
+peculiar to aqueous strata, but characterise many metamorphic and
+igneous rocks as well. To avoid repetition it was also necessary, while
+treating of _joints_, &c., to give some account of certain structures
+which are the result of metamorphic action. But, for sake of clearness,
+we have reserved special account of the structure and mode of occurrence
+of metamorphic and igneous rocks to this place. After what has been said
+as to the structure and arrangement of aqueous strata, it is hardly
+needful to say much about the crystalline schists. These the student
+will understand to be merely highly altered aqueous rocks,[E] in which
+the marks of their origin are still more or less distinctly traceable.
+As a rule, metamorphic strata are contorted, twisted, and crumpled,
+although here and there comparatively horizontal stretches of altered
+rocks may be observed. The regions in which they occur are often hilly
+and mountainous, but this is by no means invariably the case. The
+greater part of the mountainous regions of the British Islands is
+occupied by rocks which are more or less altered; the more crystalline
+rocks, such as mica-schist, gneiss, &c., being abundantly developed in
+the Scottish Highlands, and in the north and west of Ireland; while
+those which are less altered cover large areas in the south of Scotland,
+and in Wales and the north-west of England. Throughout these wide areas
+the rocks generally dip at high angles, and contortion and crumpling are
+of common occurrence. The finer-grained clay-rocks also exhibit fine
+cleavage planes, and are in some places quarried for roofing-slates--the
+Welsh quarries being the most famous. Here and there, bedding is
+entirely effaced, and the resulting rock is quite amorphous, and,
+becoming gradually more and more crystalline, passes at last into a rock
+which in many cases is true granite. The original strata have
+disappeared, and granite occupies their place, in such a way as to lead
+to the inference that the granite is merely the aqueous strata which
+have been fused up, as it were, _in situ_. At other times the granite
+would appear to have been erupted amongst the aqueous strata, for these
+are highly confused, and baked, as it were, at their junction with the
+granite, from which, also, long veins are seen protruding into the
+surrounding beds. Metamorphic granite, then, graduates, as a rule,
+almost imperceptibly into rocks which are clearly of aqueous origin;
+while on the contrary the junction-line between igneous granite and the
+surrounding rocks is always well marked. The origin of granite, however,
+is a difficult question, and one which has given rise to much
+discussion. Some further remarks upon the subject will be found in the
+sequel under the heading of _Metamorphism_.
+
+   [E] Igneous rocks have also in some cases undergone considerable
+       alteration; fine-grained tuffs, for example, occasionally
+       assume a crystalline texture.
+
+52. True _igneous rocks_ occur either in beds or as irregular amorphous
+masses. When they occur as beds interstratified with aqueous strata,
+they are said to be _contemporaneous_, because they have evidently been
+erupted at the time the series of strata among which they appear was
+being amassed. When, on the other hand, they cut across the bedding,
+they are said to be _subsequent_ or _intrusive_, because in this case
+they have been formed at a period _subsequent_ to the strata among which
+they have been _intruded_. The bed upon which a contemporaneous igneous
+rock reclines, often affords marks of having been subjected to the
+action of heat; sandstones being hardened, and frequently much jointed
+and cracked, owing to the shrinking induced by the heat of the once
+molten rock above, and clay-rocks often assuming a baked appearance.
+There is generally, also, some discoloration both in the pavement of
+rock upon which the igneous mass lies, and in the under portions of the
+latter itself. The beds overlying a contemporaneous igneous rock,
+however, do not exhibit any marks of the action of heat; the old
+lava-stream having cooled before the sediment, now forming the overlying
+strata, was accumulated over its surface. One may often notice how the
+sand and mud have quietly settled down into the irregular hollows and
+crevices of the old lava, as in the following section, where _i_
+represents the igneous rock; _a_ being the baked pavement of sandstone,
+&c.; and _b_ the overlying sedimentary deposits. When the igneous rock
+itself is examined, its upper portions are often observed to be
+scoriaceous or cinder-like, and the under portions likewise frequently
+exhibit a similar appearance. It is generally most solid towards the
+centre of the bed. The vesicles, or pores, in the upper and lower
+portions are often flattened, and are frequently filled with mineral
+matter. Sometimes these cavities may have been filled at the time the
+rock was being erupted, but in most cases the mineral matter would
+appear to have been introduced subsequently by the action of water
+percolating through the rock. Occasionally we meet with igneous rocks
+which are more or less vesicular and amygdaloidal throughout their
+entire mass. Others, again, often shew no vesicular structure, but are
+homogeneous from top to bottom. The texture is also very variable, and
+this even in the same rock-mass; some portions being compact or
+fine-grained, and others coarsely crystalline. As a rule the rock is
+most crystalline towards the centre, and gets finer-grained as the top
+and bottom of the bed are approached. Not unfrequently, however, an
+igneous rock will preserve the same texture throughout. The jointing is
+also highly irregular as a rule. But in many cases, especially when the
+rock is fine-grained, the jointing is very regular. The basaltic columns
+of the Giants' Causeway and the Isle of Staffa are well-known examples
+of such regularly jointed masses. Igneous rocks frequently decompose
+into a loose earthy mass (_wacke_), and this is most markedly the case
+with those belonging to the basic group.
+
+   [Illustration: Fig. 20.--Contemporaneous Igneous Rock.]
+
+53. Contemporaneous igneous rocks are frequently associated with more or
+less regular beds of _breccia_, _conglomerate_, _ash_, _tuff_, &c. These
+are evidently the loose volcanic ejectamenta which accompanied former
+eruptions of lava, and have been arranged by the action of water. Beds
+of such materials, however, frequently occur without any accompanying
+lava-form rocks. Nor are they always arranged in bedded masses. They
+sometimes appear filling vertical pipes which seem to have been the
+funnels of old volcanoes. The following section exhibits the general
+appearance of one of these volcanic _necks_. They are very common in
+some parts of Scotland, as in Ayrshire, and are frequently ranged along
+the line of a fault in the strata. Fig. 21 shews such a neck of
+ejectamenta, made up of fragments of various kinds of rock, such as
+sandstone, shale, limestone, coal, &c., sometimes without any admixture
+of igneous rocks. The strata through which the pipe has been pierced
+usually dip in towards the latter, and at their junction with the coarse
+agglomerate often shew marks of the action of heat, coal-seams having
+sometimes been 'burned' useless for a number of yards away from the
+'neck.'
+
+   [Illustration: Fig. 21.--Neck filled with Volcanic Agglomerate.]
+
+54. Intrusive igneous rocks occur as _sheets_, _dykes_, and _necks_. The
+sheets frequently conform for long distances to the bedding of the
+strata among which they occur, and are thus liable to be mistaken for
+contemporaneous rocks. But when they are closely examined, it will be
+seen that they not only bake or alter the beds above and below them, but
+seldom keep precisely to one horizon or level--occasionally rising to a
+higher, or sinking to a lower position in the strata, as shewn in the
+following diagram-section. Dykes are wall-like masses of igneous strata
+which cut across the strata, generally at a high angle (see _d, d_, fig.
+22). In the neighbourhood of a recent volcanic orifice, numerous dykes
+are seen ramifying in all directions. In the British Islands some dykes
+have been followed in a linear direction for very long distances.
+Sometimes these occupy the sites of large dislocations, at other times
+they have cut through the strata without displacing them. Occasionally
+they appear to have been the feeders of the great sheets of igneous rock
+which here and there occur in their vicinity. The phenomena presented by
+the _necks_ of intrusive rock do not differ from those characteristic of
+_agglomerate_ or _tuff necks_. The strata are bent down towards the
+central plug of igneous rock, and are generally more or less altered at
+the line of junction.
+
+   [Illustration: Fig. 22.--Intrusive Sheet and Dykes: _i_, igneous
+   intrusive sheet; _d_, _d_, dykes; _s_, _s_, sedimentary strata.]
+
+55. Intrusive rocks offer, as a rule, some contrasts in texture to
+contemporaneous masses. They are seldom amygdaloidal, but when they are
+so it is generally towards the centre of the mass. The kernels are
+usually minute and more or less spherical.
+
+   [Illustration: Fig. 23.--Contemporaneous and Intrusive Igneous
+   Rocks: _c_, _c_, contemporaneous trap-rocks[1]; _t_, _t_,
+   contemporaneous fragmental igneous rocks; _i_, _p_, _n_, _d_,
+   intrusive igneous rocks.]
+
+The diagram (fig. 23) shews the general mode of occurrence of igneous
+rocks on the large scale. The stratified aqueous deposits are indicated
+at _a_, _a_. These are overlaid by a series of alternating beds of
+crystalline (_c_) and fragmental (_t_) igneous rocks. An irregular
+intrusive sheet at _i_ cuts across the beds _a_, _a_. At _p_, another
+intrusive mass is seen rising in a pipe, as it were, and overflowing the
+beds _a_, _a_, so as to form a cap. A volcanic neck filled with angular
+stones intersects the strata at _n_, and two dykes, approaching the
+vertical, traverse the bedded rocks at _d_, _d_. It will be noticed that
+the contemporaneous igneous rocks form a series of escarpments rising
+one above the other.
+
+The alteration effected by igneous rocks is generally greatest in the
+case of intrusive masses. This is well seen in some of our coal-fields,
+where the coal has frequently been destroyed over large areas by the
+proximity of masses of what was once melted rock. It is curious to
+notice how the intrusive sheets in a great series of strata have forced
+their way along the lines of least resistance. Thus, in the Scottish
+coal-fields, we find again and again that intrusive sheets have been
+squirted along the planes occupied by coal-seams, these having been more
+easily attacked than beds of sandstone or shale. The coal in such cases
+is either entirely 'eaten up,' as it were, or converted into a black
+soot. At other times, however, it is changed into a kind of coke, while
+other seams at a greater distance from the intrusive mass have been
+altered into a kind of 'blind coal' or _anthracite_.
+
+These remarks on the mode of occurrence of igneous rocks are meant to
+refer chiefly to those masses which occur in regions where volcanic
+action has long been extinct, as, for instance, in the British Islands.
+In the sequel, some account will be given of the appearances presented
+by modern volcanoes and volcanic rocks.
+
+   [1] It has been usual to apply the term _trap_ or _trappean_ rock
+       to all the old igneous rocks which could neither be classed
+       with the granites and syenites, nor yet with the recent lavas,
+       &c., which are connected with a more or less well-marked
+       volcanic vent. The term _trap_ (Swedish _trappa_, a flight of
+       steps) was suggested by the terraced or step-like appearance
+       presented by hills which are built up of successive beds of
+       igneous rock. But the passage from the granitic into the
+       so-called trap rocks, and from these into the distinctly
+       volcanic, is so very gradual, that it is impossible to say
+       where the one class ends and the other begins. The term _trap_,
+       therefore, has no scientific precision, although it is
+       sometimes very convenient as a kind of broad generic term to
+       include a large number of rocks.
+
+
+MINERAL VEINS.
+
+56. The cracks and crevices and joint planes which intersect all rocks
+in a greater or less degree, are not unfrequently filled with
+subsequently introduced mineral matter, forming what are termed _veins_.
+This introduced matter may either be harder or less durable than the
+rock itself; in the former case, the veins will project from the surface
+of the stone, where that has been subjected to the weathering action of
+the atmosphere; in the latter case, the veins, under like circumstances,
+are often partially emptied of their mineral matter. Not unfrequently,
+however, the more or less irregularly ramifying, non-metalliferous veins
+appear as if they had segregated from the body of the rock in which they
+occur, as in the case of the quartz veins in granite. Besides these
+irregular veins, the rocks of certain districts are traversed in one or
+more determinate directions by fissures, extending from the surface down
+to unknown depths. These great fissures are often in like manner filled
+with mineral matter. The minerals are usually arranged in bands or
+layers which run parallel to the walls of the vein. Quartz, fluor-spar,
+barytes, calcite, &c. are among the commonest vein-minerals, and with
+these are frequently associated ores of various metals. A vein may vary
+in width from less than an inch up to many yards, and the arrangement of
+its contents is also subject to much variation. Instead of parallel
+layers of spars and ores, frequently a confused mass of clay and broken
+rocks, which are often cemented together with sparry matter, chokes up
+the vein. The ore in a vein may occur in one or more ribs, which often
+vary in thickness from a mere line up to masses several yards in width.
+Sometimes the rocks are dislocated along the line of fissure occupied by
+a great vein; at other times no dislocation can be observed. Mineral
+veins, however, do not necessarily occupy dislocation fissures. They
+often occur in cavities which have been formed by the erosive action of
+acidulated water, in the way described in pars. 59, 60, and 61. This is
+frequently the case in calcareous strata. Such veins usually coincide
+more or less with the bedding of the rocks, but in the case of thick
+limestones they not unfrequently cut across the bedding in a vertical or
+nearly vertical direction, forming what are termed _pipe-veins_.
+
+
+
+
+DYNAMICAL GEOLOGY.
+
+
+57. Having considered the composition, structure, and arrangement of the
+rock-masses which form the solid crust of our globe, we have next to
+inquire into the nature of those physical agencies by the action of
+which the rocks, as we now see them, have been produced. The work
+performed by the various forces employed in modifying the earth's crust
+is at one and the same time destructive and reconstructive. Rocks are
+being continually demolished, and out of their ruins new rocks are being
+built. In other words, matter is constantly entering into new
+relations--now existing as solid rock, or in solution in water, or
+carried as the lightest dust on the wings of the wind; now being swept
+down by rivers into the sea, or brought under the influence of
+subterranean heat--but always changing, sooner or later, slowly or
+rapidly, from one form to another. The great geological agents of change
+are these: 1. THE ATMOSPHERE; 2. WATER; 3. PLANTS AND ANIMALS; 4.
+SUBTERRANEAN FORCES. We shall consider these in succession.
+
+
+THE ATMOSPHERE.
+
+58. All rocks have a tendency to waste away under the influence of the
+atmosphere. This is termed _weathering_. Under the influence of the
+sun's heat, the external portions of a rock expand, and again contract
+when they cool at night. The effect of this alternate expansion and
+contraction is often strikingly manifest in tropical countries: some
+rocks being gradually disintegrated, and crumbling into grit and sand;
+others becoming cracked, and either exfoliating or breaking up all over
+their surface into small angular fragments. Again, in countries subject
+to alternations of extreme heat and cold, similar weathering action
+takes place. The chemical action of the atmosphere is most observable in
+the case of calcareous rocks. The carbonic acid almost invariably
+present acts as a solvent, so that dew and rain, which otherwise would
+in many cases have but feeble disintegrating power, are enabled to eat
+into such rocks as chalk and limestone, calcareous sandstones, &c. The
+oxygen of the atmosphere also unites with certain minerals, such as the
+proto-salts of iron, and converts them into peroxides. It is this action
+which produces the red and yellow ferruginous discolorations in
+sandstone. Chemical changes also take place in the case of many igneous
+rocks, the result being that a weathered 'crust' forms wherever such
+rocks are exposed to the action of the atmosphere. Of course, the rate
+at which a rock weathers depends upon its mineralogical and chemical
+composition. Limestones weather much more rapidly than clay-rocks; and
+augitic igneous rocks, as a rule, disintegrate more readily than the
+more highly silicated species. The weathering action of the atmosphere
+is also greatly aided by frost, as we shall see presently. The result of
+all this weathering is the formation of _soil_--soil being only the
+fine-grained debris of the weathered rocks. The angular debris found at
+the base of all cliffs in temperate and arctic regions, and on every
+hill and mountain which is subjected to alternations of extreme heat and
+cold, is also the effect of weathering. But these and other effects of
+frost will be treated of under the head of _Frozen Water_. The hillocks
+and ridges of loose sand (_sand dunes_) found in many places along the
+sea-margin, and even in the interior of some continents, as in Africa
+and Asia, are due to the action of the wind, which drives the loose
+grains before it, and piles them up. Sometimes also the wind carries in
+suspension the finest dust, which may be transported for vast distances
+before it falls to the ground. Thus, fine dust shot into the air by the
+volcanoes of Iceland has been blown as far as the Shetland Islands; and
+in tropical countries the dust of the dried-up and parched beds of lakes
+and rivers is often swept away during hurricanes, and carried in thick
+clouds for leagues. Rain falling through this dust soaks it up, and
+comes down highly discoloured, brown and red. This is the so-called
+_blood-rain_. Minute microscopic animal and vegetable organisms are
+often commingled with this dust, and falling into streams, lakes, or the
+sea, may thus become eventually buried in sediments very far removed
+from the place that gave them birth.
+
+
+WATER.
+
+59. The geological action of water in modifying the crust of the earth
+is twofold--namely, _chemical_ and _mechanical_.
+
+_Underground Water._--All the moisture which we see falling as rain or
+snow does not flow immediately away by brooks and rivers to the sea.
+Some portion of it soaks into the ground, and finds a passage for itself
+by cracks and fissures in the rocks below, from which it emerges at last
+as springs, either at the surface of the earth, or at the bottom of the
+sea. Such are the more obvious courses pursued by the water--it flows
+off either by sub-aerial or subterranean channels. But a not
+inconsiderable portion soaks into the solid rocks themselves, which are
+all more or less porous and pervious. Water thus slowly soaking often
+effects very considerable chemical changes. Sometimes the binding matter
+which held the separate particles of the rock together is dissolved out,
+and the rock is thus rendered soft and crumbling; at other times, the
+reverse takes place, and the water deposits, in the minute interstitial
+pores, some binding matter by which the partially or wholly incoherent
+grains are agglutinated into a solid mass. Thus what were originally
+hard and tough rocks become disintegrated to such a degree, that they
+crumble to powder soon after they are exposed to the air; while some
+again are converted into a clay, and may be dug readily with a spade.
+And, on the other hand, loose sand is glued into a hard building-stone.
+There are many other changes effected upon rocks by water, in virtue of
+the chemical agents which it holds in solution. Indeed, it may be said
+that there are very few, if any, rocks in which the chemical action of
+interstitial water has not formerly been, or is not at present being,
+carried on. Besides that which soaks through the rocks themselves, there
+is always a large proportion of underground water, which, as we have
+said above, finds a circuitous route for itself by joints, cracks, and
+crevices. After coursing for, it may be, miles underground, such water
+eventually emerges as springs, which contain in solution the various
+ingredients which the water has chemically extracted from the rocks.
+These ingredients are then deposited in proportion as the mineral water
+suffers from evaporation. Water impregnated with carbonate of lime, for
+example, deposits that compound as soon as evaporation has carried off a
+certain percentage of the water itself, and the carbonic acid gas which
+it held. This is the origin of the mineral called _travertine_ or
+_calcareous tufa_, which is so commonly met with on the margins of
+springs, rivers, and waterfalls.
+
+60. _Stalactites_ and _stalagmites_ have been formed in a similar way.
+Water slowly oozing from the roof of a limestone cavern partially
+evaporates there, and a thin pellicle of carbonate of lime is formed;
+while that portion of the water which falls to the ground, and is there
+evaporated, likewise gives rise to the formation of carbonate of lime.
+By such constant dropping and evaporating, long tongue-and icicle-like
+pendants (_stalactites_) grow downwards from the roof; while at the same
+time domes and bosses (_stalagmites_) grow upwards from the floor, so as
+sometimes to meet the former and give rise to continuous pillars and
+columns. The great solvent power of carbonated water is shewn first by
+the chemical analysis of springs, and, secondly, by the great wasting
+effects which the long-continued action of these has brought about.
+Thus, it has been estimated that the fifty springs near Carlsbad, which
+yield eight hundred thousand cubic feet of water in twenty-four hours,
+contain in solution as much lime as would go to form a mass of stone
+weighing two hundred thousand pounds. Warm, or, as they are termed,
+_thermal_ springs, frequently carry away with them, out of the bowels of
+the earth, vast quantities of mineral matter in solution. The waters at
+Bath, for instance, are estimated to bring to the surface an annual
+amount of various salts, the mass of which is not less than 554 cubic
+yards. One of the springs of Loueche, France, however, carries out with
+it no less than 8,822,400 pounds of gypsum annually, which is equal to
+about 2122 cubic yards.
+
+61. It is easy to conceive, therefore, that in the course of ages great
+alterations must be caused by springs. Caves and winding galleries, and
+irregular channels, will be worn out of the rocks which are thus being
+dissolved. Especially will this be the case in countries where
+calcareous rocks abound. It is in such regions, accordingly, where we
+meet with the most striking examples of caves and underground
+river-channels. The largest cave at present known is the Mammoth Cave,
+in Kentucky. This remarkable hollow consists of numerous winding
+galleries and passages that cross and recross, and the united length of
+which is said to be 217 miles. In calcareous countries, rivers, after
+flowing for, it may be, miles at the surface, suddenly disappear into
+the ground, and flow often for long distances before they reappear in
+the light of day. In some regions, indeed, nearly all the drainage is
+subterranean. The surface of the ground, in calcareous countries,
+frequently shews circular depressions, caused by the falling in of the
+roofs of caverns. Sometimes, also, great masses of rock, often miles in
+extent, get loosened by the dissolving action of subterranean water, and
+crash downwards into the valleys. Such _landslips_, as they are called,
+are not, however, confined to calcareous regions. In 1806, a large
+section of the Rossberg, a mountain lying to the north of the Righi,
+consisting of conglomerate overlying beds of clay, rushed down into the
+plains of Goldau, overwhelming four villages and nearly a thousand
+inhabitants. The cause of this catastrophe was undoubtedly the softening
+into mud of the clay-beds on which the conglomerate rested, for the
+season which had just terminated when the slip took place had been very
+wet. The mass of material that slid down was estimated to contain
+upwards of fifty-four millions of cubic yards; it reached not less than
+two and a half miles in length, by some three hundred and fifty yards
+wide, and thirty-five yards thick.
+
+62. _Surface-water--Rain._--Having now learned something as to the
+modifications produced by underground water, we turn next to consider
+the action of surface-water, and the results arising from that action.
+Rain, when it falls to the ground, carries with it some carbonic acid
+gas which it has absorbed from the atmosphere. Armed with this solvent,
+it attacks certain rocks, more especially limestones and chalk, a
+certain proportion of which it licks up and delivers over to brooks and
+streams. Under its influence, also, the finer particles of the soil are
+ever slowly making their way from higher to lower levels. Rocks which
+are being gradually disintegrated by weathering have their finer grains
+and particles, thus loosened, carried away by rain. Nor is this
+rain-action so inconsiderable as might be supposed. In the gentler
+hollows of an undulating country, we frequently find accumulations of
+clay, loam, and brick-earth, which often reach many feet in thickness,
+and which are undoubtedly the results of rain washing down the particles
+of soil, &c. from the adjacent slopes.
+
+63. _River-action._--The water of streams and rivers almost invariably
+contains in solution one or more chemical compounds, and in this respect
+does not differ from the water of springs. Of course, this mineral
+matter is derived in considerable measure from springs, but is also no
+doubt to a large extent taken up by the rivers themselves, as they wash
+the rocks and soils on their journey to the sea. The amount of mineral
+matter thus transported must be something enormous, as is shewn by the
+chemical analyses of river-water. Bischof calculated that the Rhine
+carries in solution as much carbonate of lime as would suffice for the
+yearly formation of three hundred and thirty-two thousand millions of
+oyster-shells of the usual size--a quantity equal to a cube five hundred
+and sixty feet in the side, or a square bed a foot thick, and upwards of
+two miles in the side. But the mechanical erosion effected by running
+water is what impresses us most with the importance of rivers as
+geological agencies. This erosive action is due to the gravel, sand, and
+mud carried along by the water. These ingredients act as files in the
+hand of a workman, and grind, polish, and reduce the rocks against which
+they are borne. The beds of some streams that flow over solid rock are
+often pitted with circular holes, at the bottom of which one invariably
+finds a few rounded stones. These stones, kept in constant motion by the
+water, are the means by which the _pot-holes_, as they are called, have
+been excavated. When pot-holes are numerous, they often unite so as to
+form curious smooth-sided trenches and gullies. The same filing action
+goes on all over the bed of the stream wherever the solid rock is
+exposed. And while the latter is being gradually reduced, the stones and
+grit which act as the files are themselves worn and reduced; so that
+stones diminish in size, and grit passes into fine sand and mud, as they
+move from higher to lower levels. No doubt the erosive action of running
+water appears to have but small effect in a short time, and we are apt,
+therefore, to underestimate its power. But when our observations extend,
+we see it is quite otherwise, and that, so far from being unimportant,
+running water is really one of the most powerful of all the geological
+agencies that are employed in modifying the earth's crust. Even within a
+comparatively short time, it is able to effect very considerable
+changes. Thus, the river Simeto, in Sicily, having become dammed by a
+stream of lava flowing from Etna, succeeded, in two hundred and fifty
+years, in cutting through hard solid basalt a new channel for itself,
+which measured from twenty to fifty metres in depth, and from twelve to
+eighteen in breadth. When, also, we remember the fact, that no river is
+absolutely free from mineral matter held in suspension, but that, on the
+contrary, all running water is more or less discoloured with sediment,
+which is merely the material derived from the disintegration of rocks,
+it will appear to us difficult to overestimate the power of watery
+erosion. To the mineral matter held in suspension, we have to add the
+coarser detritus, gravel and sand, which is being gradually pushed along
+the beds of rivers, and which, in the case of the Mississippi, has been
+estimated to equal a mass of seven hundred and fifty million cubic feet,
+discharged annually into the Gulf of Mexico. By careful measurements, it
+has also been ascertained that the same river carries down annually into
+the sea a weight of mud held in suspension which reaches the vast sum of
+812,500,000,000 pounds. The total annual amount of mineral matter,
+whether held in suspension or pushed along the bottom of this great
+river, has been estimated to equal a mass 268 feet in height, with an
+area of one square mile.
+
+64. _Alluvium._--The sediment carried along and deposited by a river is
+called _alluvium_. Sometimes this alluvium covers wide areas, forming
+broad flats on one or both sides of a river, and in such cases it is due
+to the action of the floodwaters of the stream. Every time the river
+overflows the low grounds through which it passes, a layer of sediment
+is laid down, which has the effect of gradually raising the level of the
+alluvial tract. By and by a time comes when the river, which has all the
+while been slowly deepening its channel, is unable to flood the flats,
+and thereupon it begins to cut into these, and to form new flats at a
+somewhat lower level. In this way we often observe a series of alluvial
+terraces, consisting of gravel, sand, and silt, rising one above another
+along a river valley. Such are the terraces of the Thames and other
+rivers in England, and of the Tweed, Clyde, Tay, &c. in Scotland. The
+great plains through which the Rhine flows between Basel and Bingen, are
+also well-marked examples of alluvial accumulations. There are very few
+streams, indeed, which have not formed such deposits along some portion
+of their course.
+
+65. When a river enters a lake, the motion of the water is of course
+checked, and hence the heavier detritus, such as gravel and coarse sand,
+moves more slowly forward, and at last comes to rest on the bed of the
+lake, at no great distance from the mouth of the river. Finer sand and
+mud are carried out for some distance further, but eventually they also
+cease to move, and sink to the bottom. When the lake is sufficiently
+large, it catches all or nearly all the matter brought down by the
+river, which, as it issues from the lower end of the lake, is bright and
+clear. A well-known example of this phenomenon is that of the Rhone,
+which enters the Lake of Geneva turbid and muddy, but rushes out quite
+clear at the lower end of the lake. Lakes, therefore, are all being
+slowly or more rapidly silted up, and this, of course, is most
+conspicuous at the points where they are entered by rivers. Thus, at the
+head of the Lake of Geneva, it is manifest that the wide flat through
+which the river flows before it pours into the lake, has been conquered
+by the Rhone from the latter. In the times of the Romans, the lake, as
+we know, extended for more than one mile and a half further up the
+valley.
+
+66. _Deltas._--When there are no lakes to intercept fluviatile sediment,
+this latter is borne down to the sea, where it is deposited in precisely
+the same way as in a lake: the heavier detritus comes to rest first, the
+finer sediment being swept out for some distance further. So that, in
+passing from the river-mouth outwards, we have at first gravel, which
+gradually gets finer and finer until it is replaced by sand, while this
+in turn is succeeded by mud and silt. There is this difference, however,
+between lacustrine and fluvio-marine deposits, that while the former
+accumulate in water which is comparatively still, the latter are often
+brought under the influence of waves and currents, and become shifted
+and sifted to such a degree that fine and coarse detritus are frequently
+commingled; and there is, therefore, not the same orderly succession of
+coarse and fine materials which characterises lacustrine deposits.
+Often, indeed, the currents opposite the mouth of a river are so strong,
+that little or no sediment is permitted to gather there. Usually,
+however, we find that rivers have succeeded in reclaiming more or less
+wide tracts from the dominion of the waves, or at all events have
+cumbered the bed of the sea with banks and bars of detritus. The broad
+plains formed at the mouth of a river are called _deltas_, from their
+resemblance to the Greek letter [Delta]. The deltas of the Nile, Ganges,
+and Mississippi are among the most noted. The term _delta_, however, is
+not exclusively applied to fluvio-marine deposits; rivers also form
+deltas in fresh-water lakes. It is usual, however, to restrict the term
+to extensive alluvial plains which are intersected by many winding
+channels, due to the rapid bifurcation of the river, which begins to
+take place at the very head of the great flat--that is to say, at the
+point where the river originally entered the sea (or lake).
+
+67. _Frozen Water._--We have now seen what can be done by the mechanical
+action of running water. We have next to consider what modifications are
+effected by freezing and frozen water. Water, as every one knows,
+expands in the act of freezing, and in doing so exerts great force. Let
+the reader bear in mind what has been said as to the passage of water
+through the minute and often invisible pores of rocks, and to its
+presence in cracks and crevices after every shower of rain, and he will
+readily see how excessive must be the waste caused by the action of
+frost. The water, to as great a depth as the frost extends, passes into
+the solid state, and in doing so pushes the grains of the rocks asunder,
+or wedges out large masses. No sooner does thaw ensue than the water,
+becoming melted, allows the grains of the rock to fall asunder; the
+outer skin of the rock, as it were, is disintegrated, and crumbles away,
+while fragments and masses lose their balance in many cases, and topple
+down. Hence it is, that in all regions where frost acts, the hill-tops
+and slopes are covered with angular fragments and debris, and a soil is
+readily formed by the disintegration of the rocks.
+
+River-ice is often a potent agent of geological change. Stones get
+frozen in along the margins of a river, and often debris falls down from
+cliff and scaur upon the surface of the ice; when thaw sets in, and the
+ice breaks up, stones and rubbish are frequently floated for long
+distances, and may even be carried out to sea before their support
+fails them, and they sink to the bottom. In some cases, when the ice is
+very thick, it may run aground in a river, and confuse and tumble up the
+deposits gathering at the bottom. Ice sometimes forms upon stones at the
+bottom of a river, and floats these off; and this curious action may
+take place even although no ice be forming at the time on the surface of
+the water.
+
+68. _Glaciers, Icebergs, and Ice-foot._--In certain mountainous
+districts, and in arctic and antarctic regions, snow accumulates to such
+an extent that its own weight suffices to press the lower portions into
+ice. Alternate thawing and freezing also aid in the formation of the
+ice, which soon begins to creep down the mountain-slopes into the
+valleys, where it constitutes what are called _glaciers_ or ice-rivers.
+These great masses of ice attain often a great thickness, and frequently
+extend for many miles along the course of a valley. In the Alps they
+occasionally reach as much as five hundred or six hundred feet in depth.
+In Greenland, however, there are glaciers probably not less than five
+thousand feet thick; and the glacier ice of the antarctic continent has
+been estimated even to reach twelve miles in thickness. Glaciers flow
+slowly down their valleys, at a rate which varies with the slope of
+their beds and the mass of the ice. Some move only a few inches, others
+two or three feet, in a day. Their forward motion is arrested at a point
+where the ice is melted just as fast as it comes on. A glacier is always
+more or less seamed with yawning cracks, which are called _crevasses_.
+These owe their origin to the unequal rate at which the different parts
+of the ice flow; this differential motion causing strains, to which the
+ice yields by snapping asunder. The flanks of a glacier are usually
+fringed with heaps of angular blocks and debris which fall from the
+adjacent rocky slopes, and some of this rubbish tumbling into the gaping
+crevasses must occasionally reach to the bottom of the ice. The rubbish
+heaps (_superficial moraines_) travel slowly down the valley on the
+surface of the ice, and are eventually toppled over the end of the
+glacier, where they form great banks and mounds. These are called
+_terminal moraines_. The rocky bed of a glacier is invariably smoothed
+and polished, and streaked with coarse and fine _striae_, or scratches,
+which run parallel to the direction of the ice-flow. These are due to
+the presence, at the bottom of the ice, of such angular fragments as
+become detached from the underlying rocks, or of boulders and rubbish
+which have been introduced from above. The stones are ground by the ice
+along the surface of its bed, causing ruts and scratches, while the
+finer material resulting from the grinding action forms a kind of
+polisher. The stones acting as gravers are themselves covered with
+striae, and their sharp edges get smoothed away. In alpine districts
+there is always a good deal of water circulating underneath a glacier,
+and this washes out the sand and fine clay. Thus it is that rivers
+issuing from glaciers are always more or less discoloured brown, yellow,
+green, gray, or blue, according to the nature of the rocks which the ice
+has pounded down into mud. In Greenland many of the large glaciers go
+right out to sea, and owing to their great thickness are able to
+dispossess the sea sometimes for miles. But erelong the greater specific
+gravity of the sea-water forces off large segments from the terminal
+front of the ice, which float away as _icebergs_. Large masses are also
+always falling down from the ice-front. Occasionally, big blocks and
+debris are floated away on the icebergs, but this does not appear to be
+common. In Greenland there is very little rock-surface exposed, from
+which blocks can be showered down upon the glaciers, and the surface of
+the latter is therefore generally free from superficial moraines. A kind
+of submarine terminal moraine, however, gathers in front of some
+glaciers, made up chiefly of the stones and rubbish that are dragged
+along underneath the ice, and exposed by the breaking-off of icebergs,
+but partly composed also of the sand and mud washed out by sub-glacial
+waters. A narrow belt of ice forms along the sea-coast in arctic
+regions, which often attains a thickness of thirty or forty feet. This
+is called the _ice-foot_. It becomes loaded with debris and blocks,
+which fall upon it from the cliffs above; and, as large portions are
+frequently detached from the cliffs in summer-time, they sail off with
+their cargoes of debris, and drop these over the sea-bottom as they
+gradually melt away. The ice-foot is the great distributor of _erratics_
+or wandered blocks, the part taken in this action by the huge icebergs
+which are discharged by the glaciers being, comparatively speaking,
+insignificant. But when these latter run aground, they must often cause
+great confusion among the beds of fine material accumulating upon the
+floor of the sea.
+
+69. _The Sea._--Sea-water owes its saltness to the presence of various
+more or less soluble substances, such as _common salt_, _gypsum_, _Epsom
+salts_, _chloride of magnesium_, &c. Besides these, there are other
+ingredients held in solution, which, although they can be detected in
+only minute quantities in sea-water, are yet of the very utmost
+importance to marine creatures. This is the case with _carbonate of
+lime_, vast quantities of which are carried down by many rivers to the
+sea. But it must be nearly all used up in the formation of hard shells
+and skeletons by molluscs, crustaceans, corals, &c., for very little can
+be traced in the water itself. _Silica_ is also met with sparingly, and
+is abstracted by some creatures to form their hard coverings.
+
+70. _Breaker-action--Currents._--The most conspicuous action of the sea,
+as a geological agent, takes place along its margin, where the breakers
+are hurled against the land. Stones and gravel are borne with more or
+less intense force against the rocks, and by their constant battering
+succeed eventually in undermining the cliffs, which by and by become
+top-heavy, and large masses fall down and get broken up and pounded into
+gravel and sand. The new wall of rock thus exposed becomes in turn
+assaulted, and in course of time is undermined in like manner. The waste
+of the cliffs is greatly aided by the action of frost, which loosens the
+jointed rocks, and renders them an easier prey to the force of the
+waves. Of course, the rapidity with which a coast-line is eaten into
+depends very much upon the nature of the rocks. Where these are formed
+of loose materials like sand, gravel, or clay, considerable inroads are
+effected by the sea in a comparatively short time. Thus, along some
+parts of the English coast, as between Flamborough Head and the mouth of
+the Humber, and between the Wash and the Thames, it is estimated that
+the land is wasted away at the rate of a yard per annum. Where hard
+rocks form the coast-line the rate of waste is often exceedingly slow,
+and centuries may elapse without any apparent change being effected.
+When the rocks are of unequal hardness the coast-line becomes very
+irregular, the sea carving out bays and gullies in the softer portions,
+while the more durable masses stand out as capes and bold headlands. Not
+unfrequently, such headlands are converted into sea-stacks and rocky
+islets, as one may observe along the rockier parts of our shore-lines.
+Close inshore, the bulkier debris derived from the waste of the land
+often accumulates, forming beds and banks of shingle and gravel. The
+finer materials are carried farther out to sea, and distributed over the
+sea-floor by the action of the tide and currents. Tidal and other
+currents may also have some denuding effect upon the sea-bottom, but
+this can only be in comparatively shallow water. The great bulk of the
+material derived from the waste of the coasts by the mechanical action
+of the breakers, travels for no great distance. But the fine mud brought
+down by rivers is frequently transported for vast distances before it
+settles. So fine, indeed, is some of this sedimentary material, that it
+may be carried in suspension by sea-currents for thousands of miles
+before it sinks to the bottom.
+
+71. From this short outline it becomes evident, therefore, that the
+coarser-grained the deposit, the smaller will be the area it covers;
+while conversely, the finer the accumulation, the more widely will it be
+distributed. A partial exception to this rule is that of the debris
+scattered over the bottom of the ocean by icebergs and detached portions
+of ice-foot. These are often floated for vast distances by currents
+before they finally melt away, and hence the coarse debris transported
+by them must be very widely distributed over that part of the sea-bottom
+which is traversed by currents flowing out of the Arctic and Antarctic
+Oceans. Although the deeper recesses of the ocean appear to be covered
+only with ooze and fine mud, yet in some instances coarse sand, and even
+small stones, have been brought up from depths of a hundred fathoms, so
+that currents may occasionally carry coarser materials for great
+distances from the shore. The shifting action of tidal currents succeeds
+in giving rise to very irregular deposits in shallow seas. The
+soundings often shew sudden changes from gravel to sand and mud, nor can
+there be any doubt that, could we lay bare the sea-bottom, we should
+often observe gravel shading off into sand, and sand into mud, and _vice
+versa_. But as we receded from the shore, and approached areas which
+were once deeply submerged, we should find that the change of material
+was generally from coarse to fine.
+
+
+GEOLOGICAL ACTION OF PLANTS AND ANIMALS.
+
+72. _Plants._--The disintegration of rocks is often aided by the
+action of plants, which force their roots into joints and crevices,
+and thus loosen blocks and fragments. Carbonic acid, derived from the
+decay of plants, being absorbed by rain-water, acts chemically upon
+many rocks, as in the case of limestone (see 59, 60, 61). In temperate
+regions, vegetation frequently accumulates, under certain conditions,
+to form very considerable masses. Of such a nature is _peat_, which,
+as is well known, covers many thousands of acres in the British
+Islands. This substance is composed fundamentally of the bog-moss
+(_Sphagnum palustre_), with which, however, are usually associated
+many other marsh-loving plants. The lower parts of bog-moss die and
+decay while its upper portions continue to flourish, and thus, in
+process of time, a thickness of peat is accumulated to the extent of
+six, twelve, twenty-four, or even forty feet. Many of the hill-tops
+and hill-slopes in Scotland and Ireland are covered with a few feet of
+peat, but it is only in valleys and hollows where the peat-bogs attain
+their greatest depth. In not a few cases, the bogs seem to occupy the
+sites of ancient lakes, shell-marl often occurring at the bottom of
+these. The trunks and roots of trees are also commonly met with
+underneath peat, and occasionally the remains of land animals.
+Frequently, indeed, it would seem as if the overthrow of the trees, by
+obstructing the drainage of the country, had given rise to a marsh,
+and the consequent formation of peat. Some of the most valuable peat
+closely resembles lignite, and makes a good fuel. In tropical
+countries, the rapidity with which vegetation decays prevents, as a
+rule, any great accumulation taking place; but the mangrove swamps are
+exceptions.
+
+73. _Animals._--The action of animal life is for the most part
+conservative and reconstructive. Considerable accumulations of
+shell-marl take place in fresh-water lakes, and the flat bottoms which
+mark the sites of lakes which have been drained are frequently dug to
+obtain this material. But by far the most conspicuous formations due
+to the action of animal life accumulate in the sea. Molluscs,
+crustaceans, corals, and the like, secrete from the ocean the
+carbonate of lime of which their hard shells and skeletons are
+composed, and these hard parts go to the formation of limestone. The
+most remarkable masses of modern limestone occur within intertropical
+regions. These are the coral reefs of the Pacific and Indian Oceans.
+
+   [Illustration: Fig. 24.--Formation of Coral Reefs.]
+
+74. _Coral_ is the calcareous skeleton of certain small soft-bodied
+gelatinous animals called _actinozoa_. These zoophytes flourish only in
+clear water, the temperature of which is not below 66 deg. F., and they
+cannot live at greater depths than one hundred feet. There are three
+kinds of coral reef--namely, _fringing_ reefs, _barrier_ reefs, and
+_atolls_. Fringing reefs occur, as a rule, near to the shore; but if
+this latter be gently sloping, they may extend for one or even two miles
+out to sea; as far, indeed, as the depth of water is not too great for
+the actinozoa. Barrier reefs are met with at greater distances from the
+land, and often rise from profound depths. The barrier reef which
+extends along the north-east coast of Australia, often at a distance
+from the land of fifty or sixty miles, stretches, with interruptions,
+for about 1250 miles, with a breadth varying from ten to ninety miles.
+In some places, the depth of the sea immediately outside of this reef
+exceeds 1800 feet. Sometimes barrier reefs completely encircle an island
+or islands, which are usually mountainous, as in the case of Pouynipete,
+an island in the Caroline Archipelago, and the Gambier Islands in the
+Low Archipelago. _Atolls_ are more or less irregular ring-shaped reefs
+inclosing a lagoon of quiet water. They usually rise from profound
+depths; Keeling Atoll, in the Indian Ocean, is a good example. The upper
+surface of atolls and barrier reefs often peers at separate points above
+the level of the sea, so as to form low-lying islets. In some cases, the
+land thus formed is almost co-extensive with the reef, and being clothed
+with palms and tropical verdure, resembles a beautiful chaplet floating,
+as it were, in mid-ocean. The rock of a coral reef is a solid white
+limestone, similar in composition to that of the limestones occurring in
+this country. In some places, it is quite compact, shewing few or no
+inclosed shells or other animal remains; in other places, it is made up
+of broken and comminuted corals cemented together, or of masses of coral
+standing as they slowly grew, with the spaces between the separate
+clumps filled up with coral sand and triturated fragments and grit of
+coral and shell. The thickness of the reefs is often very great,
+reaching in many cases to thousands of feet. At the Fijis, the reef can
+hardly be less than 2000 or 3000 feet thick. Below a depth of one
+hundred feet, all the coral rock is dead, and since the coral zoophytes
+do not live at greater depths than this, it follows that the bed of the
+sea in which coral reefs occur must have slowly subsided during a long
+course of ages. Mr Darwin was the first to give a reasonable explanation
+of the origin of coral reefs. Briefly stated, his explanation is as
+follows: The corals began to grow first in water not exceeding one
+hundred feet in depth, and built up to the surface of the sea, thus
+forming a fringing reef at no great distance from the land. This initial
+step is shewn at A, B, in the accompanying section across a coral
+island. A, A, are the outer edges of the fringing reef; B, B, the shores
+of the island; and S1 the level of the sea. Subsidence ensuing, the
+island and the sea-bottom sink slowly down, while the coral animals
+continue to grow to the surface--the building of the reef keeping pace
+with the subsidence. By and by the island sinks to the level S2, when
+B', B', represent the shores of the now diminished island, and A', A',
+the outer edges of the reef, which has become a barrier reef; C, C,
+being the lagoon between the reef and the central island. We have now
+only to suppose a continuance of the submergence to the level S3, when
+the island disappears, its site being occupied by a lagoon, C'--the
+reef, which has at the same time become an atoll, being shewn at A'',
+A''.
+
+75. In extra-tropical latitudes, great accumulations of carbonate of
+lime are also taking place. The bottom of the Atlantic has been found to
+be covered, over vast areas, by a fine calcareous sticky deposit called
+_ooze_, which would appear to consist for the most part of the skeletons
+of minute animal organisms, called Foraminifera. This accumulation, when
+dried, closely resembled chalk, and there can be no doubt that in the
+deep recesses of the Atlantic we have thus a gradually increasing
+deposit of carbonate of lime, which rivals, if it does not exceed, in
+extent the most widely spread calcareous rocks with which we are
+acquainted. A small percentage of siliceous materials occurs in the
+ooze, made up partly of granules of quartz, and partly of the skeletons
+and coverings of minute animal and vegetable organisms. When in process
+of time the chemical forces begin to act upon the siliceous matter
+diffused through the Atlantic ooze, _segregation_, or the gathering
+together of the particles, may take place, and nodules of flint will be
+the result, similar to the flint nodules which occur in chalk, and the
+cherty concretions in limestones. Animalcules with siliceous envelopes
+and skeletons are by no means so abundant as those that secrete
+carbonate of lime, but they are very widely diffused through the oceans,
+and in favourable places are so abundant that they may well give rise
+eventually to extensive beds of flint. Ehrenberg calculated that 17,946
+cubic feet of these organisms were formed annually in the muddy bottom
+of the harbour at Wismar, in the Baltic.
+
+It would appear from recent observations (_Challenger_ expedition) that
+the calcareous ooze at the bottom of the Atlantic and Southern Oceans,
+which occurs at a mean depth of 2250 fathoms, passes gradually as the
+ocean deepens into a gray ooze, which is less calcareous, and which
+occurs at a mean depth of 2400 fathoms. At still greater depths this
+gray ooze also disappears, and is replaced by red clay at a mean depth
+of 2700 fathoms. The minute creatures (foraminifera and pelagic
+mollusca chiefly) whose shells go to form the calcareous ooze, live for
+the most part on the surface, and swarm all over the areas in which ooze
+and red clay occur at the bottom. Hence it seems probable that the clay
+is merely the insoluble residue or _ash_, as it were, of the
+organisms--the delicate shells, as they slowly sink to the more profound
+depths, being dissolved by the free carbonic acid, which, as
+observations would seem to shew, occurs rather in excess at great
+depths. Thus we see how the organic forces may give rise to extensive
+accumulations of inorganic matter, closely resembling the finest silt or
+mud which is carried down to the sea by rivers, and distributed far and
+wide by ocean currents.
+
+
+SUBTERRANEAN FORCES.
+
+76. There have been many speculations as to the condition of the
+interior of the earth. Some have inferred that the external crust of the
+globe incloses a fluid or molten mass; others think it more probable
+that the interior is solid, but contains scattered throughout its bulk,
+especially towards the surface of the earth, irregular seas of molten
+matter, occupying large vesicles or tunnels in the solid honey-combed
+mass. At present, the facts known would appear to be best explained by
+the latter hypothesis. All that we know from observation is, that the
+temperature increases as we descend from the surface. The rate of
+increase is very variable. Thus, in the Artesian well at Neuffen, in
+Wuertemberg, it was as much as 1 deg. F. for every 19 feet. In the mines of
+Central Germany, however, the increase is only 1 deg. F. for every 76 feet;
+while in the Dukinfield coal-pit, near Manchester, the increase was
+still less, being only 1 deg. F. in 89 feet. Taking the average of many
+observations, it may be held as pretty well proved that the temperature
+of the earth's crust increases 1 deg. for every 50 or 60 feet of descent
+after the first hundred.
+
+77. The crust of the earth is subject to certain movements, which are
+either sudden and paroxysmal, or protracted and tranquil. The former are
+known as earthquakes, which may or may not result in a permanent
+alteration of the relative level of land and sea; the latter always
+effect some permanent change, either of upheaval or depression.
+
+78. _Earthquakes_ have been variously accounted for. Those who uphold
+the hypothesis of a fluid interior think the undulatory motion
+experienced at the surface is caused by movements in the underlying
+molten mass--an earthquake being thus 'the reaction of the liquid
+nucleus against the outer crust.' By others, again, earthquakes are
+supposed to be caused by the fall of large rock-masses from the roofs of
+subterranean cavities, or by any sudden impulse or blow, such as might
+be produced by the cracking of rocks in a state of tension, by a sudden
+volcanic outburst, or sudden generation or condensation of steam. In
+support of this latter hypothesis, many facts may be adduced. The
+undulatory motion communicated to the ground during gunpowder
+explosions, or by the fall of rocks from a mountain, is often propagated
+to great distances from the scene of these catastrophes, and the
+phenomena closely resemble those which accompany a true earthquake. When
+the level of a district has been permanently affected by an earthquake,
+the movement has generally resulted in a lowering of the surface. Thus,
+in 1819, the Great Runn of Cutch, in Hindustan, was depressed over an
+area of several thousand square miles, so as during the monsoons to
+become a salt lagoon. Occasionally, however, we find that elevation of
+the land has taken place during an earthquake. This was the case in New
+Zealand in 1855, when the ground on which the town of Wellington stands
+rose about two feet, and a cape in the neighbourhood nearly ten feet.
+Sometimes the ground so elevated is, after a shorter or longer period,
+again depressed to its former level. A good example of this occurred in
+South America in 1835. The shore at Concepcion was raised a yard and a
+half; and the Isle Santa Maria was pushed up two and a half yards at one
+end, and three and a half yards at the other. But only a few months
+afterwards the ground sank again, and everything returned to its old
+position. The heaving and undulatory motion of an earthquake produces
+frequently considerable changes at the surface of the ground, besides an
+alteration of level. Rocks are loosened, and sometimes hurled down from
+cliff and mountain-side, and streams are occasionally dammed with the
+soil and rubbish pitched into them. Sometimes also the ground opens, and
+swallows whatever chances to come in the way. If these chasms close
+again permanently, no change in the physiography of the land may take
+place, but sometimes they remain open, and affect the drainage of the
+country.
+
+79. _Movements of Upheaval and Depression._--Besides the permanent
+alteration of level which is sometimes the result of a great earthquake,
+it is now well known that the crust of the earth is subject to
+long-continued and tranquil movements of elevation and depression. The
+cause of these movements is at present merely matter for speculation,
+some being of opinion that they may be caused by the gradual contraction
+of the slowly cooling nucleus of the earth, which would necessarily give
+rise to depression, while this movement, again, would be accompanied by
+some degree of elevation--the result of the lateral push or thrust
+effected by the descending rock-masses. It is doubtful, however, if this
+hypothesis will explain all the appearances. The Scandinavian peninsula
+affords a fine example of the movements in question. At the extremity of
+the peninsula (Scania), the land is slowly sinking, while to the north
+of that district gradual elevation is taking place at a very variable
+rate, which in some places reaches as much as two or three feet in a
+century. Movements of elevation are also affecting Spitzbergen, Northern
+Siberia, North Greenland, the whole western borders of South America,
+Japan, the Kurile Islands, Asia Minor, and many other districts in the
+Mediterranean area, besides various islets in the great Pacific Ocean.
+The proofs of a slow movement of elevation are found in old
+_sea-beaches_ and _sea-caves_, which now stand above the level of the
+sea. In the case of Scandinavia, it has been noticed that the pine-woods
+which clothe the mountains are being slowly elevated to ungenial
+heights, and are therefore gradually dying out along their upper limits.
+The proofs of depression of the land are seen in submerged forests and
+peat, which occur frequently around our own shores, and there is also
+strong human testimony to such downward movements of the surface. The
+case of Scania has already been referred to. Several streets in some of
+its coast towns have sunk below the sea, and it is calculated that the
+Scanian coast has lost to the extent of thirty-two yards in breadth
+within the past hundred and thirty years. The coral reefs of southern
+oceans also afford striking evidence of a great movement of depression.
+
+Not long ago a theory was started by a French savant, M. Adhemar, to
+account for changes in the sea-level, without having recourse to
+subterranean agency. He pointed out that a vast ice-cap, covering the
+northern regions of our hemisphere, as was certainly the case during
+what is termed the glacial epoch, would cause a rise of the sea by
+displacing the earth's centre of gravity. Mr James Croll has recently
+strongly supported this opinion; and there can be no doubt that we have
+here a _vera causa_ of considerable mutations of level. It is
+unquestionably true, however, that great oscillatory movements, such as
+described above, and which can only be attributed to subterranean
+agencies, have frequently taken and are still taking place.
+
+80. Such movements of the earth's crust cannot take place without
+effecting some change upon the strata of which that crust is composed.
+During _depression_ of the curved surface of the earth, the under strata
+must necessarily be subjected to intense lateral pressure, since they
+are compelled to occupy less space, and contortion and plication will be
+the result. It is evident also that contortion will diminish from below
+upwards, so that we can conceive that excessive contortion may be even
+now taking place at a great depth from the surface in Greenland. During
+a movement of _elevation_, on the other hand, the strata are subjected
+to excessive tension, and must be seamed with great rents: when the
+elevating force is removed, the disrupted rocks will settle down
+unequally--in other words, they will be _faulted_, and their continuity
+will be broken. But both contortion and faulting may be due, on a small
+scale, to local causes, such as the intrusion of igneous rocks, the
+consolidation of strata, the falling in of old water-courses, &c.
+_Cleavage_ is believed to have been caused by compression, such as the
+rocks might well be subjected to during great movements of the earth's
+crust. The particles of which the rock is composed are compressed in one
+direction, and of course are at the same time drawn out at right angles
+to the pressure. This is observed not only as regards the particles of
+the rock themselves, but imbedded fossils also are distorted and
+flattened in precisely the same way.
+
+81. _Volcanoes._--Besides movements of elevation and depression, there
+are certain other phenomena due to the action of the subterranean
+forces. Such are the ejection from the interior of the earth of heated
+matters, and their accumulation upon the surface. The erupted materials
+consist of molten matter (lava), stones and dust, gases and steam--the
+lava, ashes, and stones gradually accumulating round the focus of
+ejection, and thus tending to form a conical hill or mountain. Could we
+obtain a complete section of such a volcanic cone, we should find it
+built up of successive irregular beds of lava, and layers of stones and
+ashes, dipping outwards and away from the source of eruption, but having
+round the walls of the _crater_ (that is, the cavity at the summit of
+the truncated cone) a more or less perceptible dip inwards. Fig. 25
+gives a condensed view of the general phenomena accompanying an
+eruption. In this ideal section, _a_ is the funnel or neck of the
+volcano filled with lava; _b_, _b_, the crater. The molten lava is
+highly charged with elastic fluids, which continually escape from its
+surface with violent explosions, and rise in globular clouds, _d_, _d_,
+to a certain height, after which they dilate into a dark cloud, _c_.
+From this cloud showers of rain, _e_, are frequently discharged. Large
+and small portions of the incandescent lava are shot upwards as the
+imprisoned vapour of water explodes and makes its escape, and, along
+with these, fragments of the rocks forming the walls of the crater and
+the funnel are also violently discharged; the cooled bombs, angular
+stones, and _lapilli_, as the smaller stones are called, falling in
+showers, _f_, upon the exterior parts of the cone or into the crater,
+from which they are again and again ejected. Most frequently the great
+weight of the lava inside the crater suffices to break down the side of
+the cone, and the molten rock escapes through the breach. Sometimes,
+however, it issues from beneath the base of the cone. At other times,
+finding for itself some weak place in the cone, it may flow out by a
+lateral fissure, _g_. In the diagram, _i_, _i_ represents the lava
+streaming down the outward slopes, jets of steam and fumaroles escaping
+from almost every part of its surface. Forked lightning often
+accompanies an eruption, and is supposed to be generated by the intense
+mutual friction in the air of the ejected stones. The trituration to
+which these are subjected reduces them, first, to a kind of coarse
+gravel (_lapillo_); then to sand (_puzzolana_); and lastly, to fine dust
+or ashes (_ceneri_).
+
+   [Illustration: Fig. 25.--Diagrammatic Section of Volcano.]
+
+82. _Lava._--Any rock which has been erupted from a volcano in a molten
+state is called _lava_. Some modern lava-streams cover a great extent of
+surface. One of two streams which issued from the volcano of Skaptur
+Jokul (Iceland) in 1783 overflowed an area fifty miles in length, with a
+breadth in places of fifteen; the other was not much less extensive,
+being forty miles in length, with an occasional breadth of seven. In
+some places the lava exceeded 500 feet in thickness. Again, in 1855, an
+eruption in the island of Hawaii sent forth a stream of lava sixty-five
+miles long, and from one to ten miles wide. The surface of a stream
+quickly cools and consolidates, and in doing so shrinks, so as to become
+seamed with cracks, through which the incandescent matter underneath can
+be seen. As the current flows on, the upper crust separates into rough
+ragged scoriform blocks, which are rolled over each other and jammed
+into confused masses. The slags that cake upon the face or front of the
+stream roll down before it, and thus a kind of rude pavement is formed,
+upon which the lava advances and is eventually consolidated. Thus, in
+most cases, a bed of lava is scoriaceous as well below as above. Other
+kinds of lava are much more ductile and viscous, and coagulate
+superficially in glossy or wrinkled crusts. When lava has inclosed
+fragments of aqueous rocks, such as limestone, clay, or sandstone, these
+are observed to have undergone some alteration. The sandstone is often
+much hardened, the clay is porcelainised, and the limestone, still
+retaining its carbonic acid, assumes a crystalline texture. But the
+aqueous rock upon which lava has cooled does not usually exhibit much
+change, nor does the alteration, as a rule, extend more than a few feet
+(often only a few inches) into the rock. A lava-current which entered a
+lake or the sea, however, has sometimes caught up much of the sediment
+gathering there, and become so commingled with it, that in some parts it
+is hard to say whether the resulting rock is more igneous or aqueous.
+Lava which has been squirted up from below into cracks and crevices, and
+there consolidated so as to form _dykes_, sometimes, but not often,
+produces considerable alteration upon the rocks which it intersects. The
+basaltic structure is believed to be due to the contraction of lava
+consequent upon its cooling. The axes of the prisms are always
+perpendicular to the cooling surface or surfaces, and in some cases the
+columns are wonderfully regular. There are numerous varieties of lava,
+such as _basalt_, _obsidian_, _pitchstone_, _pearlstone_, _trachyte_,
+&c.; some are heavy compact rocks, others are light and porous. Many are
+finely or coarsely crystalline; others have a glassy and resinous or
+waxy texture. Some shew a flaky or laminated structure; others are
+concretionary. Most of the lava rocks, however, are granularly
+crystalline. In many, a vesicular character is observed. These vesicles,
+being due to the bubbles of vapour that gathered in the molten rock,
+usually occur in greatest abundance towards the upper surface of a bed
+of lava. They are also more or less well developed near the bottom of a
+bed, which, as already explained, is frequently scoriaceous.
+Occasionally the vesicles are disseminated throughout the entire rock.
+As a rule, those lavas which are of inferior specific gravity are much
+more vesicular than the denser and heavier varieties. The vesicles are
+usually more or less flattened, having been drawn out in the direction
+in which the lava-current flowed. Sometimes they are filled, or
+partially filled, with mineral matter introduced at the time of
+eruption, or subsequently brought in a state of solution and deposited
+there by water filtering through the rock: this forms what is called
+_amygdaloidal lava_. In volcanic districts, the rocks are often
+traversed by more or less vertical dykes or veins of igneous matter.
+These dykes appear in some cases to have been formed by the filling up
+of crevices from above--the liquid lava having filtered downwards from
+an overflowing mass. In most cases, however, the lava has been injected
+from below, and not unfrequently the 'dykes' seem to have been the
+feeders from which lava-streams have been supplied--the feeders having
+now become exposed to the light of day either by some violent eruption
+which has torn the rocks asunder, or else by the gradual wearing away of
+the latter by atmospheric and aqueous agencies.
+
+
+METAMORPHISM.
+
+83. Mention has already been made of the fact, that the heated matters
+ejected from volcanoes, or forcibly intruded into cracks, crevices, &c.,
+occasionally _alter_ the rocks with which they come in contact. When
+this alteration has proceeded so far as to induce a crystalline or
+semi-crystalline character, the rock so altered is said to be
+metamorphosed. Metamorphism has likewise been produced by the chemical
+action of percolating water, which frequently dissolves out certain
+minerals, and replaces these with others having often a very different
+chemical composition. But metamorphism on the large scale--that is to
+say, metamorphism which has affected wide areas, such as the northern
+Highlands of Scotland and wide regions in Scandinavia, or the still
+vaster areas in North America--has most probably been effected both by
+the agency of heat and chemical action, at considerable depths, and
+under great pressure. When we observe what effect can be produced by
+heat upon rocks, under little or no pressure, and how water percolating
+from above gradually changes the composition of some rock-masses, we may
+readily believe that at great depths, where the heat is excessive, such
+metamorphic action must often be intensified. Thus, for example,
+limestone heated in the usual way gives off its carbonic acid gas, and
+is reduced to quicklime; but, under sufficient pressure, this gas is not
+evolved, the limestone becoming converted into a crystalline marble.
+Some crystalline limestones, indeed, have all the appearance of having
+at one time been actually melted and squirted under great pressure into
+seams and cracks of the surrounding strata. Heated water would appear to
+have been the agent to which much of the metamorphism which affects the
+rocky strata must be attributed. But the mode or modes in which it has
+acted are still somewhat obscure; as may be readily understood when it
+is remembered how difficult, and often how impossible it is to realise
+or reproduce in our laboratories the conditions under which deep-seated
+metamorphic action must frequently have taken place. In foliated rocks,
+the minerals are chiefly quartz, felspar, and mica, talc, or chlorite.
+The ingredients of these minerals undoubtedly existed in a diffused
+state in the original rocks, and heated water charged with alkaline
+carbonates, as it percolated through the strata, either along the
+layers of bedding or lines of cleavage, slowly acted upon these,
+dissolving and redepositing them, and thus inducing segregation. There
+is every kind of gradation in metamorphism. Thus, we find certain rocks
+which are but slightly altered--their original character being still
+quite apparent; while, in other cases, the original character is so
+entirely effaced that we can only conjecture what that may have been.
+When we have a considerable thickness of metamorphic rocks which still
+exhibit more or less distinct traces of bedding, like the successive
+beds of gneiss, mica-schist, and quartz rock of the Scottish Highlands,
+we can hardly doubt that the now crystalline masses are merely highly
+altered aqueous strata. But there are cases where even the bedding
+becomes obliterated, and it is then much more difficult to determine the
+origin of the rocks. Thus, we find bedded gneiss passes often, by
+insensible gradations, into true amorphous granite. There has been much
+difference of opinion as to the origin of granite--some holding it to be
+an igneous rock, others maintaining its metamorphic origin. It is
+probably both igneous and metamorphic, however. If we conceive of
+certain aqueous rocks becoming metamorphosed into gneiss, we may surely
+conceive of the metamorphism being still further continued until the
+mass is reduced to a semi-fluid or pasty condition, when all trace of
+foliation and bedding might readily disappear, and the weight of the
+superincumbent strata would be sufficient to force portions of the
+softened mass into cracks and crevices of the still solid rocks above
+and around it. Hence we might expect to find the same mass of granite
+passing gradually in some places into gneiss, and in other places
+protruding as _veins_ and _dykes_ into the surrounding rocks; and this
+is precisely what occurs in nature.
+
+84. _Mineral veins_ have, as a rule, been formed by water depositing
+along the walls of fissures the various matters which they held in
+solution, but certain kinds of veins (such as quartz veins in granite)
+probably owe their origin to chemical action which has induced the
+quartz to segregate from the rock mass. Some have maintained that the
+metallic substances met with in many veins owe their deposition to the
+action of currents of voltaic electricity; while others have attributed
+their presence to sublimation from below, the metals having been
+deposited in the fissures very much as lead is deposited in the chimney
+of a leadmill. But in many cases there seems little reason to doubt that
+the ores have merely been extracted from the rocks, and re-deposited in
+fissures, by water, in the same way as the other minerals with which
+they are associated.
+
+
+PHYSIOGRAPHY.
+
+85. _Denudation._--By the combined action of all the geological agencies
+which have been described in the preceding sections, the earth has
+acquired its present diversified surface. Valleys, lacustrine hollows,
+table-lands, and mountains have all been more or less slowly formed by
+the forces which we see even now at work in the world around us. When we
+reflect upon the fact that all the inclined strata which crop out at the
+surface of the ground are but the truncated portions of beds that were
+once continuous, and formed complete anticlinal arches or curves, we
+must be impressed with the degree of _denudation_, or wearing-away,
+which the solid strata have experienced. If we protract in imagination
+the outcrop of a given set of strata, we shall find them curving upwards
+into the air to a height of, it may be, hundreds or even thousands of
+feet, before they roll over to come down and fit on to the truncated
+ends of the beds on the further side of the anticline (see figs. 9 and
+11, pages 33, 34). _Dislocations_ or _faults_ afford further striking
+evidence in the same direction. Sometimes these have displaced the
+strata for hundreds and even thousands of feet--that is to say, that a
+bed occurring at, for example, a few feet from the surface upon one side
+of a fault, has sunk hundreds or thousands of feet on the other side.
+Yet it often happens that there is no irregularity at the surface to
+betray the existence of a dislocation. The ground may be flat as a
+bowling-green, and yet, owing to some great fault, the rocks underneath
+one end of the flat may be geologically many hundred feet, or even
+yards, higher or lower than the strata underneath the other end of the
+same level space. What has become of the missing strata? They have been
+carried away grain by grain by the denuding forces--by weathering, rain,
+frost, and fluviatile and marine action. The whole surface of a country
+is exposed to the abrading action of the subaerial forces, and has been
+carved by them into hills and valleys, the position of which depends
+partly upon the geological structure of the country, and partly upon the
+texture and composition of the rocks. The original slope of the surface,
+when it was first elevated out of the sea, would be determined by the
+action of the subterraneous forces--the dominant parts, whether
+table-lands or undulating ridges, forming the centres from which the
+waters would begin to flow. After the land had been subjected for many
+long ages to the wearing action of the denuding agents, it is evident
+that the softer rocks--those which were least capable of withstanding
+weathering and erosion--would be more worn away than the less easily
+decomposed masses. The latter would, therefore, tend to form elevations,
+and the former hollows. This is precisely what we find in nature. The
+great majority of isolated hills and hilly tracts owe their existence as
+such merely to the fact that they are formed of more durable materials
+than the rock-masses by which they are surrounded. When a line of
+dislocation is visible at the surface, it is simply because rocks of
+unequal durability have been brought into juxtaposition. The more easily
+denuded strata have wasted away to a greater extent than the tougher
+masses on the other side of the dislocation. Nearly all elevations,
+therefore, may be looked upon as monuments of the denudation of the
+land; they form hills for the simple reason that they have been better
+able to withstand the attacks of the denuding agents than the rocks out
+of which the hollows have been eroded.
+
+86. To this general rule there are exceptions, the most obvious being
+hills and mountains of volcanic origin, such as Hecla, Etna, Vesuvius,
+&c., and, on a larger scale, the rocky ridge of the Andes. Again, it is
+evident that the great mountain-chains of the world are due in the first
+place to upheaval; but these mountains, as we now see them--peaks,
+cliffs, precipices, gorges, ravines--have been carved out of the solid
+block, as it were, by the ceaseless action of the subaerial forces. The
+direction of river-valleys has in like manner been determined in the
+first place by the original slope of the land; but the deep dells, the
+broad valleys and straths, have all been scooped out by running water.
+The northern Highlands of Scotland, for example, evidently formed at one
+time a broad table-land, elevated above the level of the sea by the
+subterranean forces. Out of this old table-land the denuding agents,
+acting through untold ages, have carved out all the numerous ravines,
+glens, and valleys, the intervening ridges left behind now forming the
+mountains. It is true that now and again streams are found flowing in
+the direction of a fault, but that is simply because the dislocation is
+a line of weakness, along which it is easier for the denuding forces to
+act. For one fault that we find running parallel to the course of a
+river, we may observe hundreds cutting across its course at all angles.
+The great rocky basins occupied by lakes, which are so abundant in the
+mountainous districts of temperate regions and in northern latitudes,
+are believed to have been excavated by the erosive power of glacier-ice;
+and they point, therefore, to a time when our hemisphere must have been
+subjected to a climate severe enough to nourish massive glaciers in the
+British Islands and similar latitudes. It may be concluded that the
+present physiography of the land is proximately due solely to the action
+of the denuding agents--rain, frost, rivers, and the sea. But the lines
+along which these agents act with greatest intensity have been
+determined in the first place by the subterranean forces which upheaved
+the solid crust into great table-lands or mountain undulations. Both the
+remote and the proximate causes of the earth's surface-features,
+however, have acted in concert and contemporaneously, for no sooner
+would new land emerge above the sea-level than the breakers would assail
+it, and all the forces of the atmosphere would be brought to bear upon
+it--rain, frost, and rivers--so that the beginning of the sculpturing of
+hill and valley dates back to the period when the present lands were
+slowly emerging from the ocean. So great is the denudation of the land,
+that in process of time the whole would be planed down to the level of
+the sea, if it were not for the subterranean forces, which from time to
+time depress and elevate different portions of the earth's crust. It
+can be proved that strata miles in thickness have been removed bodily
+from the surface of our own country by the seemingly feeble agents of
+denudation. All the denuded material--mud, sand, and gravel--carried
+down into the sea has been re-arranged into new beds, and these have
+ever and anon been pushed up to the light of day, and scarped and
+channelled by the denuding forces, the resulting detritus being swept
+down as before into the sea, to form fresh deposits, and so on. It
+follows, therefore, that the present arrangement of land and sea has not
+always existed. There was a time before the present distribution of land
+obtained, and a time will yet arrive when, after infinite modifications
+of surface and level, the continents and islands may be entirely
+re-arranged, the sea replacing the land, and _vice versa_. To trace the
+history of such changes in the past is one of the great aims of the
+scientific geologist.
+
+
+
+
+PALAEONTOLOGY.[F]
+
+   [F] _Palaios_, ancient, _onta_, beings, and _logos_, a discourse.
+
+
+87. _Fossils._--In our description of rock-masses, and again in our
+account of geological agencies, we referred to the fact that certain
+rocks are composed in large measure, or exclusively, of animal or
+vegetable organisms, or of both together; and we saw that analogous
+organic formations were being accumulated at the present time. But we
+have deferred to this place any special account of the organic remains
+which are entombed in rocks. _Fossils_, as these are called, consist
+generally of the harder and more durable parts of animals and plants,
+such as bones, shells, teeth, seeds, bark, and ligneous tissues, &c. But
+it is usual to extend the term fossil to even the _casts_ or
+_impressions_ of such remains, and to foot-marks and tracks, whether of
+vertebrates, molluscs, crustaceans, or annelids. The organic remains
+met with in the rocks have usually undergone some chemical change. They
+have become _petrified_ wholly or in part. The gelatine which originally
+gave flexibility to some of them has disappeared, and even the carbonate
+and phosphate of lime of the harder parts have frequently been replaced
+by other mineral matter, by flint, pyrites, or the like. So perfect is
+the petrifaction in many cases, that the most minute structures have
+been entirely preserved--the original matter having been replaced atom
+by atom. As a rule, fossils occur most abundantly and in the best state
+in clay-rocks, like shale; while in porous rocks, like sandstone, they
+are generally poorly preserved, and not of so frequent occurrence. One
+reason for this is, that clay-rocks are much less pervious than
+sandstone, and their imbedded fossils have consequently escaped in
+greater measure the solvent powers of percolating water. But there are
+other reasons for the comparative paucity of fossils in arenaceous
+strata, as we shall see presently.
+
+88. _Proofs of varied Physical Conditions._--Organic remains are either
+of terrestrial, fresh-water, or marine origin, and they are therefore of
+the utmost value to the geologist in deciphering the history of those
+great changes which have culminated in the present. But we can go a step
+further than this. We know that at the present day the distribution of
+animal and vegetable life is due to a variety of causes--to climatic and
+physical conditions. The creatures inhabiting arctic and temperate
+regions contrast strongly with those that tenant the tropics. So also we
+observe a change in animal and vegetable forms as we ascend from the low
+grounds of a country to its mountain heights. Similar changes take place
+in the sea. The animals and plants of littoral regions differ from those
+whose habitat is in deeper water. Now, the fossiliferous strata of our
+globe afford similar proofs of varying climatic and physical conditions.
+There are littoral deposits and deep-sea accumulations: the former are
+generally coarse-grained (conglomerates, grit, and sandstone); the
+latter are for the most part finer-grained (clay, shale, limestone,
+chalk, &c.); and both inshore and deep-water formations have each their
+peculiar organic remains. Again, we know that some parts of the
+sea-bottom are not so prolific in life as others--where, for example,
+any considerable deposit of sand is taking place, or where sediment is
+being constantly washed to and fro upon the bottom, shells and other
+creatures do not appear in such numbers as where there is less
+commotion, and a finer and more equable deposit is taking place. It is
+partly for the same reason that certain rocks are more barren of organic
+remains than others.
+
+89. _Fossil Genera and Species frequently extinct._--It might perhaps at
+first be supposed that similar rocks would contain similar fossils. For
+example, we might expect that formations resembling in their origin
+those which are now forming in our coral seas would also, like the
+latter, contain corals in abundance, with some commingling of shells,
+crustaceans, fish, &c., such as are peculiar to the warm seas in which
+corals flourish. And this in some measure holds good. But when we
+examined carefully the fossils in certain of the limestones of our own
+country, we should find that while the same great orders and classes
+were actually present, yet the genera and species were frequently
+entirely different; and not only so, but that often none of these were
+now living on the earth. Moreover, if we extended our research, we
+should soon discover that similar wide differences actually obtained
+between many of the limestones themselves and other fossiliferous strata
+of our country.
+
+90. _Fossiliferous Strata of Different Ages._--Another fact would also
+gradually dawn upon us--this, namely, that in certain rocks the fossils
+depart much more widely from analogous living forms, than the organic
+remains in certain other rocks do. The cause of this lies in the fact
+that the fossiliferous strata are of different ages; they have not all
+been formed at approximately the same time. On the contrary, they have
+been slowly amassed, as we have seen, during a long succession of eras.
+While they have been accumulating, great vicissitudes in the
+distribution of land and sea have taken place, climates have frequently
+altered, and the whole organic life of the globe has slowly changed
+again and again--successive races of plants and animals flourishing each
+for its allotted period, and then becoming extinct for ever.[G] Thus,
+strata formed at approximately the same time contain generally the same
+fossils; while, on the other hand, sedimentary deposits accumulated at
+different periods are charged with different fossils. Fossils in this
+way become invaluable to the geologist. They enable him to identify
+formations in separate districts, and to assign to them their relative
+antiquity.[H] If, for example, we have a series of formations, A, B, C,
+piled one on the top of the other, A being the lowest, and C the
+highest, and each charged with its own peculiar fossils, we may compare
+the fossils met with in other sets of strata with the organic remains
+found in A, B, C. Should the former be found to correspond with the
+fossil contents of B, we conclude that the rocks in which they occur are
+approximately of contemporaneous origin with B, even although the
+equivalents of the formations A and C should be entirely wanting.
+Further, we soon learn that the order of the series A, B, C, is never
+inverted. If A be the lowest, and C the highest stratum in one place, it
+is quite certain that the same order of succession will obtain wherever
+the equivalents of these strata happen to occur together. But the
+succession of strata is not invariably the same all the world over; in
+some countries, we may have dozens of separate formations piled one on
+the top of the other; in other countries, many members of the series are
+absent; in brief, _blanks in the succession_ are of constant occurrence.
+But by dovetailing, as it were, all the formations known to us, we are
+enabled to form a more or less complete series of rocks arranged in the
+order of their age. A little reflection will serve to shew that the
+partial mode in which the strata are distributed over the globe arises
+chiefly from two causes. We have to remember, _first_, that the deposits
+themselves were laid down only here and there in irregular spreads and
+patches--opposite the mouths of rivers, at various points along the
+ancient coast-lines, and over certain areas in the deeper abysses of the
+ocean--the coarser accumulations being of much less extent than those
+formed of finer materials. And, _second_, we must not forget the intense
+denudation which they have experienced, so that miles and miles of
+strata which once existed have been swept away, and their materials
+built up into new formations.
+
+   [G] To this there are some exceptions. Certain small foraminifers,
+       for example, met with in some of the oldest formations, do not
+       seem to differ from species which are still living. The genus
+       _Lingula_ (Mollusca) has also come down from remotest ages,
+       having outlived all its earlier associates.
+
+   [H] This holds strictly true, however, only in regard to comparatively
+       limited areas. The student must remember that strata occurring
+       in widely separate regions of the earth, even although they
+       contain very much the same assemblage of fossils, are not
+       necessarily contemporaneous, in the strict meaning of the word;
+       for the _fauna_ and _flora_ (the animal and plant life) may have
+       died out, and become replaced by new forms more rapidly in one
+       place than another. The term 'contemporaneous,' therefore, is a
+       very lax one, and may sometimes group together deposits which,
+       for aught that we can tell, may really have been accumulated at
+       widely separated times.
+
+91. _Gradual Extinction of Species._--When a sufficient number of
+fossils has been diligently compared, we discover that those in the
+younger strata approach most nearly to the present living forms, and
+that the older the strata are, the more widely do their organic remains
+depart from existing types of animals and plants. We may notice also,
+that when a series of beds graduate up into each other, so that no
+strongly marked line separates the overlying from the underlying strata,
+there is also a similar gradation amongst the fossils. The fossils in
+the highest beds may differ entirely from those in the lowest; but in
+the middle beds there is an intermingling of forms. In short, it is
+evident that the creatures gradually became extinct, and were just as
+gradually replaced by new forms, until a time came when all the species
+that were living while the lowest beds were being amassed, at last died
+out, and a complete change was effected.
+
+92. _Proofs of Cosmical Changes of Climate._--From the preceding remarks
+it will be also apparent that fossils teach us much regarding the
+climatology of past ages. They tell us how the area of the British
+Islands has experienced many vicissitudes of climate, sometimes
+rejoicing in a warm or almost tropical temperature, at other times
+visited with a climate as severe as is now experienced in arctic and
+antarctic regions. Not only so, but we learn from fossils that
+Greenland once supported myrtles and other plants which are now only
+found growing under mild and genial climatic conditions; while, on the
+other hand, remains of arctic mammals are met with in the south of
+France. Such great changes of climate are due, according to Mr Croll, to
+variations in the eccentricity of the earth's orbit combined with the
+precession of the equinox. It is well known that the orbit of our earth
+becomes much more elliptical at certain irregularly recurring periods
+than it is at present. During a period of extreme ellipticity, the earth
+is, of course, much further away from the sun in _aphelion_[I] than it
+is at a time of moderate ellipticity, while, in _perihelion_,[J] it is
+considerably nearer. Now, let us suppose that, at a time when the
+ellipticity is great, the movement known as the precession of the
+equinox has changed the incidence of our seasons, so that our summer
+happens in perihelion and not in aphelion, while that of the southern
+hemisphere occurs in aphelion, and not, as at present, in perihelion.
+Under such conditions, the climate of the globe would experience a
+complete change. In the northern hemisphere, so long and intensely cold
+would the winter be, that all the moisture that fell would fall as rain,
+and although the summer would be very warm, it would nevertheless be
+very short, and the heat then received would be insufficient to melt the
+snow and ice which had accumulated during the winter. Thus gradually
+snow and ice would cover all the lands down to temperate latitudes. In
+the southern hemisphere, the reverse of all this would obtain. The
+winter there would be short and mild, and the summer, although cool,
+would be very long. But such changes would bring into action a whole
+series of physical agencies, every one of which would tend still further
+to increase the difference between the climates of the two hemispheres.
+Owing to the vast accumulation of snow and ice in the northern
+hemisphere, the difference of temperature between equatorial and
+temperate and polar regions would be greater in that hemisphere than in
+the southern. Hence the winds blowing from the north would be more
+powerful than those coming from the southern and warmer hemisphere, and
+consequently the warm water of the tropics would necessarily be impelled
+into the southern ocean. This would tend still further to lower the
+temperature of our hemisphere, while, at the same time, it would raise
+correspondingly the temperature at our antipodes. The general result
+would be, that in our hemisphere ice and snow would cover the ground
+down to low temperate latitudes--the British Islands being completely
+smothered under a great sea of confluent glaciers. In the southern
+hemisphere, on the contrary, a kind of perennial summer would reign even
+up to the pole. Such conditions would last for some ten or twelve
+thousand years, and then, owing to the precession of the equinox, a
+complete change would come about--the ice-cap would disappear from the
+north, and be replaced by continuous summer, while at the same time an
+excessively severe or glacial climate would characterise the south; and
+such great changes would occur several times during each prolonged epoch
+of great eccentricity. This, in few words, is an outline of Mr Croll's
+theory. That theory is at present _sub judice_, but there can be no
+doubt that it gives a reasonable explanation of many geological facts
+which have hitherto been inexplicable. Of course, it is not maintained
+that all changes of climate are due directly or indirectly to
+astronomical causes. Local changes of climate--changes affecting limited
+regions--may be induced by mutations of land and sea, resulting in the
+partial deflection of ocean currents, which are the chief secondary
+means employed by nature for the distribution of heat over the globe's
+surface.
+
+   [I] _Apo_, away from; _helios_, the sun.
+
+   [J] _Peri_, round about or near by; _helios_, the sun.
+
+From what has been stated in the foregoing paragraphs, it is clear that
+in our endeavours to decipher the geological history of our planet,
+palaeontological must go hand in hand with stratigraphical evidence. We
+may indeed learn much from the mode of arrangement of the rocks
+themselves. But the test of superposition does not always avail us. It
+is often hard, and sometimes quite impossible, to tell from
+stratigraphical evidence which are the older rocks of a district. In the
+absence of fossils we must frequently be in doubt. But physical evidence
+alone will often afford us much and varied information. It will shew us
+what was land and what sea at some former period; it will indicate to us
+the sites of ancient igneous action; it will tell us of rivers, and
+lakes, and seas which have long since passed away. Nay, in some cases,
+it will even convince us that certain great climatic changes have taken
+place, by pointing out to us the markings, and debris, and wandered
+blocks which are the sure traces of ice action, whether of glaciers or
+icebergs. The results obtained by combining physical and palaeontological
+evidence form what is termed Historical Geology.
+
+
+
+
+HISTORICAL GEOLOGY.
+
+
+93. The fossiliferous strata, as they are generally termed, have been
+chronologically arranged in a series of _formations_, each of which is
+characterised by its own peculiar suites of fossils. Their relative age
+has been determined, as we have indicated above, by their fossils, and
+also by certain physical tests, the chief of these being
+_superposition_. It holds invariably true that a formation, A, found
+resting upon another series of strata, B, will always occur in precisely
+the same position, wherever these two deposits occur together. If B
+should appear in some place as resting upon A, we may be sure that the
+beds have been inverted during the contortion of the strata consequent
+upon subterranean action (see fig. 11, page 34). Again, another useful
+test of the relative age of strata lies in the circumstance that one is
+often made up or contains fragments of the other. In this case, then, it
+is quite clear which is the more recent accumulation. These tests have
+now been applied to the strata in many parts of the world, and the
+result is that geologists have been able to arrive at a chronological
+arrangement or classification, and so to construct a table shewing the
+relative position which would be occupied by all the different
+formations, if these occurred together in one place. In the British
+Islands the long series of strata is well developed, but many of the
+formations are much more meagrely represented than their equivalents in
+other countries. But even when we attempt to fill up the blanks in our
+own series by dovetailing with them the strata of foreign countries,
+there yet remain numerous breaks in the succession, pointing to the fact
+that the stony record is a very fragmentary one at the best. No doubt
+there are many large tracts of the earth's surface which have not yet
+been investigated, and when these are known we may hope to have our
+knowledge greatly increased. But no one who reflects upon the mode of
+origin of the fossiliferous strata, and the wonderful mutations which
+the earth has undergone, can reasonably anticipate that a perfect and
+complete record of the geological history of our planet shall ever be
+compiled from the broken and fragmentary testimony of the rocks.
+
+94. The following table gives the names of the different formations
+arranged in the order of their superposition, the youngest being at the
+top, and the oldest known at the bottom:
+
+  IV. POST-TERTIARY OR QUATERNARY--
+        Historical or Recent.
+        Pleistocene.
+
+  III. TERTIARY OR CAINOZOIC--
+        Pliocene.
+        Miocene.
+        Eocene.
+
+  II. SECONDARY OR MESOZOIC--
+        Cretaceous.
+        Jurassic.
+        Triassic.
+
+  I. PRIMARY OR PALAEOZOIC--
+        Permian.
+        Carboniferous.
+        Devonian and Old Red Sandstone.
+        Silurian.
+        Cambrian.
+        Laurentian or Pre-Cambrian.
+
+95. The PRIMARY formations are so called because they are the oldest
+known to us: they are not necessarily the first-formed aqueous deposits.
+Dr Hutton said truly: There is no trace of a beginning, and no signs of
+an end. In the PRIMARY or PALAEOZOIC (ancient-life) formations are found
+the earliest traces of life. The forms as a rule depart very widely from
+those with which we are acquainted now. The _Laurentian_ rocks have
+yielded only one fossil--a large foraminifer named _Eozoon Canadense_.
+The _Cambrian_ formation contains but few fossils--crustaceans,
+molluscs, zoophytes, and worm-tracks. The _Silurian_ strata are often
+abundantly fossiliferous. All the great classes of invertebrates are
+represented, and fish remains also occur. The _Devonian_ and _Old Red
+Sandstone_ are also characterised by the presence of an abundant fauna.
+In the Old Red Sandstone are numerous fish remains; it appears to have
+been an estuarine or lacustrine deposit; the Devonian, on the other
+hand, was marine, like the Silurian and Cambrian. The _Carboniferous_
+formation is the chief repository of coal in Britain. It consists of
+terrestrial, fresh or brackish water, and marine deposits. The fauna and
+flora of the _Permian_, which is partly a marine and partly a
+fresh-water formation, are allied, upon the whole, to those of the
+Carboniferous, but offer at the same time many contrasts.
+
+96. The SECONDARY OR MESOZOIC (middle-life) formations contain
+assemblages of fossils which do not depart so widely from analogous
+living forms as those belonging to Palaeozoic times. The _Triassic_
+strata yield abundance of rock-salt. In Britain they contain very few
+fossils, but these are more abundant in the Triassic deposits of foreign
+countries. The oldest known mammals first appear in this formation. The
+_Jurassic_ formation is very highly fossiliferous. It is distinguished
+by the occurrence of numerous reptilian remains. Nearly all the beds of
+this formation are marine, but there are associated with these the
+remains of a forest or old land surface, and a considerable accumulation
+of estuarine or fresh-water deposits; impure coals also occur in this
+formation. The _Cretaceous_ strata are almost wholly marine, and chiefly
+of deep-water origin. But some land-plants are found, chiefly ferns,
+conifers, and cycads. Near the base of the formation occurs a great
+river deposit (Weald clay) with numerous remains of reptiles.
+
+97. Among the oldest strata of the TERTIARY or CAINOZOIC (recent-life)
+division we meet with the _dawn_ of the existing state of the testaceous
+fauna--the _Eocene_ (_eos_, dawn, and _kainos_, recent) containing three
+and a half per cent. of recent species among its shells. The proportion
+of recent species increases in the _Miocene_ (_meion_, less, and
+_kainos_, recent), although the majority of the molluscs entombed in
+that formation belong to extinct species. In the _Pliocene_ (_pleion_,
+more, and _kainos_, recent), however, the extinct species are in a
+minority.
+
+The POST-TERTIARY or QUATERNARY division comprises the concluding
+chapters of geological history. The _Pleistocene_ (_pleistos_, most, and
+_kainos_, recent) contains no extinct species of shells, but a number of
+extinct mammalia. In the _Recent_ deposits all the species of animals
+and plants are living. The Tertiary and Quaternary formations are partly
+of marine and partly of terrestrial and fresh-water origin. At the close
+of the Tertiary period the 'glacial epoch' of Pleistocene times began,
+and the British Islands and a large part of northern Europe and North
+America were then cased in snow and ice. Traces of glacial conditions
+have also been met with in the Eocene and Miocene. The evidence
+furnished by Palaeozoic and Mesozoic formations points chiefly to mild,
+genial, and sometimes tropical conditions. But traces of ice action are
+occasionally noted (namely, in the Silurian, Old Red Sandstone,
+Carboniferous, Permian, and Cretaceous formations), pointing, perhaps,
+in some of the cases, to former alternations of cold and warm periods.
+Indeed, the belief is now gaining ground, that the so-called glacial
+epoch of Pleistocene times was not one long continuous age of ice, but
+rather consisted of an alternation of warm and cold periods. And it is
+not improbable, but highly likely, that similar alternations of climate
+have happened during every period of great eccentricity of the earth's
+orbit.
+
+
+
+
+QUESTIONS.
+
+
+Section 1. What is Geology?
+
+2. Define the term _rock_. How many classes of rock are there?
+
+3, 4, 5. Into what groups are the mechanically formed rocks divided?
+Define the terms _conglomerate_, _sandstone_, and _shale_.
+
+6. What is the nature of the rocks belonging to the Aerial or Eolian
+group?
+
+7. Give an example of a chemically formed rock.
+
+8. Give examples of organically derived rocks.
+
+9. What kinds of rocks are embraced by the Metamorphic class?
+
+10. What are igneous rocks?
+
+12. What is the mineralogical composition of granite?
+
+13. What is meant by a _mineral_?
+
+14. Name five minerals which do not contain oxygen. Where does
+_fluor-spar_ occur? What is the element that enters most largely into
+the composition of the earth's crust?
+
+15. Name the forms under which the mineral _quartz_ occurs. Name some
+of the oxides of iron. What is _iron pyrites_?
+
+16. Name two _sulphates_. Name two _carbonates_. Name some of the
+_silicates_. In what kinds of rock is _augite_ found? Where does it
+never occur? In what kinds of rock does _hornblende_ usually occur?
+Mention three species of felspar. What is one of the most
+distinguishing characteristics of mica? Name three silicates of
+magnesia. Mention some of their distinguishing peculiarities. Where do
+_zeolites_ commonly occur?
+
+17. What is a _quartzose conglomerate_? What is a _calcareous
+conglomerate_?
+
+18. What is _grit_? What is _freestone_? To what are the various
+colours of sandstone due? What is _shale_?
+
+19. Name some typical Eolian rocks, and tell where they occur.
+
+20. How do _stalactites_ and _stalagmites_ occur? What is _siliceous
+sinter_, and how does it occur? How does _rock-salt_ occur?
+
+21. Mention some of the varieties of limestone. What is _cornstone_?
+What is the composition of _dolomite_?
+
+22. Name some of the varieties of coal.
+
+23. What is _quartzite_?
+
+24. Describe _clay-slate_.
+
+25. Mention some altered limestones.
+
+26. What are _schists_? Name and give the mineralogical composition of
+three schists.
+
+27. What is the general character of metamorphic rocks?
+
+28. How would you classify granite?
+
+29. What is the mineralogical composition of _syenite_ and _diorite_?
+
+30. How do we distinguish the two groups into which igneous rocks are
+subdivided? What is meant by the terms _amygdaloidal_ and
+_porphyritic_?
+
+31. Name some rocks that belong to the acidic group. What is
+_quartz-porphyry_?
+
+32. Give examples of augitic igneous rocks. Name a hornblendic igneous
+rock.
+
+33. What are fragmental igneous rocks? What is the difference between
+_trappean breccia_ and _trappean conglomerate_?
+
+34. What is meant by the terms _stratum_, _strata_, and _stratified_?
+What is the difference between _lamination_ and _bedding_? What is a
+section?
+
+35. What is _false bedding_?
+
+36. Briefly describe the general appearance of _mud-cracks_ and
+_rain-prints_, and say how these have been formed.
+
+37. What is meant by a _succession of strata_?
+
+38. Which kinds of stratified rocks generally have the greatest
+extension?
+
+39. How do beds terminate?
+
+40. How may planes of bedding sometimes indicate a break in the
+succession of strata?
+
+41. What is the nature of _joints_? What are _master-joints_, and what
+is their probable cause?
+
+42. What is _cleavage_, and what is its effect upon the bedding of
+rocks?
+
+43. What is _foliation_?
+
+44. Give examples of concretionary rocks. What is the nature of chert
+and flint nodules?
+
+45. Define the terms _dip_ and _strike_. What is the _crop_ of a bed?
+What are _anticlines_ and _synclines_?
+
+46. What is meant by an _inversion of strata_?
+
+47. How does contemporaneous erosion indicate a pause in the
+deposition of a series of strata?
+
+48. What is meant by _unconformability_? How does unconformability
+prove a lapse of time between the accumulation of the underlying and
+overlying strata?
+
+49. What is _overlap_?
+
+50. What is a _fault_? What is _hade_? How are the strata affected on
+either side of a fault? What is the appearance called _slickensides_?
+Under what circumstances should we term a fault a _downthrow_? and
+when should we term it an _upcast_? How is the approximate age of a
+fault sometimes shewn?
+
+51. What are metamorphic rocks, and what is their general appearance?
+In what districts of the British Islands are they most abundantly
+developed? What are some of the appearances relied upon for
+distinguishing metamorphic from igneous granite?
+
+52. How do igneous rocks occur? Define what is meant by
+_contemporaneous_ and _subsequent_ or _intrusive_ igneous rocks. How
+does a contemporaneous igneous rock affect the beds upon which it
+rests? What is the character of the bed overlying a contemporaneous
+rock? What is the general structure of a contemporaneous igneous rock?
+What is meant by _vesicular structure_? What is the general texture of
+a contemporaneous igneous rock? What is the nature of the jointing in
+igneous rocks? What is _wacke_?
+
+53. What is the nature of the beds of _breccia_, _conglomerate_,
+_ash_, and _tuff_, with which contemporaneous igneous rocks are often
+associated? What is a _neck_ of _volcanic agglomerate_? How are the
+strata affected at their junction with a 'neck'?
+
+54. How do intrusive igneous rocks occur? How do intrusive _sheets_
+occur? What effect have they produced upon the strata above and below
+them? What is a _dyke_? What relation do they occasionally bear to
+_sheets_ of igneous rock? What is a _neck_ of intrusive igneous rock,
+and how have the strata surrounding it been affected?
+
+55. Mention some of the contrasts between _intrusive_ and
+contemporaneous igneous rocks. What alteration is produced upon coal
+with which an intrusive sheet has come in contact?
+
+56. What are _mineral veins_? What is the nature of the quartz veins
+in granite? How are the minerals usually arranged in the great
+metalliferous veins? What is a _pipe-vein_?
+
+57. What are the great geological agents of change?
+
+58. What is meant by _weathering_? How are rocks affected at the
+surface in tropical countries? What chemical effect has the atmosphere
+on calcareous rocks? How is soil formed? How are sand dunes formed?
+Mention some effects of the transporting power of the atmosphere.
+
+59. Mention some of the chemical effects of interstitial water. What
+is the origin of _travertine_ or _calcareous tufa_?
+
+60. How have _stalactites_ and _stalagmites_ been formed? Give some
+instances of the solvent power of springs.
+
+61. How are caves in limestone formed? Describe some of the
+appearances of a country composed of calcareous rocks. Describe
+briefly how a river erodes its channel.
+
+62. Describe the geological action of rain.
+
+63. What do chemical analyses of river-water prove? Give an example.
+What are pot-holes? Give an example of the erosive power of running
+water. What amount of mud is carried in suspension by the Mississippi,
+and discharged annually into the sea? What estimate has been formed of
+the total amount of mineral matter annually transported by that river?
+
+64. What is _alluvium_? How is it formed? and mention some examples of
+its occurrence.
+
+65. How is sediment deposited by a river in a lake?
+
+66. What is the difference between lacustrine and fluvio-marine
+deposits? What is a _delta_?
+
+67. Describe the geological action of frost. Describe the geological
+action of river-ice.
+
+68. What are _glaciers_? What thickness do they attain in the Alps?
+What is their rate of motion? What are _crevasses_, and how do they
+originate? What are _superficial moraines_? What are _terminal
+moraines_? What changes does a glacier effect upon its bed, and how
+are these modifications produced? What is the character of a glacial
+river? What is the origin of _icebergs_? How is the general absence of
+blocks and stones in Greenland icebergs to be explained? What is the
+nature of a submarine terminal moraine? What is the _ice-foot_? What
+is the chief agent in distributing erratic stones and blocks over the
+sea-bottom? What effect upon the sea-bed must stranding icebergs
+produce?
+
+69. What are some of the chemical compounds held in solution in
+sea-water? Which of these go to form the shells and skeletons of
+marine animals?
+
+70. Describe the action of breakers on a sea-coast. How does frost aid
+the wasting action of breakers? What effect has the nature of the
+rocks in the production of inequalities in a coast-line? Upon what
+part of the sea-bottom does the material derived by the action of the
+breakers chiefly accumulate? What effect have the tides and ocean
+currents in the distribution of sediment?
+
+71. What is the general rule as regards fine-grained and
+coarse-grained deposits? Mention a partial exception to this rule.
+What effect have tidal currents in shallow seas?
+
+72. How are rocks disintegrated through the action of plants? What is
+peat? What may be inferred from the occurrence of shell-marl
+underneath peat? What does the appearance of roots and trunks of
+trees, and of remains of land animals under peat, indicate?
+
+73. What, generally, is the geological action of animal life?
+
+74. What is coral? What is a _fringing_ reef? What is the general
+character of a _barrier_ reef? Give an example of one. What is an
+_atoll_? What is the nature of coral rock? What is Mr Darwin's theory
+of the formation of coral reefs?
+
+75. What is the nature of the Atlantic ooze? In what respects may it
+eventually come to resemble chalk and limestone? Mention an instance
+of the abundant occurrence in the sea of animalcules with siliceous
+coverings and skeletons. What is the nature of the red clay found at
+great depths in the Atlantic and Southern Oceans?
+
+76. What are some of the notions held in regard to the internal
+condition of the earth? At what (average) rate does the temperature of
+the earth's crust increase as we descend from the surface?
+
+77. What is the nature of the movements to which the earth's crust is
+subjected?
+
+78. Describe the hypotheses advanced to account for earthquakes.
+Mention some of the effects of earthquakes--1_st_, as regards
+alterations of level; and 2_d_, as regards modifications of the
+surface.
+
+79. Mention a good example of tranquil elevation and depression of the
+earth's crust. Mention some of the proofs of an elevatory movement.
+Give proofs that shew depression of the land. How may certain former
+changes of sea-level be accounted for without inferring any movement
+of the land?
+
+80. What effect must _depression_ have upon the strata forming the
+earth's crust? What is the result of a movement of elevation? What is
+the cause of _cleavage_?
+
+81. What is the nature of the materials thrown out during volcanic
+eruptions? What is the general structure of a volcanic cone? How does
+molten rock make its escape from the orifice of eruption? What is the
+meaning of the terms _lapillo_, _puzzolana_, and _ceneri_?
+
+82. What is lava? Describe the general appearance and mode of
+progression of a stream of lava. What effect is produced upon
+fragments of rock caught up and inclosed in lava; and what changes are
+caused in the pavement upon which it cools? How does a lava stream
+entering a lake or the sea behave in regard to the sediment gathering
+therein? To what is the basaltic structure due? How are the axes of
+the prisms in a columnar igneous rock arranged? Name some of the
+varieties of lava. What is the origin of the vesicular structure in
+igneous rocks? What portions of a bed of lava are most frequently
+scoriaceous? In what kinds of lava is the vesicular structure most
+abundantly met with? How have the vesicles become flattened? In what
+manner have they been filled with mineral matter? What is the origin
+of the dykes of modern volcanic districts?
+
+83. How is metamorphism on the large scale supposed to have been
+induced? How may granite be at one and the same time a metamorphic and
+igneous rock?
+
+84. Mention some of the views held with regard to the origin of
+mineral veins.
+
+85. What is _denudation_? How do inclined strata prove that the strata
+have been denuded? How do _faults_ afford proof of denudation? What
+have been the general effects produced by denudation on the face of
+the land?
+
+86. What part have the subterranean forces acted in the formation of
+mountains? To what geological action is the present aspect of these
+mountains due? What has determined the direction of river valleys? How
+have the valleys, dells, &c. been formed? What effect have faults had
+in determining the direction of river valleys? What is supposed to be
+the origin of the deep rock-basins occupied by many fresh-water lakes?
+How is the waste of land by denudation compensated?
+
+87. What are _fossils_? What is meant by _petrifaction_? In what kind
+of rocks do fossils occur most abundantly, and in the best state of
+preservation? and what reason can be given for this?
+
+88. How do fossils afford proof of varied physical conditions? Give a
+reason for some rocks being more barren of fossils than others.
+
+89. State some of the characters which distinguish broadly the older
+fossiliferous strata from those similar accumulations which are being
+formed in our own day.
+
+90. How may we identify formations in separate districts? How is the
+interrupted and partial distribution of strata to be accounted for?
+
+91. In what respect do the fossils in younger strata differ from those
+in older strata? What general proof can be adduced to shew that
+species have become gradually extinct?
+
+92. Give an instance how fossils prove changes of climate in the past.
+What is supposed to be the cause of great cosmical changes of climate?
+Describe Mr Croll's theory of cosmical changes of climate.
+
+93. What is the test of _superposition_? Mention another test of the
+relative age of strata. 94. Name the four great divisions under which
+the fossiliferous rocks are arranged.
+
+95. Name the Primary or Palaeozoic formations. What are the principal
+kinds of fossils found in the Old Red Sandstone? Which formation is
+the chief repository of coal in Britain?
+
+96. In what other formations do coals occur? In which formation do the
+oldest known mammals occur? Name the Secondary formations.
+
+97. Name the Tertiary formations. What kind of climate characterised
+the northern hemisphere at the beginning of Pleistocene times? What
+kinds of climate would appear from the evidence to have chiefly
+prevailed in Primary, Secondary, and Tertiary ages? Have we any trace
+of frigid conditions during these ages? What is the growing opinion
+with regard to the climatic conditions during the glacial period of
+Pleistocene times?
+
+
+
+
+    THE END.
+
+
+
+
+    Edinburgh: Printed by W. & R. Chambers.
+
+
+
+
+     *     *     *     *     *     *     *
+
+
+Transcriber's Notes
+
+  The only minor correction that was noted in converting this document
+  from a printed version into an electronic version was an unpaired
+  parenthesis on the first line of the Title Page.
+
+     *     *     *     *     *     *     *
+
+
+
+
+
+End of the Project Gutenberg EBook of Geology, by James Geikie
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