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diff --git a/old/3772-h/files/ch4.html b/old/3772-h/files/ch4.html new file mode 100644 index 0000000..0c88df6 --- /dev/null +++ b/old/3772-h/files/ch4.html @@ -0,0 +1,503 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> +<!-- saved from url=(0036)http://../Lyell/The Student's Elements of Geology --> +<html> +<head> +<meta name="generator" content="HTML Tidy, see www.w3.org"> +<title>The Student's Elements of Geology: Title</title> +<meta content="text/html; charset=iso-8859-1" http-equiv= +"Content-Type"> +<meta content="MSHTML 5.00.2919.6307" name="GENERATOR"> +<link rel="stylesheet" href="geology.css" type="text/css"> +</head> +<body> +<p><b>The Student’s Elements of Geology</b></p> + +<hr> +<p class="page"><a name="page 60">[ 60 ]</a></p> + +<p> </p> + +<center><b>Chapter IV</b><br> +<br> +CONSOLIDATION OF STRATA AND PETRIFACTION OF FOSSILS.</center> + +<p class="intro">Chemical and Mechanical Deposits. — Cementing +together of Particles. — Hardening by Exposure to +Air. — Concretionary Nodules. — Consolidating Effects of +Pressure. — Mineralization of Organic +Remains. — Impressions and Casts: how formed. — Fossil +Wood. — Goppert’s Experiments. — Precipitation of Stony +Matter most rapid where Putrefaction is going on. — Sources of +Lime and Silex in Solution.</p> + +<p>Having spoken in the preceding chapters of the characters of +sedimentary formations, both as dependent on the deposition of +inorganic matter and the distribution of fossils, I may next treat +of the consolidation of stratified rocks, and the petrifaction of +imbedded organic remains.</p> + +<p><b>Chemical and Mechanical +Deposits.</b>— A distinction has been made by +geologists between deposits of a mechanical, and those of a +chemical, origin. By the name mechanical are designated beds of +mud, sand, or pebbles produced by the action of running water, also +accumulations of stones and scoriæ thrown out by a volcano, +which have fallen into their present place by the force of +gravitation. But the matter which forms a chemical deposit has not +been mechanically suspended in water, but in a state of solution +until separated by chemical action. In this manner carbonate of +lime is occasionally precipitated upon the bottom of lakes in a +solid form, as may be well seen in many parts of Italy, where +mineral springs abound, and where the calcareous stone, called +travertin, is deposited. In these springs the lime is usually held +in solution by an excess of carbonic acid, or by heat if it be a +hot spring, until the water, on issuing from the earth, cools or +loses part of its acid. The calcareous matter then falls down in a +solid state, incrusting shells, fragments of wood and leaves, and +binding them together.</p> + +<p>That similar travertin is formed at some points in the bed of +the sea where calcareous springs issue can not be doubted, but as a +general rule the quantity of lime, according to Bischoff, spread +through the waters of the ocean is very small, the free carbonic +acid gas in the same waters being five times as much as is +necessary to keep the lime in a fluid state. Carbonate of lime, +therefore, can rarely be precipitated at the bottom of the sea by +chemical action alone, but</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 61">[ 61 ]</a></p> + +<p>must be produced by vital agency as in the case of coral +reefs.</p> + +<p>In such reefs, large masses of limestone are formed by the stony +skeletons of zoophytes; and these, together with shells, become +cemented together by carbonate of lime, part of which is probably +furnished to the sea-water by the decomposition of dead corals. +Even shells, of which the animals are still living on these reefs, +are very commonly found to be incrusted over with a hard coating of +limestone.</p> + +<p>If sand and pebbles are carried by a river into the sea, and +these are bound together immediately by carbonate of lime, the +deposit may be described as of a mixed origin, partly chemical, and +partly mechanical.</p> + +<p>Now, the remarks already made in Chapter II, on the original +horizontality of strata are strictly applicable to mechanical +deposits, and only partially to those of a mixed nature. Such as +are purely chemical may be formed on a very steep slope, or may +even incrust the vertical walls of a fissure, and be of equal +thickness throughout; but such deposits are of small extent, and +for the most part confined to vein-stones.</p> + +<p><b>Consolidation of Strata.</b>—It is chiefly in the case of calcareous rocks that solidification +takes place at the time of deposition. But there are many deposits +in which a cementing process comes into operation long afterwards. +We may sometimes observe, where the water of ferruginous or +calcareous springs has flowed through a bed of sand or gravel, that +iron or carbonate of lime has been deposited in the interstices +between the grains or pebbles, so that in certain places the whole +has been bound together into a stone, the same set of strata +remaining in other parts loose and incoherent.</p> + +<p>Proofs of a similar cementing action are seen in a rock at +Kelloway, in Wiltshire. A peculiar band of sandy strata belonging +to the group called Oolite by geologists may be traced through +several counties, the sand being for the most part loose and +unconsolidated, but becoming stony near Kelloway. In this district +there are numerous fossil shells which have decomposed, having for +the most part left only their casts. The calcareous matter hence +derived has evidently served, at some former period, as a cement to +the siliceous grains of sand, and thus a solid sandstone has been +produced. If we take fragments of many other argillaceous grits, +retaining the casts of shells, and plunge them into dilute muriatic +or other acid, we see them immediately changed into common sand and +mud; the cement of lime, derived from the shells, having been +dissolved by the acid.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 62">[ 62 ]</a></p> + +<p>Traces of impressions and casts are often extremely faint. In +some loose sands of recent date we meet with shells in so advanced +a stage of decomposition as to crumble into powder when touched. It +is clear that water percolating such strata may soon remove the +calcareous matter of the shell; and unless circumstances cause the +carbonate of lime to be again deposited, the grains of sand will +not be cemented together; in which case no memorial of the fossil +will remain.</p> + +<p>In what manner silex and carbonate of lime may become widely +diffused in small quantities through the waters which permeate the +earth’s crust will be spoken of presently, when the petrifaction of +fossil bodies is considered; but I may remark here that such waters +are always passing in the case of thermal springs from hotter to +colder parts of the interior of the earth; and, as often as the +temperature of the solvent is lowered, mineral matter has a +tendency to separate from it and solidify. Thus a stony cement is +often supplied to sand, pebbles, or any fragmentary mixture. In +some conglomerates, like the pudding-stone of Hertfordshire (a +Lower Eocene deposit), pebbles of flint and grains of sand are +united by a siliceous cement so firmly, that if a block be +fractured, the rent passes as readily through the pebbles as +through the cement.</p> + +<p>It is probable that many strata became solid at the time when +they emerged from the waters in which they were deposited, and when +they first formed a part of the dry land. A well-known fact seems +to confirm this idea: by far the greater number of the stones used +for building and road-making are much softer when first taken from +the quarry than after they have been long exposed to the air; and +these, when once dried, may afterwards be immersed for any length +of time in water without becoming soft again. Hence it is found +desirable to shape the stones which are to be used in architecture +while they are yet soft and wet, and while they contain their +“quarry-water,” as it is called; also to break up stone intended +for roads when soft, and then leave it to dry in the air for months +that it may harden. Such induration may perhaps be accounted for by +supposing the water, which penetrates the minutest pores of rocks, +to deposit, on evaporation, carbonate of lime, iron, silex, and +other minerals previously held in solution, and thereby to fill up +the pores partially. These particles, on crystallising, would not +only be themselves deprived of freedom of motion, but would also +bind together other portions of the rock which before were loosely +aggregated. On the same principle wet sand and mud become as hard +as stone when</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 63">[ 63 ]</a></p> + +<p>frozen; because one ingredient of the mass, namely, the water, +has crystallised, so as to hold firmly together all the separate +particles of which the loose mud and sand were composed.</p> + +<p>Dr. MacCulloch mentions a sandstone in Skye, which may be +moulded like dough when first found; and some simple minerals, +which are rigid and as hard as glass in our cabinets, are often +flexible and soft in their native beds: this is the case with +asbestos, sahlite, tremolite, and chalcedony, and it is reported +also to happen in the case of the beryl.*</p> + +<p>The marl recently deposited at the bottom of Lake Superior, in +North America, is soft, and often filled with fresh-water shells; +but if a piece be taken up and dried, it becomes so hard that it +can only be broken by a smart blow of the hammer. If the lake, +therefore, was drained, such a deposit would be found to consist of +strata of marlstone, like that observed in many ancient European +formations, and, like them, containing fresh-water shells.</p> + +<img src="../images/fig48.jpg" width="195" height="97" alt= +"Fig. 48: Calcareous nodules in Lias." align="right"> + +<p><b>Concretionary +Structure.</b>—It is probable that some of the +heterogeneous materials which rivers transport to the sea may at +once set under water, like the artificial mixture called pozzolana, +which consists of fine volcanic sand charged with about twenty per +cent of oxide of iron, and the addition of a small quantity of +lime. This substance hardens, and becomes a solid stone in water, +and was used by the Romans in constructing the foundations of +buildings in the sea. Consolidation in such cases is brought about +by the action of chemical affinity on finely comminuted matter +previously suspended in water. After deposition similar particles +seem often to exert a mutual attraction on each other, and +congregate together in particular spots, forming lumps, nodules, +and concretions. Thus in many argillaceous deposits there are +calcareous balls, or spherical concretions, ranged in layers +parallel to the general stratification; an arrangement which took +place after the shale or marl had been thrown down in successive +laminæ; for these laminæ are often traceable through +the concretions, remaining parallel to those of the surrounding +unconsolidated rock. (See Fig. 48.) Such nodules of limestone have +often a shell or other foreign body in the centre.</p> + +<p>Among the most remarkable examples of concretionary structure +are those described by Professor Sedgwick as</p> + +<p class="fnote">* Dr. MacCulloch, Syst. of Geol., vol. i, p. +123.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 64">[ 64 ]</a></p> + +<p>abounding in the magnesian limestone of the north of England. +The spherical balls are of various sizes, from that of a pea to a +diameter of several feet, and they have both a concentric and +radiated structure, while at the same time the laminæ of +original deposition pass uninterruptedly through them. In some +cliffs this limestone resembles a great irregular pile of +cannon-balls. Some of the globular masses have their centre in one +stratum, while a portion of their exterior passes through to the +stratum above or below. Thus the larger spheroid in the section +(Fig. 49) passes from the stratum <i>b</i> upward into <i>a.</i> In +this instance we must suppose the deposition of a series of minor +layers, first forming the stratum <i>b,</i> and afterwards the +incumbent stratum <i>a</i>; then a movement of the particles took +place, and the carbonates of lime and magnesia separated from the +more impure and mixed matter forming the still unconsolidated parts +of the stratum. Crystallisation, beginning at the centre, must have +gone on forming concentric coats around the original nucleus +without interfering with the laminated structure of the rock.</p> + +<img src="../images/fig49.jpg" width="181" height="201" alt= +"Fig. 49: Spheroidal concretions in magnesian limestone. Fig. 50: Section through strata of grit." + align="left"> + +<p>When the particles of rocks have been thus rearranged by +chemical forces, it is sometimes difficult or impossible to +ascertain whether certain lines of division are due to original +deposition or to the subsequent aggregation of several particles. +Thus suppose three strata of grit, A, B, C, are charged unequally +with calcareous matter, and that B is the most calcareous. If +consolidation takes place in B, the concretionary action may spread +upward into a part of A, where the carbonate of lime is more +abundant than in the rest; so that a mass, <i>d e f,</i> forming a +portion of the superior stratum, becomes united with B into one +solid mass of stone. The original line of division, <i>d e,</i> +being thus effaced, the line <i>d f</i> would generally be +considered as the surface of the bed B, though not strictly a true +plane of stratification.</p> + +<p><b>Pressure and Heat.</b>—When +sand and mud sink to the bottom of a deep sea, the particles are +not pressed down by the enormous weight of the incumbent ocean; for +the water, which becomes mingled with the sand and mud, resists +pressure with a force equal to that of the column of fluid above. +The same happens in regard to organic remains which are</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 65">[ 65 ]</a></p> + +<p>filled with water under great pressure as they sink, otherwise +they would be immediately crushed to pieces and flattened. +Nevertheless, if the materials of a stratum remain in a yielding +state, and do not set or solidify, they will be gradually squeezed +down by the weight of other materials successively heaped upon +them, just as soft clay or loose sand on which a house is built may +give way. By such downward pressure particles of clay, sand, and +marl may become packed into a smaller space, and be made to cohere +together permanently.</p> + +<p>Analogous effects of condensation may arise when the solid parts +of the earth’s crust are forced in various directions by those +mechanical movements hereafter to be described, by which strata +have been bent, broken, and raised above the level of the sea. +Rocks of more yielding materials must often have been forced +against others previously consolidated, and may thus by compression +have acquired a new structure. A recent discovery may help us to +comprehend how fine sediment derived from the detritus of rocks may +be solidified by mere pressure. The graphite or "black lead” of +commerce having become very scarce, Mr. Brockedon contrived a +method by which the dust of the purer portions of the mineral found +in Borrowdale might be recomposed into a mass as dense and compact +as native graphite. The powder of graphite is first carefully +prepared and freed from air, and placed under a powerful press on a +strong steel die, with air-tight fittings. It is then struck +several blows, each of a power of 1000 tons; after which operation +the powder is so perfectly solidified that it can be cut for +pencils, and exhibits when broken the same texture as native +graphite.</p> + +<p>But the action of heat at various depths in the earth is +probably the most powerful of all causes in hardening sedimentary +strata. To this subject I shall refer again when treating of the +metamorphic rocks, and of the slaty and jointed structure.</p> + +<p><b>Mineralisation of Organic +Remains.</b>—The changes which fossil organic bodies +have undergone since they were first imbedded in rocks, throw much +light on the consolidation of strata. Fossil shells in some modern +deposits have been scarcely altered in the course of centuries, +having simply lost a part of their animal matter. But in other +cases the shell has disappeared, and left an impression only of its +exterior, or, secondly, a cast of its interior form, or, thirdly, a +cast of the shell itself, the original matter of which has been +removed. These different forms of fossilisation may easily</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 66">[ 66 ]</a></p> + +<center><img src="../images/fig51.jpg" width="314" height="197" alt= +"Fig. 51: Phasianella Heddingtonensis. Fig. 52: Pleurotomaria Anglica."> +</center> + +<p>be understood if we examine the mud recently thrown out from a +pond or canal in which there are shells. If the mud be +argillaceous, it acquires consistency on drying, and on breaking +open a portion of it we find that each shell has left impressions +of its external form. If we then remove the shell itself, we find +within a solid nucleus of clay, having the form of the interior of +the shell. This form is often very different from that of the outer +shell. Thus a cast such as <i>a,</i> Fig. 51, commonly called a +fossil screw, would never be suspected by an inexperienced +conchologist to be the internal shape of the fossil univalve, <i> +b,</i> Fig. 51. Nor should we have imagined at first sight that the +shell a and the cast <i>b,</i> Fig. 52, belong to one and the same +fossil. The reader will observe, in the last-mentioned figure +(<i>b,</i> Fig. 52), that an empty space shaded dark, which the <i> +shell itself</i> once occupied, now intervenes between the +enveloping stone and the cast of the smooth interior of the whorls. +In such cases the shell has been dissolved and the component +particles removed by water percolating the rock. If the nucleus +were taken out, a hollow mould would remain, on which the external +form of the shell with its tubercles and striæ, as seen in +<i>a,</i> Fig. 52, would be seen embossed. Now if the space alluded +to between the nucleus and the impression, instead of being left +empty, has been filled up with calcareous spar, flint, pyrites, or +other mineral, we then obtain from the mould an exact cast both of +the external and internal form of the original shell. In this +manner silicified casts of shells have been formed; and if the mud +or sand of the nucleus happen to be incoherent, or soluble in acid, +we can then procure in flint an empty shell, which in shape is the +exact counterpart of the original. This cast may be compared to a +bronze statue, representing merely the superficial form,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 67">[ 67 ]</a></p> + +<p>and not the internal organisation; but there is another +description of petrifaction by no means uncommon, and of a much +more wonderful kind, which may be compared to certain anatomical +models in wax, where not only the outward forms and features, but +the nerves, blood-vessels, and other internal organs are also +shown. Thus we find corals, originally calcareous, in which not +only the general shape, but also the minute and complicated +internal organisation is retained in flint.</p> + +<img src="../images/fig53.jpg" width="137" height="167" alt="Fig. 53: Section of a tree from the coal-measures." align="right"> + +<p>Such a process of petrifaction is still more remarkably +exhibited in fossil wood, in which we often perceive not only the +rings of annual growth, but all the minute vessels and medullary +rays. Many of the minute cells and fibres of plants, and even those +spiral vessels which in the living vegetable can only be discovered +by the microscope, are preserved. Among many instances, I may +mention a fossil tree, seventy-two feet in length, found at +Gosforth, near Newcastle, in sandstone strata associated with coal. +By cutting a transverse slice so thin as to transmit light, and +magnifying it about fifty-five times, the texture, as seen in Fig. +53, is exhibited. A texture equally minute and complicated has been +observed in the wood of large trunks of fossil trees found in the +Craigleith quarry near Edinburgh, where the stone was not in the +slightest degree siliceous, but consisted chiefly of carbonate of +lime, with oxide of iron, alumina, and carbon. The parallel rows of +vessels here seen are the rings of annual growth, but in one part +they are imperfectly preserved, the wood having probably decayed +before the mineralising matter had penetrated to that portion of +the tree.</p> + +<p>In attempting to explain the process of petrifaction in such +cases, we may first assume that strata are very generally permeated +by water charged with minute portions of calcareous, siliceous, and +other earths in solution. In what manner they become so impregnated +will be afterwards considered. If an organic substance is exposed +in the open air to the action of the sun and rain, it will in time +putrefy, or be dissolved into its component elements, consisting +usually of oxygen, hydrogen, nitrogen, and carbon. These will +readily be absorbed by the atmosphere or be washed away by rain, so +that all vestiges of the dead animal or plant disappear. But if the +same substances be submerged in water, they decompose more +gradually; and if buried in earth, still more</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 68">[ 68 ]</a></p> + +<p>slowly; as in the familiar example of wooden piles or other +buried timber. Now, if as fast as each particle is set free by +putrefaction in a fluid or gaseous state, a particle equally minute +of carbonate of lime, flint, or other mineral, is at hand ready to +be precipitated, we may imagine this inorganic matter to take the +place just before left unoccupied by the organic molecule. In this +manner a cast of the interior of certain vessels may first be +taken, and afterwards the more solid walls of the same may decay +and suffer a like transmutation. Yet when the whole is lapidified, +it may not form one homogeneous mass of stone or metal. Some of the +original ligneous, osseous, or other organic elements may remain +mingled in certain parts, or the lapidifying substance itself may +be differently coloured at different times, or so crystallised as +to reflect light differently, and thus the texture of the original +body may be faithfully exhibited.</p> + +<p>The student may perhaps ask whether, on chemical principles, we +have any ground to expect that mineral matter will be thrown down +precisely in those spots where organic decomposition is in +progress? The following curious experiments may serve to illustrate +this point: Professor Goppert of Breslau, with a view of imitating +the natural process of petrifaction, steeped a variety of animal +and vegetable substances in waters, some holding siliceous, others +calcareous, others metallic matter in solution. He found that in +the period of a few weeks, or sometimes even days, the organic +bodies thus immersed were mineralised to a certain extent. Thus, +for example, thin vertical slices of deal, taken from the Scotch +fir (<i>Pinus sylvestris</i>), were immersed in a moderately strong +solution of sulphate of iron. When they had been thoroughly soaked +in the liquid for several days they were dried and exposed to a +red-heat until the vegetable matter was burnt up and nothing +remained but an oxide of iron, which was found to have taken the +form of the deal so exactly that casts even of the dotted vessels +peculiar to this family of plants were distinctly visible under the +microscope.</p> + +<p>The late Dr. Turner observes, that when mineral matter is in a +“ nascent state,” that is to say, just liberated from a previous +state of chemical combination, it is most ready to unite with other +matter, and form a new chemical compound. Probably the particles or +atoms just set free are of extreme minuteness, and therefore move +more freely, and are more ready to obey any impulse of chemical +affinity. Whatever be the cause, it clearly follows, as before +stated, that where organic matter newly imbedded in sediment is +decomposing, there will chemical changes take place most +actively.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 69">[ 69 ]</a></p> + +<p>An analysis was lately made of the water which was flowing off +from the rich mud deposited by the Hooghly River in the Delta of +the Ganges after the annual inundation. This water was found to be +highly charged with carbonic acid holding lime in solution.* Now if +newly-deposited mud is thus proved to be permeated by mineral +matter in a state of solution, it is not difficult to perceive that +decomposing organic bodies, naturally imbedded in sediment, may as +readily become petrified as the substances artificially immersed by +Professor Goppert in various fluid mixtures.</p> + +<p>It is well known that the waters of all springs are more or less +charged with earthy, alkaline, or metallic ingredients derived from +the rocks and mineral veins through which they percolate. Silex is +especially abundant in hot springs, and carbonate of lime is almost +always present in greater or less quantity. The materials for the +petrifaction of organic remains are, therefore, usually at hand in +a state of chemical solution wherever organic remains are imbedded +in new strata.</p> + +<p class="fnote">* Piddington, Asiat. Research., vol. xviii, p. +226.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch3.html"> +Chapter III</a> / <a href="ch5.html">Chapter V</a></small> +</body> +</html> + |