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diff --git a/old/3772-h/files/ch3.html b/old/3772-h/files/ch3.html new file mode 100644 index 0000000..420af80 --- /dev/null +++ b/old/3772-h/files/ch3.html @@ -0,0 +1,610 @@ +<!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 47">[ 47 ]</a></p> + +<p> </p> + +<center><b>Chapter III</b><br> +<br> +ARRANGEMENT OF FOSSILS IN STRATA.—FRESH-WATER AND MARINE +FOSSILS.</center> + +<p class="intro">Successive Deposition indicated by +Fossils. — Limestones formed of Corals and Shells. — Proofs +of gradual Increase of Strata derived from Fossils. — Serpula +attached to Spatangus. — Wood bored by Teredina. — Tripoli +formed of Infusoria. — Chalk derived principally from Organic +Bodies. — Distinction of Fresh-water from Marine +Formations. — Genera of Fresh-water and Land +Shells. — Rules for recognising Marine +Testacea. — Gyrogonite and Chara. — Fresh-water +Fishes. — Alternation of Marine and Fresh-water +Deposits. — Lym-Fiord.</p> + +<p>Having in the last chapter considered the forms of +stratification so far as they are determined by the arrangement of +inorganic matter, we may now turn our attention to the manner in +which organic remains are distributed through stratified deposits. +We should often be unable to detect any signs of stratification or +of successive deposition, if particular kinds of fossils did not +occur here and there at certain depths in the mass. At one level, +for example, univalve shells of some one or more species +predominate; at another, bivalve shells; and at a third, corals; +while in some formations we find layers of vegetable matter, +commonly derived from land plants, separating strata.</p> + +<p>It may appear inconceivable to a beginner how mountains, several +thousand feet thick, can have become full of fossils from top to +bottom; but the difficulty is removed, when he reflects on the +origin of stratification, as explained in the last chapter, and +allows sufficient time for the accumulation of sediment. He must +never lose sight of the fact that, during the process of +deposition, each separate layer was once the uppermost, and +immediately in contact with the water in which aquatic animals +lived. Each stratum, in fact, however far it may now lie beneath +the surface, was once in the state of shingle, or loose sand or +soft mud at the bottom of the sea, in which shells and other bodies +easily became enveloped.</p> + +<p><b>Rate of Deposition indicated by +Fossils.</b>—By attending to the nature of these +remains, we are often enabled to determine whether the deposition +was slow or rapid, whether it took place in a deep or shallow sea, +near the shore or far</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 48">[ 48 ]</a></p> + +<p>from land, and whether the water was salt, brackish, or fresh. +Some limestones consist almost exclusively of corals, and in many +cases it is evident that the present position of each fossil +zoophyte has been determined by the manner in which it grew +originally. The axis of the coral, for example, if its natural +growth is erect, still remains at right angles to the plane of +stratification. If the stratum be now horizontal, the round +spherical heads of certain species continue uppermost, and their +points of attachment are directed downward. This arrangement is +sometimes repeated throughout a great succession of strata. From +what we know of the growth of similar zoophytes in modern reefs, we +infer that the rate of increase was extremely slow, and some of the +fossils must have flourished for ages like forest-trees, before +they attained so large a size. During these ages, the water must +have been clear and transparent, for such corals can not live in +turbid water.</p> + +<img src="../images/fig9.jpg" width="212" height="359" alt= +"Fossil Gryphæ, covered both on the outside and inside with fossil serpulæ." align="left"> + +<p>In like manner, when we see thousands of full-grown shells +dispersed everywhere throughout a long series of strata, we can not +doubt that time was required for the multiplication of successive +generations; and the evidence of slow accumulation is rendered more +striking from the proofs, so often discovered, of fossil bodies +having lain for a time on the floor of the ocean after death before +they were imbedded in sediment. Nothing, for example, is more +common than to see fossil oysters in clay, with Serpulæ, or +barnacles (acorn-shells), or corals, and other creatures, attached +to the inside of the valves, so that the mollusk was certainly not +buried in argillaceous mud the moment it died. There must have been +an interval during which it was still surrounded with clear water, +when the creatures whose remains now adhere to it grew from an +embryonic to a mature state. Attached shells which are merely +external, like some of the Serpulæ (<i>a</i>) in Fig. 9, may +often have grown upon an</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 49">[ 49 ]</a></p> + +<p>oyster or other shell while the animal within was still living; +but if they are found on the inside, it could only happen after the +death of the inhabitant of the shell which affords the support. +Thus, in Fig. 9, it will be seen that two Serpulæ have grown +on the interior, one of them exactly on the place where the +adductor muscle of the <i>Gryphæa</i> (a kind of oyster) was +fixed.</p> + +<img src="../images/fig10.jpg" width="168" height="342" alt= +"Fig. 10: Serpula attached to a fossil. Fig. 11: Recent Spatangus with spines removed from one side." + align="left"> + +<p>Some fossil shells, even if simply attached to the <i> +outside</i> of others, bear full testimony to the conclusion above +alluded to, namely, that an interval elapsed between the death of +the creature to whose shell they adhere, and the burial of the same +in mud or sand. The sea-urchins, or <i>Echini</i>, so abundant in +white chalk, afford a good illustration. It is well known that +these animals, when living, are invariably covered with spines +supported by rows of tubercles. These last are only seen after the +death of the sea-urchin, when the spines have dropped off. In Fig. +11 a living species of <i>Spatangus</i>, common on our coast, is +represented with one half of its shell stripped of the spines. In +Fig. 10 a fossil of a similar and allied genus from the white chalk +of England shows the naked surface which the individuals of this +family exhibit when denuded of their bristles. The full-grown <i> +Serpula</i>, therefore, which now adheres externally, could not +have begun to grow till the <i>Micraster</i> had died, and the +spines became detached.</p> + +<img src="../images/fig12.jpg" width="120" height="190" alt= +"Fig. 12: Ananchytes from the chalk." align= +"right"> + +<p>Now the series of events here attested by a single fossil may be +carried a step farther. Thus, for example, we often meet with a +sea-urchin (<i>Ananchytes</i>) in the chalk (see Fig. 12) which has +fixed to it the lower valve of a <i>Crania</i>, a genus of bivalve +mollusca. The upper valve (<i>b</i>, Fig. 12) is almost invariably +wanting, though occasionally found in a perfect state of +preservation in white chalk at some distance. In this case, we see +clearly that the sea-urchin first</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 50">[ 50 ]</a></p> + +<p>lived from youth to age, then died and lost its spines, which +were carried away. Then the young <i>Crania</i> adhered to the +bared shell, grew and perished in its turn; after which the upper +valve was separated from the lower before the <i>Ananchytes</i> +became enveloped in chalky mud.</p> + +<center><img src="../images/fig13.jpg" width="313" height="187" alt= +"Fig. 13: Fossil wood bored by Teredina."></center> + +<center><img src="../images/fig14.jpg" width="351" height="184" alt= +"Fig. 14: Recent wood bored by Teredo."></center> + +<p>It may be well to mention one more illustration of the manner in +which single fossils may sometimes throw light on a former state of +things, both in the bed of the ocean and on some adjoining land. We +meet with many fragments of wood bored by ship-worms at various +depths in the clay on which London is built. Entire branches and +stems of trees, several feet in length, are sometimes found drilled +all over by the holes of these borers, the tubes and shells of the +mollusk still remaining in the cylindrical hollows. In Fig. 14, <i> +e</i>, a representation is given of a piece of recent wood pierced +by the <i>Teredo navalis</i>, or common ship-worm, which destroys +wooden piles and ships. When the cylindrical tube <i>d</i> has been +extracted from the wood, the valves are seen at the larger or +anterior extremity, as shown at <i>c.</i> In like manner, a piece +of fossil wood (<i>a</i>, Fig. 13) has been perforated by a kindred +but extinct genus, the <i>Teredina</i> of Lamarck. The calcareous +tube of this mollusk was united and, as it were, soldered on to the +valves of the shell (<i>b</i>), which therefore can not be detached +from the tube, like the valves of</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 51">[ 51 ]</a></p> + +<p>the recent <i>Teredo.</i> The wood in this fossil specimen is +now converted into a stony mass, a mixture of clay and lime; but it +must once have been buoyant and floating in the sea, when the <i> +Teredinæ</i> lived upon, and perforated it. Again, before the +infant colony settled upon the drift wood, part of a tree must have +been floated down to the sea by a river, uprooted, perhaps, by a +flood, or torn off and cast into the waves by the wind: and thus +our thoughts are carried back to a prior period, when the tree grew +for years on dry land, enjoying a fit soil and climate.</p> + +<p><b>Strata of Organic +Origin.</b>—It has been already remarked that there +are rocks in the interior of continents, at various depths in the +earth, and at great heights above the sea, almost entirely made up +of the remains of zoophytes and testacea. Such masses may be +compared to modern oyster-beds and coral-reefs; and, like them, the +rate of increase must have been extremely gradual. But there are a +variety of stone deposits in the earth’s crust, now proved to have +been derived from plants and animals of which the organic origin +was not suspected until of late years, even by naturalists. Great +surprise was therefore created some years since by the discovery of +Professor Ehrenberg, of Berlin, that a certain kind of siliceous +stone, called tripoli, was entirely composed of millions of the +remains of organic beings, which were formerly referred to +microscopic Infusoria, but which are now admitted to be plants. +They abound in rivulets, lakes, and ponds in England and other +countries, and are termed Diatomaceæ by those naturalists who +believe in their vegetable origin. The subject alluded to has long +been well-known in the arts, under the name of infusorial earth or +mountain meal, and is used in the form of powder for polishing +stones and metals. It has been procured, among other places, from +the mud of a lake at Dolgelly, in North Wales, and from Bilin, in +Bohemia, in which latter place a single stratum, extending over a +wide area, is no less than fourteen feet thick. This stone, when +examined with a powerful microscope, is found to consist of the +siliceous plates or frustules of the above-figured +Diatomaceæ, united together without any visible cement. It is +difficult to convey an idea of their extreme minuteness; but +Ehrenberg estimates that in the Bilin tripoli there are 41,000 +millions of individuals of the <i>Gaillonella distans</i> (see Fig. +16) in every cubic inch (which weighs about 220 grains), or about +187 millions in a single grain. At every stroke, therefore, that we +make with this polishing powder, several millions, perhaps tens of +millions, of perfect fossils are crushed to atoms.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 52">[ 52 ]</a></p> + +<img src="../images/fig15.jpg" width="135" height="267" alt= +"Figs 15 and 16: Gaillonella; Fig. 17: Bacillaria parodoxa" + align="left"> + +<p>A well-known substance, called bog-iron ore, often met with in +peat-mosses, has often been shown by Ehrenberg to consist of +innumerable articulated threads, of a yellow ochre colour, composed +of silica, argillaceous matter, and peroxide of iron. These threads +are the cases of a minute microscopic body, called <i>Gaillonella +ferruginea</i> (Fig. 15), associated with the siliceous frustules +of other fresh-water algæ. Layers of this iron ore occurring +in Scotch peat bogs are often called “the pan,” and are sometimes +of economical value.</p> + +<p>It is clear much time must have been required for the +accumulation of strata to which countless generations of +Diatomaceæ have contributed their remains; and these +discoveries lead us naturally to suspect that other deposits, of +which the materials have been supposed to be inorganic, may in +reality be composed chiefly of microscopic organic bodies. That +this is the case with the white chalk, has often been imagined, and +is now proved to be the fact. It has, moreover, been lately +discovered that the chambers into which these Foraminifera are +divided are actually often filled with thousands of well-preserved +organic bodies, which abound in every minute grain of chalk, and +are especially apparent in the white coating of flints, often +accompanied by innumerable needle-shaped spiculæ of sponges +(see Chapter XVII).</p> + +<center>“The dust we tread upon was once +alive!”—B<small>YRON.</small></center> + +<p>How faint an idea does this exclamation of the poet convey of +the real wonders of nature! for here we discover proofs that the +calcareous and siliceous dust of which hills are composed has not +only been once alive, but almost every particle, albeit invisible +to the naked eye, still retains the organic structure which, at +periods of time incalculably remote, was impressed upon it by the +powers of life.</p> + +<p><b>Fresh-water and Marine +Fossils.</b>—Strata, whether deposited in salt or +fresh water, have the same forms; but the imbedded fossils are very +different in the two cases, because the aquatic animals which +frequent lakes and rivers are distinct from those inhabiting the +sea. In the northern part of the Isle of Wight formations of marl +and limestone, more than</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 53">[ 53 ]</a></p> + +<p>50 feet thick occur, in which the shells are of extinct species. +Yet we recognise their fresh-water origin, because they are of the +same genera as those now abounding in ponds, lakes, and rivers, +either in our own country or in warmer latitudes.</p> + +<p>In many parts of France—in Auvergne, for +example—strata occur of limestone, marl, and sandstone +hundreds of feet thick, which contain exclusively fresh-water and +land shells, together with the remains of terrestrial quadrupeds. +The number of land-shells scattered through some of these +fresh-water deposits is exceedingly great; and there are districts +in Germany where the rocks scarcely contain any other fossils +except snail-shells (<i>helices</i>); as, for instance, the +limestone on the left bank of the Rhine, between Mayence and Worms, +at Oppenheim, Findheim, Budenheim, and other places. In order to +account for this phenomenon, the geologist has only to examine the +small deltas of torrents which enter the Swiss lakes when the +waters are low, such as the newly-formed plain where the Kander +enters the Lake of Thun. He there sees sand and mud strewn over +with innumerable dead land-shells, which have been brought down +from the valleys in the Alps in the preceding spring, during the +melting of the snows. Again, if we search the sands on the borders +of the Rhine, in the lower part of its course, we find countless +land-shells mixed with others of species belonging to lakes, +stagnant pools, and marshes. These individuals have been washed +away from the alluvial plains of the great river and its +tributaries, some from mountainous regions, others from the low +country.</p> + +<p>Although fresh-water formations are often of great thickness, +yet they are usually very limited in area when compared to marine +deposits, just as lakes and estuaries are of small dimensions in +comparison with seas.</p> + +<p>The absence of many fossil forms usually met with in marine +strata, affords a useful negative indication of the fresh-water +origin of a formation. For example, there are no sea-urchins, no +corals, no chambered shells, such as the nautilus, nor microscopic +Foraminifera in lacustrine or fluviatile deposits. In +distinguishing the latter from formations accumulated in the sea, +we are chiefly guided by the forms of the mollusca. In a +fresh-water deposit, the number of individual shells is often as +great as in a marine stratum, if not greater; but there is a +smaller variety of species and genera. This might be anticipated +from the fact that the genera and species of recent fresh-water and +land shells are few when contrasted with the marine. Thus, the +genera of true mollusca according to Woodward’s system, excluding +those</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 54">[ 54 ]</a></p> + +<center><img src="../images/fig18.jpg" width="345" height="126" alt= +"Fig. 18: Cyrena obovata. Fig. 19: Cyrena fluminatis."></center> + +<p>altogether extinct and those without shells, amount to 446 in +number, of which the terrestrial and fresh-water genera scarcely +form more than a fifth.*</p> + +<center><img src="../images/fig20.jpg" width="360" height="208" alt= +"Fig. 20: Anodonta Cordierii. Fig. 21: Anodonta latimarginata. Fig. 22: Unio littoralis."> +</center> + +<p>Almost all bivalve shells, or those of acephalous mollusca, are +marine, about sixteen only out of 140 genera being fresh-water. +Among these last, the four most common forms, both recent and +fossil, are <i>Cyclas, Cyrena, Unio,</i> and <i>Anodonta</i> (see +Figures); the two first and two last of which are so nearly allied +as to pass into each other.</p> + +<img src="../images/fig23.jpg" width="142" height="204" alt="Fig. 23: +Gryphæa incurva." align="right"> + +<p>Lamarck divided the bivalve mollusca into the Dimyary, or those +having two large muscular impressions in each valve, as <i>a b</i> +in the Cyclas, Fig. 18, and Unio, Fig. 22, and the <i> +Monomyary,</i> such as the oyster and scallop, in which there is +only one of these impressions, as is seen in Fig. 23. Now, as none +of these last, or the unimuscular bivalves, are +fresh-water,† we may at once presume a deposit containing +any of them to be marine.</p> + +<p class="fnote">* See Woodward’s Manual of Mollusca, 1856.<br> +† The fresh-water Mulleria, when young, forms a single +exception to the rule, as it then has two muscular impressions, but +it has only one in the adult state.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 55">[ 55 ]</a></p> + +<center><img src="../images/fig24.jpg" width="413" height="486" alt= +"Fig. 24: Planorbis enomphalus. Fig. 25: Limnæa longiscala. Fig. 26: Pauldina lenta. Fig. 27: Succinea amphibia. Fig. 28: Ancylus velletia. Fig. 29: Valvata piscinalis. Fig. 30: Physa hypnorum. Fig. 31: Auricula. Fig. 32: Melania inquinata. Fig. 33: Physa columnaris. Fig. 34: Melanopsis buccinoidea."> +</center> + +<img src="../images/fig35.jpg" width="159" height="202" alt= +"Fig. 35: Neritina globulud. Fig. 36: Nerita granulosa." align="left"> + + +<p>The univalve shells most characteristic of fresh-water deposits +are, <i>Planorbis, Limnæa,</i> and <i>Paludina.</i> (See +Figures.) But to these are occasionally added <i>Physa, Succinea, +Ancylus, Valvata, Melanopsis, Melania, Potamides,</i> and <i> +Neritina</i> (see Figures), the four last being usually found in +estuaries.</p> + +<p>Some naturalists include <i>Neritina</i> (Fig. 35) and the +marine <i>Nerita</i> (Fig. 36) in the same genus, it being scarcely +possible to distinguish the two by good generic characters. But, as +a general rule, the</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 56">[ 56 ]</a></p> + +<img src="../images/fig37.jpg" width="80" height="257" alt="Fig. 37: +Potamides cinctus." align="left"> + +<p>fluviatile species are smaller, smoother, and more globular than +the marine; and they have never, like the <i>Neritæ,</i> the +inner margin of the outer lip toothed or crenulated. (See Fig. +36.)</p> + +<p>The Potamides inhabit the mouths of rivers in warm latitudes, +and are distinguishable from the marine Cerithia by their orbicular +and multispiral opercula. The genus Auricula (Fig. 31) is +amphibious, frequenting swamps and marshes within the influence of +the tide.</p> + +<p>The terrestrial shells are all univalves. The most important +genera among these, both in a recent and fossil state, are <i> +Helix</i> (Fig. 38), <i>Cyclostoma</i> (Fig. 39), <i>Pupa</i> (Fig. +40), <i>Clausilia</i> (Fig. 41), <i>Bulimus</i> (Fig. 42), <i> +Glandina</i> and <i>Achatina.</i></p> + +<center><img src="../images/fig38.jpg" width="414" height="153" alt= +"Fig. 38: Helix Turomensis. Fig. 39: Cyclostoma elegans. Fig. 40: Pupa tridens. Fig. 41: Clausilia bidens. Fig. 42: Bulimus lubricus."> +</center> + +<p><i>Ampullaria</i> (Fig. 43) is another genus of shells +inhabiting rivers and ponds in hot countries. Many fossil species +formerly referred to this genus, and which have been met with +chiefly in marine formations, are now considered by conchologists +to belong to <i>Natica</i> and other marine genera.</p> + +<img src="../images/fig43.jpg" width="111" height="154" alt="Fig. 43: +Ampullaria glauca." align="left"> + +<p>All univalve shells of land and fresh-water species, with the +exception of <i>Melanopsis</i> (Fig. 34), and <i>Achatina,</i> +which has a slight indentation, have entire mouths; and this +circumstance may often serve as a convenient rule for +distinguishing fresh-water from marine strata; since, if any +univalves occur of which the mouths are not entire, we may presume +that the formation is marine. The aperture is said to be entire in +such shells as the fresh-water <i>Ampullaria</i> and the +land-shells (Figs 38-42), when its outline is not interrupted by an +indentation or notch, such as that seen at <i>b</i> in <i> +Ancillaria</i> (Fig. 45); or is not prolonged into a canal, as that +seen at <i>a</i> in <i>Pleurotoma</i> (Fig. 44).</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 57">[ 57 ]</a></p> + +<center><img src="../images/fig44.jpg" width="319" height="218" alt= +"Fig. 44: Pleurotoma exorta. Fig. 45: Ancillaria subulata."></center> + +<p>The mouths of a large proportion of the marine univalves have +these notches or canals, and almost all species are carnivorous; +whereas nearly all testacea having entire mouths are plant-eaters, +whether the species be marine, fresh-water, or terrestrial.</p> + +<p>There is, however, one genus which affords an occasional +exception to one of the above rules. The <i>Potamides</i> (Fig. +37), a subgenus of Cerithium, although provided with a short canal, +comprises some species which inhabit salt, others brackish, and +others fresh-water, and they are said to be all plant-eaters.</p> + +<p>Among the fossils very common in fresh-water deposits are the +shells of <i>Cypris,</i> a minute bivalve crustaceous animal.* Many +minute living species of this genus swarm in lakes and stagnant +pools in Great Britain; but their shells are not, if considered +separately, conclusive as to the fresh-water origin of a deposit, +because the majority of species in another kindred genus of the +same order, the <i>Cytherina</i> of Lamarck, inhabit salt-water; +and, although the animal differs slightly, the shell is scarcely +distinguishable from that of the Cypris.</p> + +<p><b>Fresh-water Fossil +Plants.</b>—The seed-vessels and stems of <i> +Chara,</i> a genus of aquatic plants, are very frequent in +fresh-water strata. These seed-vessels were called, before their +true nature was known, gyrogonites, and were supposed to be +foraminiferous shells. (See Fig. 46, <i>a</i>.)</p> + +<p>The <i>Charæ</i> inhabit the bottom of lakes and ponds, +and flourish mostly where the water is charged with carbonate of +lime. Their seed-vessels are covered with a very tough integument, +capable of resisting decomposition; to which circumstance we may +attribute their abundance in a fossil</p> + +<p class="fnote">* For figures of fossil species of Purbeck, see <a +href="ch19.html">Chapter XIX</a></p> + +<p> </p> + +<hr> +<p class="page"><a name="page 58">[ 58 ]</a></p> + +<p>state. The annexed figure (Fig. 47) represents a branch of one +of many new species found by Professor Amici in the lakes of +Northern Italy. The seed-vessel in this plant is more globular than +in the British <i>Charæ,</i>) and therefore more nearly +resembles in form the extinct fossil species found in England, +France, and other countries. The stems, as well as the +seed-vessels, of these plants occur both in modern shell-marl and +in ancient fresh-water formations. They are generally composed of a +large central tube surrounded by smaller ones; the whole stem being +divided at certain intervals by transverse partitions or joints. +(See <i>b,</i> Fig. 46.)</p> + +<center><img src="../images/fig46.jpg" width="391" height="248" alt= +"Fig. 46: Chara medicaginula. Fig. 47: Chara elastica."></center> + +<p>It is not uncommon to meet with layers of vegetable matter, +impressions of leaves, and branches of trees, in strata containing +fresh-water shells; and we also find occasionally the teeth and +bones of land quadrupeds, of species now unknown. The manner in +which such remains are occasionally carried by rivers into lakes, +especially during floods, has been fully treated of in the +“Principles of Geology.”</p> + +<p><b>Fresh-water and Marine +Fish.</b>—The remains of fish are occasionally useful +in determining the fresh-water origin of strata. Certain genera, +such as carp, perch, pike, and loach (<i>Cyprinus, Perca, Esox,</i> +and <i>Cobitis</i>), as also <i>Lebias,</i> being peculiar to +fresh-water. Other genera contain some fresh-water and some marine +species, as <i>Cottus, Mugil,</i> and <i>Anguilla,</i> or eel. The +rest are either common to rivers and the sea, as the salmon; or are +exclusively characteristic of salt-water. The above observations +respecting fossil fishes are applicable only to the more modern or +tertiary deposits;</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 59">[ 59 ]</a></p> + +<p>for in the more ancient rocks the forms depart so widely from +those of existing fishes, that it is very difficult, at least in +the present state of science, to derive any positive information +from ichthyolites respecting the element in which strata were +deposited.</p> + +<p>The alternation of marine and fresh-water formations, both on a +small and large scale, are facts well ascertained in geology. When +it occurs on a small scale, it may have arisen from the alternate +occupation of certain spaces by river-water and the sea; for in the +flood season the river forces back the ocean and freshens it over a +large area, depositing at the same time its sediment; after which +the salt-water again returns, and, on resuming its former place, +brings with it sand, mud, and marine shells.</p> + +<p>There are also lagoons at the mouth of many rivers, as the Nile +and Mississippi, which are divided off by bars of sand from the +sea, and which are filled with salt and fresh water by turns. They +often communicate exclusively with the river for months, years, or +even centuries; and then a breach being made in the bar of sand, +they are for long periods filled with salt-water.</p> + +<p><b>Lym-Fiord.</b>—The Lym-Fiord +in Jutland offers an excellent illustration of analogous changes; +for, in the course of the last thousand years, the western +extremity of this long frith, which is 120 miles in length, +including its windings, has been four times fresh and four times +salt, a bar of sand between it and the ocean having been often +formed and removed. The last irruption of salt water happened in +1824, when the North Sea entered, killing all the fresh-water +shells, fish, and plants; and from that time to the present, the +sea-weed <i>Fucus vesiculosus,</i> together with oysters and other +marine mollusca, have succeeded the <i>Cyclas, Lymnæa, +Paludina,</i> and <i>Charæ.</i>*</p> + +<p>But changes like these in the Lym-Fiord, and those before +mentioned as occurring at the mouths of great rivers, will only +account for some cases of marine deposits of partial extent resting +on fresh-water strata. When we find, as in the south-east of +England (Chapter XVIII), a great series of fresh-water beds, 1000 +feet in thickness, resting upon marine formations and again covered +by other rocks, such as the Cretaceous, more than 1000 feet thick, +and of deep-sea origin, we shall find it necessary to seek for a +different explanation of the phenomena.</p> + +<p class="fnote">* See Principles, Index, “Lym-Fiord.”</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch2.html"> +Chapter II</a> / <a href="ch4.html">Chapter IV</a></small> +</body> +</html> + |