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PART VI
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See also:PART VI .—PALAEONTOLOGICAL See also:GEOLOGY This See also:division of the See also:science deals with fossils, or the traces of See also:plants and animals preserved in the rocks of the See also:earth's crust, and endeavours to gather from them See also:information as to the See also:history of the globe and its inhabitants. The See also:term " fossil " (See also:Lat. fossilis, from fodere, to dig up), meaning literally anything " dug up," was formerly applied indiscriminately to any See also:mineral substance taken out of the earth's crust, whether organized or not. Since the See also:time of See also:Lamarck, however, the meaning of the word has been restricted, so as to include only the remains or traces of plants and animals preserved in any natural formation whether hard See also:rock or superficial See also:deposit. It includes not merely the petrified structures of organisms, but whatever was directly connected with or produced by these organisms. Thus the See also:resin which was exuded from trees of See also:long-perished forests is as much a fossil as any portion of the See also:stem, leaves, See also:flowers or See also:fruit, and in some respects is even more valuable to the geologist than more determinable remains of its See also:parent trees, because it has often preserved in admirable perfection the See also:insects which flitted about in the woodlands. The burrows and trails of a See also:worm preserved in See also:sandstone and shale claim recognition as fossils, and indeed are commonly the only indications to be met with of the existence of annelid See also:life among old See also:geological formations. The droppings of fishes and See also:reptiles, called See also:coprolites, are excellent fossils, and tell their See also:tale as to the presence and See also:food of vertebrate life in See also:ancient See also:waters. The little agglutinated cases of the caddis-worm remain as fossils in formations from which, perchance, most other traces of life may have passed away. See also:Nay, the very handiwork of See also:man, when preserved in any natural manner, is entitled to See also:rank among fossils; as where his See also:flint-implements have been dropped into the pre-historic gravels of See also:river-valleys or where his canoes have been buried in the silt of See also:lake-bottoms. A study of the See also:land-surfaces and See also:sea-floors of the See also:present time shows that there are so many chances against the conservation of the remains of either terrestrial or marine animals and plants that if, as is probable, the same conditions existed in former geological periods, we should regard the occurrence of organic remains among the stratified formations of the earth's crust as generally the result of various fortunate accidents. Let us consider, in the first See also:place, the chances for the preservation of remains of the present See also:fauna and See also:flora of a See also:country. The See also:surface of the land may be densely clothed with See also:forest and abundantly peopled with See also:animal life. But the trees See also:die and moulder into See also:soil. times. In the marine See also:Mollusca, therefore, we have a See also:common ground of comparison between the stratified formations of different periods. They have been styled the See also:alphabet of palaeontological inquiry. There are two See also:main purposes to which fossils may be put in geological See also:research: (r) to throw See also:light upon former conditions of See also:physical See also:geography, such as the presence of land, See also:rivers, lakes and seas, in places where they do not now exist, changes of See also:climate, and the former See also:distribution of plants and animals; and (2) to furnish a See also:guide in geological See also:chronology whereby rocks may be classified according, to relative date, and the facts of geological history may be arranged and interpreted as a connected See also:record of the earth's progress. I. As examples of the first of these two directions of inquiry reference may be made to (a) former land-surfaces revealed by the occurrence of layers of soil with See also:tree-stumps and roots still in the position of growth (see See also:PURBEcKIAN); (b) ancient lakes proved by beds of See also:marl or See also:limestone full of lacustrine shells; (c) old sea-bottoms marked by the occurrence of marine organisms; (d) See also:variations in the quality of the See also:water, such as freshness or, saltness, indicated by changes in the See also:size and shape of the fossils; (e) proximity to former land, suggested by the occurrence of abundant See also:drift-See also:wood in the strata; (f) former conditions of climate, different from the present, as evidenced by such organisms as tropical types of plants and animals intercalated among the strata of temperate or See also:northern The animals, too, disappear, See also:generation after generation, and leave few or no perceptible traces of their existence. If we were not aware from See also:authentic records that central and northern See also:Europe were covered with vast forests at the beginning of our era, how could we know this fact? What has become of the herds of See also:wild oxen, the bears, wolves and other denizens of primeval Europe? How could we prove from the examination of the surface soil of any country that those creatures had once abounded there? The conditions for the preservation of any See also:relics of the plant and animal life of a terrestrial surface must obviously be always exceptional. They are supplied only where the organic remains can be protected from the See also:air and superficial decay. Hence they may be observed in (1) the deposits on the floors of lakes; (2) in See also:peat-mosses; (3) in deltas at river-mouths; and (4) under the stalagmite of caverns in limestone districts. But in these and other favourable places a See also:mere infinitesimal fraction of the fauna or flora of a land-surface is likely to be entombed or preserved. In the second place, although in the sea the conditions for the preservation of organic remains are in many respects more favourable than on land, they are See also:apt to be frustrated by many adverse circumstances. While the level of the land remains stationary, there can be but little effective entombment of marine organisms. in littoral deposits; for only a limited See also:accumulation of sediment will be formed until subsidence of the sea-See also:floor takes place. In the trifling beds of See also:sand or See also:gravel thrown up on a stationary See also:shore, only the harder and more durable forms of life, such as gastropods and lamellibranchs, which can withstand the triturating effects of the See also:beach waves, are likely to remain uneffaced. Below See also:tide-marks, along the margin of the land where sediment is gradually deposited, the conditions are more favourable for the preservation of marine organisms. In the sheets of sand and mud there laid down the harder parts of many forms of life may be entombed and protected from decay. But only a small proportion of the See also:total marine fauna may be expected to appear in such deposits. At the best, merely littoral and shallow-water forms will occur, and, even under the most favourable conditions, they will represent but a fraction of the whole assemblage of life in these juxta-terrestrial parts of the ocean. As we recede from the land. the See also:rate of deposition of sediment on the sea-floor must become feebler, until, in the remote central abysses, it reaches a hardly appreciable minimum. Except, therefore, where some See also:kind of See also:ooze or other deposit is accumulating in these more pelagic regions, the conditions must be on the whole unfavourable for the preservation of any adequate See also:representation of the deep-sea fauna. Hard durable See also:objects, such as See also:teeth and bones, may slowly accumulate, and be protected by a coating of peroxide of See also:manganese, or of some of the silicates now forming here and there over the deep-sea bottom; or the rate of growth of the abysmal deposit may be so tardy that most of the remains of at least the larger animals will disappear, owing to decay, before they can be covered up and preserved. Any such deep-sea formation, if raised into land, would See also:supply but a meagre picture of the whole life of the sea. It would thus appear that the portion of the sea-floor best suited for receiving and preserving the most varied assemblage of marine organic remains is the See also:area in front of the land, to which rivers and currents bring continual supplies of sediment. The most favourable conditions for the accumulation of a thick See also:mass of marine fossiliferous strata will arise when the area of deposit is undergoing a See also:gradual subsidence.. If the rate of depression and that of deposit were equal, or nearly so, the See also:movement might proceed for a vast 'See also:period without producing any See also:great apparent See also:change in marine geography, and even without .seriously affecting the distribution of life over the sea-floor within the area of subsidence. Hundreds or thousands of feet of sedimentary strata might in this way be heaped up See also:round the continents, containing a fragmentary See also:series of organic remains belonging to those forms of comparatively shallow-water life which had hard parts capable of preservation. There can be little doubt that such has, in fact, been the history of the main mass of stratified formations in the earth's crust. By far the largest proportion of these piles of marine strata has unquestionably been laid down in water of no great See also:depth within the area of deposit of terrestrial sediment. The enormous thickness to which they attain seems only explicable by prolonged and repeated movements of subsidence, interrupted, however, as we know, by other movements of a contrary kind. Since the conditions for the preservation of organic remains exist more favourably under the sea than on land, marine organisms must be far more abundantly conserved than those of the land. This is true to-See also:day, and has, as far as known, been true in all past geological time. Hence for the purposes of the geologist the fossil remains of marine forms of life far surpass all others in value. Among them there will necessarily be a gradation of importance, regulated chie% by their relative abundance. Now, of all the marine tribes which live within the juxta-terrestrial See also:belt of sedimentation, unquestionably the Mollusca stand in the place of pre-See also:eminence as regards their aptitude for becoming fossils. They almost all possess a hard, durable See also:shell, capable of resisting considerable See also:abrasion and readily passing into a mineralized See also:condition. They are extremely abundant both as to individuals and genera. They occur on the shore within tide See also:mark, and range thence down into the abysses. Moreover, they appear to have possessed these qualifications from See also:early geologicalcountries. 2. In applying fossils to the determination of geological chronology it is first necessary to ascertain the See also:order of superposition of the rocks. 'Obviously, in a continuous series of undisturbed sedimentary deposits the lowest must necessarily be the See also:oldest, and the plants or animals which they contain must have lived and died before any of the organisms that occur in the overlying strata. This order of superposition having been settled in a series of formations, it is found that the fossils at the bottom are not quite the same as those at the See also:top of the series. Tracing the beds upward, we discover that See also:species after species of the lowest platforms disappears, until perhaps not one of them is found. With the cessation of these older species others make their entrance. These, in turn, are found to die out, and to be replaced by newer forms. After patient examination of the rocks, it has been ascertained that every well-marked " formation," or See also:group of strata, is characterized by its own species or genera, or by a See also:general assemblage, or ,fdcies, of organic forms. Such a generalization can only, of course, be determined by actual See also:practical experience over an area of some size. When the typical fossils of a formation are known, they serve to identify thatformation in its progress across a country. Thus, in tracts where the true order of superposition cannot be determined, owing to the want of sections or to the disturbed condition of the rocks, fossils serve as a means of See also:identification and furnish a guide to the See also:succession of the rocks. They even demonstrate that in some mountainous ground the beds have been turned completely upside down, where it can be shown that the fossils in what are now the uppermost strata ought properly to See also:lie underneath those in the beds below them. It is by their characteristic fossils that the stratified rocks of the earth's crust can be most satisfactorily subdivided into convenient See also:groups of strata and classed in See also:chronological order. Each " formation is distinguished by its own See also:peculiar assemblage of organic remains, by means of which it can be followed and recognized, even amid the crumplings and dislocations of a disturbed region. The same general succession of organic types can be observed over a large part of the See also:world, though, of course, with important modifications in different countries. This similarity of succession has been termed homotaxis, a term which expresses the fact that the order in which the leading types of organized existence have appeared upon the earth has been similar even in widely separated regions. It is evident that, in this way, a reliable method of comparison is furnished, whereby the stratified formations of different parts of the earth's crust can be brought into relation with each other. Had the geologist continued to remain, as in the days of See also:Werner, hampered by the limitations imposed by a reliance on mere litho-logical characters, he would have made little or no progress in deciphering the record of the successive phases of the history of the globe chronicled in the crust. Just as, at the present time, sheets of gravel in one place are contemporaneous with sheets of mud at another, so in the past all kinds of sedimentation have been in progress simultaneously, and those of one period may not be distinguishable in themselves from those of another. Little or no reliance can be placed upon lithological resemblances or See also:differences in comparing the sedimentary formations of different countries. In making use of fossil See also:evidence for the purpose of subdividing the stratified rocks of the earth's crust, it is found to be applicable to- the smaller details of stratigraphy as well as to the See also:definition of large groups of strata. Thus a particular stratum may be marked by the occurrence in it of various fossils, one or more of which may he distinctive, either from occurring in no other See also:bed above and below or from See also:special abundance in that stratum. One or more of these species is therefore used as a guide to the occurrence of the bed GEOLOGY 667 temporaneous disturbance or denudation, but succeed each other as if they had been accumulated by one continuous See also:process of deposit. It must be admitted that the problem of life-zones in stratigraphical geology has not yet been solved. As See also:Darwin first cogently showed, the history of life has been very imperfectly registered in the stratified parts of the earth's crust. Apart from the fact that, even under the most favourable conditions, only a small proportion of the total flora and fauna of any period would be preserved in the fossil See also:state, enormous gaps occur' where no record has survived at all. It is as if whole chapters and books were missing from a See also:historical See also:work. Some of these lacunae are sufficiently obvious. Thus, in some cases, powerful dislocations have thrown considerable portions of the rocks out of sight. Sometimes extensive See also:metamorphism has so affected them that their See also:original characters, including their organic contents, have been destroyed. Oftenest of all, denudation has come into See also:play, and vast masses of fossiliferous rock have been entirely worn away, as is demonstrated by the abundant unconformabilities in the structure of the earth's crust. While the mere fact that one series of rocks lies unconformably on another proves the See also:lapse of a considerable See also:interval between their respective See also:dates, the relative length of this interval may sometimes be proved by means of fossil evidence, and by this alone. Let us suppose, for example, that a certain group of formations has been disturbed, upraised, denuded and covered unconformably by a second group. In lithological characters the two may closely resemble each other, and there may be nothing to show that the See also:gap represented by their unconformability is of an important See also:character. In many cases, indeed, it would be quite impossible to pronounce any well-grounded See also:judgment as to the amount of interval, even measured by the vague relative See also:standards of geological chronology. But if each group contains a well-preserved See also:suite of organic remains, it may not only be possible, but easy, to say exactly how much of the geological record has been See also:left out between the two sets of formations. By comparing the fossils with those obtained from regions where the geological record is more See also:complete, it may be ascertained, perhaps, that the See also:lower rocks belong to a certain See also:platform or See also:stage in geological history which for our present purpose we may See also:call D, and that the upper rocks can in like manner be paralleled with stage H. It would be then apparent that at this locality the See also:chronicles of three great geological periods E, F, and G were wanting, which are elsewhere found to be intercalated between D and H. The lapse of time represented by this unconformability would thus be See also:equivalent to that required for the accumulation of the three missing formations in those regions where sedimentation was more continuous. Fossil evidence may be made to prove the existence of gaps which are not otherwise apparent. As has been already remarked, changes in organic forms must, on the whole, have been extremely slow in the geological past. The whole species of a sea-floor could not pass entirely away, and be replaced by other forms, without the lapse of long periods of time. If then among the conformable stratified formations of former ages we encounter sudden and abrupt changes in the facies of the fossils, we may be certain that these must mark omissions in the record, which we may See also:hope to fill in from a more perfect series elsewhere. The complete biological contrasts between the fossil contents of unconformable strata are sufficiently explicable. It is not so easy to give a satisfactory See also:account of those which occur where the beds are strictly conformable, and where no evidence can be observed of any considerable change of physical conditions at the time of deposit. A group of strata having the same general litho-logical characters throughout may be marked by a great discrepance between the fossils above and below a certain See also:line. A few species may pass from the one into the other, or perhaps every species may be different. In cases of this kind, when proved to be not merely See also:local but persistent over wide areas, we must admit, notwithstanding the apparently undisturbed and continuous character of the original deposition of the strata, that the abrupt transition from the one facies of fossils to the other represents a long interval of time which has not been recorded by the deposit of strata. A. C. See also:Ramsay, who called See also:attention to these gaps, termed them " breaks in the succession of organic remains." He showed that they occur abundantly among the Palaeozoic and Secondary rocks of See also:England. It is obvious, of course, ;that such breaks, even though traceable over wide regions, were not general over the whole globe. There have never been.any universal interruptions in the continuity of the See also:chain of being, so far as geological evidence can show. But the physical changes which caused the breaks may have been general over a zoological See also:district or See also:minor region. They no doubt often caused the complete extinction of genera and species which had a small See also:geographical range. From all these facts it is clear that the geological record, as it now exists, is at the best but an imperfect See also:chronicle of geological history, In no country is it complete. The lacunae of one region must be supplied from another. Yet in proportion to the geographical distance between the localities where the gaps occur and those whence the missing intervals are supplied, the See also:element of uncertainty in our See also:reading of the record is increased. The most (desirable method of research is to exhaust the evidence for each area or See also:province, and to compare the general order of its succession as a whole with that which can be established for other provinces.
in question, which is called by the name of the most abundant species. In this way what is called a " geological See also:horizon," or "See also:zone," is marked off, and its exact position in the seriesof formations is fixed.
Perhaps the most distinctive feature in the progress of palaeontological geology during the last See also:half See also:century has been the recognition and wide application of this method of zonal stratigraphy, which, in itself, was only a further development of See also: Combining what had been done by various observers with his own laborious researches in France, England, See also:Wurttemberg and See also:Bavaria, he See also:drew up a See also:classification of the Jurassic See also:system, grouping its several formations into zones, each characterized by some distinctly predominant fossil after which it was named (see See also:LIAS). The same method of classification was afterwards extended to the Cretaceous series by A. D. d'Orbigny, E. See also:Hebert and others, until the whole Mesozoic rocks from the Trias to the top of the See also:Chalk has now been partitioned into zones, each named after some characteristic species or genus of fossils. More recently the principle has been extended to the Palaeozoic formations, though as yet less fully than to the younger parts of the geological record. It has been successfully applied by See also:Professor C. See also:Lapworth to the investigation of the See also:Silurian series (see SILURIAN; ORDOVICIAN SYSTEM). He found that the species of See also:graptolites have each a comparatively narrow See also:vertical range, .and they may consequently be used for stratigraphical purposes. Applying the method, in the first instance, to the highly plicated Silurian rocks of the See also:south of See also:Scotland, he found that by means of graptolites he was able to work out the structure of the ground. Each great group of strata was seen to possess its own graptolitic zones, and by their means could be identified not only in the original complex Scottish area, but in England and See also:Wales and in See also:Ireland. It was eventually ascertained that the succession of zones in Great See also:Britain could be recognized on the See also:Continent, in See also:North See also:America and even in See also:Australia; The brachiopods and See also:trilobites have likewise been made use of for zonal purposes among the oldest sedimentary formations. The most ancient of the Palaeozoic systems has as its fitting See also:base the Olenellus zone. Within undefined and no doubt variable geographical limits palaeontological zones have been found to be remarkably persistent. They follow each other in the same general order, but not always with equal definiteness. The type fossil may appear in some districts on a higher or a lower platform than it does in others. Only to a limited degree is there any coincidence between lithological variations in the strata and the sequence of the zones. In the Jurassic formations, indeed, where frequent alternations of different sedimentary materials are to be met with, it is in some cases possible to trace a definite upward or downward limit for a zone by some abrupt change in the sedimentation, such as from limestone to shale. But such a precise demarcation is impossible where no distinct bands of different sediments are to be seen. The zones can then only be vaguely determined by finding their characteristic fossils, and noting where these begin to appear in the strata and where they cease. It would seem, therefore, that the sequence of palaeontological zones, or life-horizons, has not depended merely upon changes in the nature of the conditions under which the organisms lived. We should naturally expect that these changes would have had a marked See also:influence; that, for instance, a difference should be perceptible between the character of the fossils in a limestone and that of those in a shale or a sandstone. The environment, when a limestone was in course of deposition, would generally be one of clear water, favourable for a more vigorous and more varied fauna than where a shale series was accumulating, when the water would be discoloured, and only such animals would continue to live in it, or on the bottom, as could maintain themselves in the midst of mud. But no such lithological See also:reason, betokening geographical changes that would affect living creatures, can be adduced as a universally applicable explanation of the occurrence and See also:limitation of palaeontological zones. One of these zones may be only a few inches, or feet or yards in vertical extent, and no obvious lithological or other cause can be seen why its specially characteristic fossils should not be found just as frequently in the similar strata above and below. There is often little or no evidence of any serious change in the conditions of sedimentation, still less of any widespread physical disturbance, such as the catastrophes by which the older, geologists explained the extinction of successive types of life. It has been suggested that, where the life-zones are well defined, sedimentation has been extremely slow, and that though these zones follow each other with no break in the sedimentation, they were really separated by prolonged intervals of time during which organic See also:evolution could come effectively into play. Additional information and CommentsThere are no comments yet for this article.
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