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| author | Roger Frank <rfrank@pglaf.org> | 2025-10-15 05:22:20 -0700 |
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| committer | Roger Frank <rfrank@pglaf.org> | 2025-10-15 05:22:20 -0700 |
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diff --git a/old/3772-h/files/ch9.html b/old/3772-h/files/ch9.html new file mode 100644 index 0000000..33f4f18 --- /dev/null +++ b/old/3772-h/files/ch9.html @@ -0,0 +1,417 @@ +<!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 137">[ 137 ]</a></p> + +<p> </p> + +<center><b>Chapter IX</b><br> +<br> +CLASSIFICATION OF TERTIARY FORMATIONS.</center> + +<p class="intro">Order of Succession of Sedimentary Formations. +— Frequent Unconformability of Strata. — Imperfection +of the Record. — Defectiveness of the Monuments greater in +Proportion to their Antiquity. — Reasons for studying the +newer Groups first. — Nomenclature of Formations. — +Detached Tertiary Formations scattered over Europe. — Value +of the Shell-bearing Mollusca in Classification. — +Classification of Tertiary Strata. — Eocene, Miocene, and +Pliocene Terms explained.</p> + +<p>By reference to the tables given at the end of the last chapter +the reader will see that when the fossiliferous rocks are arranged +chronologically, we have first to consider the Post-tertiary and +then the Tertiary or Cainozoic formations, and afterwards to pass +on to those of older date.</p> + +<center><img src="../images/fig86.jpg" width="372" height="139" alt= +"Fig. 86: Order of Superposition of Deposits"></center> + +<p><b>Order of +Superposition.</b>—The diagram (Fig. 86) will show the +order of superposition of these deposits, assuming them all to be +visible in one continuous section. In nature, as before hinted +<a href="ch6.html#page 107">(p. 107)</a>, we have never an opportunity of seeing the +whole of them so displayed in a single region; first, because +sedimentary deposition is confined, during any one geological +period, to limited areas; and secondly, because strata, after they +have been formed, are liable to be utterly annihilated over wide +areas by denudation. But wherever certain members of the series are +present, they overlie one another in the order indicated in the +diagram, though not always in the exact manner there represented, +because some of them repose occasionally in unconformable +stratification on others. This mode of superposition has been +already explained <a href="ch5.html#page 94">(p. 94,</a> <a href="ch7.html#page 111">p. 111</a>), where I pointed out that the discordance which implies a considerable lapse +of time between two</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 138">[ 138 ]</a></p> + +<p>formations in juxtaposition is almost invariably accompanied by +a great dissimilarity in the species of organic remains.</p> + +<p><b>Frequent Unconformability of +Strata.</b>—Where the widest gaps appear in the +sequence of the fossil forms, as between the Permian and Triassic +rocks, or between the Cretaceous and Eocene, examples of such +unconformability are very frequent. But they are also met with in +some part or other of the world at the junction of almost all the +other principal formations, and sometimes the subordinate divisions +of any one of the leading groups may be found lying unconformably +on another subordinate member of the same—the Upper, for +example, on the Lower Silurian, or the superior division of the Old +Red Sandstone on a lower member of the same, and so forth. +Instances of such irregularities in the mode of succession of the +strata are the more intelligible the more we extend our survey of +the fossiliferous formations, for we are continually bringing to +light deposits of intermediate date, which have to be intercalated +between those previously known, and which reveal to us a long +series of events, of which antecedently to such discoveries we had +no knowledge.</p> + +<p>But while unconformability invariably bears testimony to a lapse +of unrepresented time, the conformability of two sets of strata in +contact by no means implies that the newer formation immediately +succeeded the older one. It simply implies that the ancient rocks +were subjected to no movements of such a nature as to tilt, bend, +or break them before the more modern formation was superimposed. It +does not show that the earth’s crust was motionless in the region +in question, for there may have been a gradual sinking or rising, +extending uniformly over a large surface, and yet, during such +movement, the stratified rocks may have retained their original +horizontality of position. There may have been a conversion of a +wide area from sea into land and from land into sea, and during +these changes of level some strata may have been slowly removed by +aqueous action, and after this new strata may be superimposed, +differing perhaps in date by thousands of years or centuries, and +yet resting conformably on the older set. There may even be a +blending of the materials constituting the older deposit with those +of the newer, so as to give rise to a passage in the mineral +character of the one rock into the other as if there had been no +break or interruption in the depositing process.</p> + +<p><b>Imperfection of the +Record.</b>—Although by the frequent discovery of new +sets of intermediate strata the transition from one type of organic +remains to another is becoming less and</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 139">[ 139 ]</a></p> + +<p>less abrupt, yet the entire series of records appears to the +geologists now living far more fragmentary and defective than it +seemed to their predecessors half a century ago. The earlier +inquirers, as often as they encountered a break in the regular +sequence of formations, connected it theoretically with a sudden +and violent catastrophe, which had put an end to the regular course +of events that had been going on uninterruptedly for ages, +annihilating at the same time all or nearly all the organic beings +which had previously flourished, after which, order being +re-established, a new series of events was initiated. In proportion +as our faith in these views grows weaker, and the phenomena of the +organic or inorganic world presented to us by geology seem +explicable on the hypothesis of gradual and insensible changes, +varied only by occasional convulsions, on a scale comparable to +that witnessed in historical times; and in proportion as it is +thought possible that former fluctuations in the organic world may +be due to the indefinite modifiability of species without the +necessity of assuming new and independent acts of creation, the +number and magnitude of the gaps which still remain, or the extreme +imperfection of the record, become more and more striking, and what +we possess of the ancient annals of the earth’s history appears as +nothing when contrasted with that which has been lost.</p> + +<p>When we examine a large area such as Europe, the average as well +as the extreme height above the sea attained by the older +formations is usually found to exceed that reached by the more +modern ones, the primary or palaeozoic rising higher than the +secondary, and these in their turn than the tertiary; while in +reference to the three divisions of the tertiary, the lowest or +Eocene group attains a higher summit-level than the Miocene, and +these again a greater height than the Pliocene formations. Lastly, +the post-tertiary deposits, such, at least, as are of marine +origin, are most commonly restricted to much more moderate +elevations above the sea-level than the tertiary strata.</p> + +<p>It is also observed that strata, in proportion as they are of +newer date, bear the nearest resemblance in mineral character to +those which are now in the progress of formation in seas or lakes, +the newest of all consisting principally of soft mud or loose sand, +in some places full of shells, corals, or other organic bodies, +animal or vegetable, in others wholly devoid of such remains. The +farther we recede from the present time, and the higher the +antiquity of the formations which we examine, the greater are the +changes which the sedimentary deposits have undergone. Time, as I +have</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 140">[ 140 ]</a></p> + +<p>explained in Chapters V, VI, and VII, has multiplied the effects +of condensation by pressure and cementation, and the modification +produced by heat, fracture, contortion, upheaval, and denudation. +The organic remains also have sometimes been obliterated entirely, +or the mineral matter of which they were composed has been removed +and replaced by other substances.</p> + +<p><b>Why newer Groups should be studied +first.</b>—We likewise observe that the older the +rocks the more widely do their organic remains depart from the +types of the living creation. First, we find in the newer tertiary +rocks a few species which no longer exist, mixed with many living +ones, and then, as we go farther back, many genera and families at +present unknown make their appearance, until we come to strata in +which the fossil relics of existing species are nowhere to be +detected, except a few of the lowest forms of invertebrate, while +some orders of animals and plants wholly unrepresented in the +living world begin to be conspicuous.</p> + +<p>When we study, therefore, the geological records of the earth +and its inhabitants, we find, as in human history, the +defectiveness and obscurity of the monuments always increasing the +remoter the era to which we refer, and the difficulty of +determining the true chronological relations of rocks is more and +more enhanced, especially when we are comparing those which were +formed simultaneously in very distant regions of the globe. Hence +we advance with securer steps when we begin with the study of the +geological records of later times, proceeding from the newer to the +older, or from the more to the less known.</p> + +<p>In thus inverting what might at first seem to be the more +natural order of historical research, we must bear in mind that +each of the periods above enumerated, even the shortest, such as +the Post-tertiary, or the Pliocene, Miocene, or Eocene, embrace a +succession of events of vast extent, so that to give a satisfactory +account of what we already know of any one of them would require +many volumes. When, therefore, we approach one of the newer groups +before endeavouring to decipher the monuments of an older one, it +is like endeavouring to master the history of our own country and +that of some contemporary nations, before we enter upon Roman +History, or like investigating the annals of Ancient Italy and +Greece before we approach those of Egypt and Assyria.</p> + +<p><b>Nomenclature.</b>—The origin +of the terms Primary and Secondary, and the synonymous terms +Palaeozoic, and Mesozoic, were explained in Chapter VIII, p. +123.</p> + +<p>The Tertiary or Cainozoic strata (see <a href="ch8.html#page 123">p. 123</a>) were +so called</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 141">[ 141 ]</a></p> + +<p>because they were all posterior in date to the Secondary series, +of which last the Chalk of Cretaceous, No. 9, Fig. 86, constitutes +the newest group. The whole of them were at first confounded with +the superficial alluviums of Europe; and it was long before their +real extent and thickness, and the various ages to which they +belong, were fully recognised. They were observed to occur in +patches, some of fresh-water, others of marine origin, their +geographical area being usually small as compared to the secondary +formations, and their position often suggesting the idea of their +having been deposited in different bays, lakes, estuaries, or +inland seas, after a large portion of the space now occupied by +Europe had already been converted into dry land.</p> + +<p>The first deposits of this class, of which the characters were +accurately determined, were those occurring in the neighbourhood of +Paris, described in 1810 by MM. Cuvier and Brongniart. They were +ascertained to consist of successive sets of strata, some of +marine, others of fresh-water origin, lying one upon the other. The +fossil shells and corals were perceived to be almost all of unknown +species, and to have in general a near affinity to those now +inhabiting warmer seas. The bones and skeletons of land animals, +some of them of large size, and belonging to more than forty +distinct species, were examined by Cuvier, and declared by him not +to agree specifically, nor most of them even generically, with any +hitherto observed in the living creation.</p> + +<p>Strata were soon afterwards brought to light in the vicinity of +London, and in Hampshire, which, although dissimilar in mineral +composition, were justly inferred by Mr. T. Webster to be of the +same age as those of Paris, because the greater number of the +fossil shells were specifically identical. For the same reason, +rocks found on the Gironde, in the South of France, and at certain +points in the North of Italy, were suspected to be of +contemporaneous origin.</p> + +<p>Another important discovery was soon afterwards made by Brocchi +in Italy, who investigated the argillaceous and sandy deposits, +replete with shells, which form a low range of hills, flanking the +Apennines on both sides, from the plains of the Po to Calabria. +These lower hills were called by him the Subapennines, and were +formed of strata chiefly marine, and newer than those of Paris and +London.</p> + +<p>Another tertiary group occurring in the neighbourhood of +Bordeaux and Dax, in the South of France, was examined by M. de +Basterot in 1825, who described and figured several hundred species +of shells, which differed for the most part both from the Parisian +series and those of the Subapennine hills.</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 142">[ 142 ]</a></p> + +<p>It was soon, therefore, suspected that this fauna might belong +to a period intermediate between that of the Parisian and +Subapennine strata, and it was not long before the evidence of +superposition was brought to bear in support of this opinion; for +other strata, contemporaneous with those of Bordeaux, were observed +in one district (the Valley of the Loire), to overlie the Parisian +formation, and in another (in Piedmont) to underlie the Subapennine +beds. The first example of these was pointed out in 1829 by M. +Desnoyers, who ascertained that the sand and marl of marine origin +called faluns, near Tours, in the basin of the Loire, full of +sea-shells and corals, rested upon a lacustrine formation, which +constitutes the uppermost subdivision of the Parisian group, +extending continuously throughout a great table-land intervening +between the basin of the Seine and that of the Loire. The other +example occurs in Italy, where strata containing many fossils +similar to those of Bordeaux were observed by Bonelli and others in +the environs of Turin, subjacent to strata belonging to the +Subapennine group of Brocchi.</p> + +<p><b>Value of Testacean Fossils in +Classification.</b>—It will be observed that in the +foregoing allusions to organic remains, the testacea or the +shell-bearing mollusca are selected as the most useful and +convenient class for the purposes of general classification. In the +first place, they are more universally distributed through strata +of every age than any other organic bodies. Those families of +fossils which are of rare and casual occurrence are absolutely of +no avail in establishing a chronological arrangement. If we have +plants alone in one group of strata and the bones of mammalia in +another, we can draw no conclusion respecting the affinity or +discordance of the organic beings of the two epochs compared; and +the same may be said if we have plants and vertebrated animals in +one series and only shells in another. Although corals are more +abundant, in a fossil state, than plants, reptiles, or fish, they +are still rare when contrasted with shells, because they are more +dependent for their well-being on the constant clearness of the +water, and are, therefore, less likely to be included in rocks +which endure in consequence of their thickness and the copiousness +of sediment which prevailed when they originated. The utility of +the testacea is, moreover, enhanced by the circumstance that some +forms are proper to the sea, others to the land, and others to +fresh water. Rivers scarcely ever fail to carry down into their +deltas some land-shells, together with species which are at once +fluviatile and lacustrine. By this means we learn what terrestrial, +fresh-water, and marine</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 143">[ 143 ]</a></p> + +<p>species coexisted at particular eras of the past: and having +thus identified strata formed in seas with others which originated +contemporaneously in inland lakes, we are then enabled to advance a +step farther, and show that certain quadrupeds or aquatic plants, +found fossil in lacustrine formations, inhabited the globe at the +same period when certain fish, reptiles, and zoophytes lived in the +ocean.</p> + +<p>Among other characters of the molluscous animals, which render +them extremely valuable in settling chronological questions in +geology, may be mentioned, first, the wide geographical range of +many species; and, secondly, what is probably a consequence of the +former, the great duration of species in this class, for they +appear to have surpassed in longevity the greater number of the +mammalia and fish. Had each species inhabited a very limited space, +it could never, when imbedded in strata, have enabled the geologist +to identify deposits at distant points; or had they each lasted but +for a brief period, they could have thrown no light on the +connection of rocks placed far from each other in the +chronological, or, as it is often termed, vertical series.</p> + +<p><b>Classification of Tertiary +Strata.</b>—Many authors have divided the European +Tertiary strata into three groups—lower, middle, and upper; +the lower comprising the oldest formations of Paris and London +before mentioned; the middle those of Bordeaux and Touraine; and +the upper all those newer than the middle group.</p> + +<p>In the first edition of the Principles of Geology, I divided the +whole of the Tertiary formations into four groups, characterised by +the percentage of recent shells which they contained. The lower +tertiary strata of London and Paris were thought by M. Deshayes to +contain only 3½ per cent of recent species, and were termed +Eocene. The middle tertiary of the Loire and Gironde had, according +to the specific determinations of the same conchologist, 17 per +cent, and formed the Miocene division. The Subapennine beds +contained 35 to 50 per cent, and were termed Older Pliocene, while +still more recent beds in Sicily, which had from 90 to 95 per cent +of species identical with those now living, were called Newer +Pliocene. The first of the above terms, Eocene, is derived from +eos, <i>dawn</i>, and cainos, <i>recent</i>, because the fossil +shells of this period contain an extremely small proportion of +living species, which may be looked upon as indicating the dawn of +the existing state of the testaceous fauna, no recent species +having been detected in the older or secondary rocks.</p> + +<p>The term Miocene (from meion, <i>less</i>, and cainos,</p> + +<p> </p> + +<hr> +<p class="page"><a name="page 144">[ 144 ]</a></p> + +<p><i>recent</i>) is intended to express a minor proportion of +recent species (of testacea), the term Pliocene (from pleion, <i> +more</i>, and cainos, <i>recent</i>) a comparative plurality of the +same. It may assist the memory of students to remind them, that the +<i>Mi</i>ocene contain a <i>mi</i>nor proportion, and <i> +Pl</i>iocene a comparative <i>pl</i>urality of recent species; and +that the greater number of recent species always implies the more +modern origin of the strata.</p> + +<p>It has sometimes been objected to this nomenclature that certain +species of infusoria found in the chalk are still existing, and, on +the other hand, the Miocene and Older Pliocene deposits often +contain the remains of mammalia, reptiles, and fish, exclusively of +extinct species. But the reader must bear in mind that the terms +Eocene, Miocene, and Pliocene were originally invented with +reference purely to conchological data, and in that sense have +always been and are still used by me.</p> + +<p>Since the year 1830 the number of known shells, both recent and +fossil, has largely increased, and their identification has been +more accurately determined. Hence some modifications have been +required in the classifications founded on less perfect materials. +The Eocene, Miocene, and Pliocene periods have been made to +comprehend certain sets of strata of which the fossils do not +always conform strictly in the proportion of recent to extinct +species with the definitions first given by me, or which are +implied in the etymology of those terms.</p> + +<br> +<hr> +<small><a href="contents.html">Contents</a> / <a href="ch8.html"> +Chapter VIII</a> / <a href="ch10.html">Chapter X</a></small> +</body> +</html> + |