NA TURE 



[juLv 18, 1895 



Slime references lo the character of the water in which each 

 turmation was dejx)sited. Stiulies of the seilimenlary formations, 

 esix^ially those made from a biological standpoint, have demon- 

 strated that the bodies of water in which they were dejwsited 

 were of the various kinds that are now known ; that is, some 

 were marine, some fresh, and some brackish. 



L'lxin physical evidence alone, it is not practicable to satis- 

 factorily classify' the setlimentary formations of the earth in such 

 a manner as to serve the pur]X)se of thorough geological investi- 

 gation. Therefore such data are in this, as in most other cases, 

 chiefly valuable as being accessory to the evidence afforded by 

 biolc^cal data. 



The biolt^cal criteria which are relied u|X)n by geologists to 

 distinguish from one another the sedinientar)' formations which 

 have been pnxiuced in marine waters, or in those of inland seas, 

 lakes, rivers, or estuaries, relate to the characteristics of faunas 

 which now inhabit those waters res|iectively. and to the differ- 

 ences from one another of such faunas. That is, the conclusions 

 w hich geologists reach concerning the questions just indicated 

 are Ijasetl ui>on now-existing physical conditions, ujion the known 

 character, structure, and habits of animals with relation to tho.se 

 conilitions, and upon the assumption that in |>ast geological 

 ei»chs animals of a given character and stnicture had similar 

 habits, and lived under conditions similar to those which are 

 congenial to their living congeners. 



The various Ixxlies of water which existed during geological 

 time, and which constituted the habitat of aquatic animals, were 

 of the same kinds that now exist, namely, marine and fresh, 

 together with those of the various intervening grades of saltness. 

 .\lthough it is |irof«able that the marine w.iters of early geological 

 time were not so salt as those of the present oceans, it is believed 

 that this difference in saltness has not lx;en so great as to make 

 any apprc-ciable difference as to legitimate conclusions of the 

 kind that have l>een indicated. It seems to Ik; es|x;cially evident 

 that this difference has Ix-en thus inappreciable since the close 

 of pal.vozoic time, since which time the greater jMirt of the 

 known unmistakably non-marine formations were de|x>sited. 



If all the known now living memlxjrs of a given family 

 are confined to niarine, or to fresh waters, as the case may be, it 

 is assumed that the habitat of the extinct niembers of such 

 families were similarly restricted, and that the presence of fossil 

 remains of such animals in a given formation, is, in the absence 

 of conflicting facts, sufficient evidence of its marine origin on the 

 one hand, or of its fresh-water origin on the other. Again, if a 

 given family is known to have (representatives now living in 

 marine, brackish, and fresh waters, respectively, it is assumed 

 that it had a similar range of habitat during past geological 

 e|x>chs. Therefore, the discovery in a given fi>rmation of fossil 

 remains of a single representative of a family having such a 

 varied range of habitat is not of itself sufficient to enable one to 

 decide whether it wxs of marine, brackish, or fresh-water origin, 

 and other evidence must Ik; s<jught. 



The criteria of past aqueous* conditions here discussed 

 arc, of course, only such as may lie derived from sedi- 

 mentary formations and their contents. It cannot Ik; said thiit 

 there are any fully trustworthy physical criteria because a non- 

 marine fonnation rarely presents any condition of stratification, 

 «ir any lithological character, which is not oliservable in .some 

 marine formations. Still, there arc many more or less valuable 

 indications which may lie observed and lo some degree relied 

 o|xin in the absence of fossil remains. 



I'cir example, although consideral)le accumulations of 

 calcareous strata arc sometimes found among the generally 

 arenaceous strata of fresh-water formations, they have never been 

 found lo contain any im|xirtant accunudalions of regularly bedded 

 limestones. I-'urthermore, esluarine dejxisits are often still more 

 of a <letrital character than are fresh-water formations, an<l .'ilso 

 nore rarely conL-iin calcareous layers. Therefore, if one 

 ■nrcdinter a series fif regularly bedded limestones, either 

 lan f<r fully calcareous, he will rarely, if ever, Ix: at fault 

 'r<ling them as of marine origin even without biological 

 I' e. 

 Ill a large pro|x>rlion of the non-marine formations, the si ratifica- 

 tion is less regular ihan is usually Ihe case with marine forma- 

 tion-. .Siill, this is by nf» means a certain criterion, and in some 

 ras/'s non-marine formations are foiHul Ut rest s<» ("(mforniably 

 nji'in Ihe mjirine and to Ix; so conformably overlain by them .i-s lo 

 give lillle indication of Ihe great difference in the condition of 

 Ihcir origin. 



These examples »cr\c lo show how indefinite is the ch.xracler 



of physical evidence as to the p-ist aqueous conditions under 

 which the various sedimentary formations have been [jroduced, 

 but they serve to emphasise a statement of the fact that almost 

 entire reliance must be placed u|X>n the evidence furnished by 

 fossil remains. 



With reference to general indications of difference Ix'tween 

 marine and non-marine formations which are furnished by their 

 fossil remains, we observe that a conspicutnis difference lies in the 

 comparative abundance and variety of forms of life which the 

 fossil faunas of the formations respectively represent. Marine 

 waters have always teemed with life in a wonderful \ariely of 

 forms, and their fossil remains are proix>rtionaIly abundant. The 

 variety is less in brackish waters, and least of all in lacustrine 

 waters. It is true that ichthyic life is abundant in some fre.sh 

 waters, but never so generally abundant or .so various as in marine 

 waters. It is also true that molluscan life is often locally abun- 

 dant in shallow fresh waters, but, as already several times 

 mentioned, the variety is extremely meagre. All these peculiar- 

 ities are distinctly observable imong the fossil faunas of the 

 non-ntarine formations. 



Other general indications of difference between marine and 

 non-marine formations are furnished by remains of land plants 

 and animals. Open-sea formations are nalvirally free from any 

 vegetable remains derived from the land, although coal and other 

 materials of vegetal origin are not unfrequenlly found alternat- 

 ing with layers containing marine fossil remains. These, 

 however, are regarded as cases of emergence of Ihe bottom of 

 shallow sea waters and the subsequent subsidence of the .same as 

 plant-laden marshy land. It is a matter of fact, Ihe rea.son for 

 which has been suggested in jireceding sections, that plant 

 remains of any kind, especially such as are in a classifiable con- 

 dition, have so rarely been finnul as.s<.)ciated with remains of 

 denizens of marine waters, that the discovery of fossil plants in 

 any formation is of itself |)resumptive evidence of its non-marine 

 origin. 



It has already been shown that the remains of land animals 

 have so seldom reached marine waters, or, having reached them, 

 they were probably so generally destroyed by the triturating 

 action of co.asi waves, that the di.scovery of any of this kind of 

 fo.ssil remains in any formation may also be regarded as presump- 

 tive evidence of its non-marine origin. 



The foregoing statements have been made with reference lo 

 indications which are either of a general character t)r without 

 direct relation to the quality of the waters in which .sedimentary 

 formations have been deposited. .\11 the direct evidence, as 

 has been alreatiy fully stated, is derivai)le from the fossil remains 

 of the denizens, especially the gill-bearing kinds, of the waters in 

 which were deix)sited the formations untier investigation. 



Referring to the previous review of the animal kingdom, it 

 will be seen that a large number of families of both fishes and 

 invertebrates are confined to a marine habitat, and that every 

 mcnilx;r of even .some of the higher divisions is similarly 

 restricted. For example, every known member of the classes 

 Cephalopoda and Hrachiopoda is confined to a marine habitat. 

 It will also be seen that a certain small number of families, 

 especially of ihe mollusca, are equally reslricled to fresh waters. 

 The significance of such cases as these has already been |K)inled 

 out, but it is desirable to refer lo them ag.iin. 



I''ossil remains representing any one of these kind t)f animals 

 may be taken .as ]xwitive evidence of the quality of the water in 

 which Ihe fornialion containing them w.as deposited, provided 

 there shall be no room for reasonable doubl that Ihe animals 

 were really denizens of that waler. That is, caution is necessity 

 even in these more |«)silive cases, es|)ecially when the amount of 

 discovered fossil material is meagre. 



Not only caution but the exercise of careful judgment is 

 neccssjiry in other Civses. I'or example, it will also be seen by 

 referring lo Ihe foregoing review that certain families, while most 

 of ils members are confined to one kind of waler, may have one 

 or more represenlalives in other kinds ; and again, thai certain 

 families may have representatives in all the known kinds of 

 habitable waters. In such ca.ses as these it is plain that all 

 evidence afforded by fossil remains, to be of any value, must be 

 corrolKtraled by other evidence. 



.Still, the cases are very few in which .serious doulil neefl be 

 enlerlained as to the true charaiier of the waler in which a given 

 formaliim was deposile<l. This is especially true if Ihe fossil 

 remains are Mifficient in <juantity and perfection lo approximately 

 represent the whole fauna Ihal livei! in those waters. Indeed, 

 if Ihe facts which are recordeil in this review are borne in 



NO. 1342, VOL. 52] 



