July 1 1, 1895] 



NATURE 



259 



They do not necessarily bear any close relation to one another 

 as to geographical area, thickness, or the duration of time in 

 their accumulation. 



Although they are thus unequal to one another, they constitute 

 the only availalile physical units for local or regional stratigraphic 

 classification. 



Because of their limited geographical extent they cannot be 

 used as units of the universal classification of the stratified rocks. 



The Rki.atio.n oi' r"ossii, Rf.m.-mns to Striitir.\i. 



Gf,oi,o<;v. 

 There are two methods by which the study of fossils may legiti- 

 mately be applied to geological investigation, and the following 

 statement of the character of these is in jjart ex])lanatory of the 

 results that may be obtained by their aid. For convenience, one 

 of them may i)e termed empirical and the other philosophical, 

 because in the one case results are obtained by experience, and 

 in the other by reasoning upon the various results thus obtained. 

 Still, discrimination between these two methods cannot usually 

 be sharply drawn, because, while all geological investigation is 

 largely empirical, it is always more or less philosphical. Such a 

 division of the subject, however, besides being a convenience, 

 gi\es an op|)ortunity to emijhasise the fact that a large 

 ljro|iortion of the work that is done in structural geology is based 

 mainly upon the empirical observation and collection ()f biological 

 data. 



Both these methods are not only important but indispensable, 

 the cine not less so than the other. Both may be, and aften are, 

 used together, but the em])irical method is more largely used in 

 jiractical field studies than in others, because in such studies 

 fossils are to a large extent treated as characteristic tokens of 

 formations, or as arbitrary means of identif}ing them and distin- 

 guishing them from one another. Such identificatinn necessarily 

 constitutes one of the first stei>s in the practical study of structural 

 geolog}", but the suiisequent study of the fossils thus empirically 

 used is necessarily more jihilosophical. 



The ])hiIosophical method of treating fossil remains, however, is 

 largely applicable to systematic geology or those branches which per- 

 tain to the universal chronological classification of the sedimentary 

 fi'miations and to their correlation in different parts of the world. 

 The naturalist studies fo.ssil remains as representatives of the 

 l<"'ng succession of jjrogressively and differentially developed 

 organic forms which, during geological time, have existed and 

 become extinct, and of which succession the now existing forms 

 I 'f life constitute only the terminal portion. It is the results of 

 >uch studies as these that the geologist uses in the philosophical 

 studies referred to. 



Of the two ways in which formations are naturally characleris- 

 ;il)le, one is physical and the other biological. Physical charac- 

 lerisation may be direct or general, that is, it may be by identity 

 of kind or kinds of rock of which the formation is composed, or 

 by its |X)ssession of that more general or indefinite jjroperty or 

 condition which indicates honmgeny. 



Formations are biologically characterised only by the fossil 

 remains of animals and plants which lived while they were in 

 process of deposition, and the more intimate the n.atural relation 

 of any of those animals and plants to the physical conditions 

 which produced a formation, the more characteristic of it are 

 their remains. This inqjlies that, while no kind of fossil remains 

 is to be rejected in practical studies of structural geolog)', there 

 is much difference in the value i>f the different kinds for this 

 pur|X)se. These differences in value will be S|x;cially discussed 

 later on. 



Much has been written on methods of <listinguishing 

 Ijctween formations of marine and non-marine origin, and the 

 legitimate inferences that may be drawn from them, respectively, 

 as to the physical conditions which prevailed while they were 

 accumulating. It is desirable here to present some remarks upon 

 the relative value in practical geological field work of the fossils 

 found in marine and non-marine formations, respectively. 



That the fossil remains of marine faunas arc far more valuable 

 as indicators of the chronological divisions of the geological scale 

 and of the correlation of its divisions in different parts of the 

 world than are those of non-niarine faimas, is apparent to every 

 one who is familiar with even the general facts of biological 

 geolog)-, but it does not follow, and it is not true, that the latter 

 are intrinsically less valuable than are the former in field studies 

 of practical geology. For this practical work, both marine and 

 non-marine fossils are treated by the empirical method already 

 explaitied, and both are found to characterise the respective 

 formations in the same maimer. 



NO. 1 34 1, VOL. 52] 



Certain conditions, however, give each an advantage over the 

 other under different circumstances. For example, the geo- 

 graphical range of the non-marine invertebrate fossil faunas, 

 especially those of fresh water, having been sharply defined by 

 shore lines, the species which constituted them are to that extent 

 more characteristic of the formations in which they occur than 

 is the case with marine faunas. Certain species of the latter 

 faunas, as already shown, usually ranged beyond the limits of the 

 area which was occupied by each fauna as a whole. 



Non-marine formations, as a rule, occur singly in a series of 

 marine formations, in which case the vertical as well as the 

 geographical range of their invertebrate species is sharply defined. 

 It is tnie that in the interior portion of Xorth America there is 

 a continuous series of fresh-water formations, and that certain of 

 the species range from one into another. These, however, are 

 notable exceptions to the nile referred to, and they at most only 

 make such non-marine faunas equal to the average marine fauna 

 as regards exceptional vertical range of species. Again, non- 

 marine formations usually have the advantage of the presence of 

 remains of plants and of land vertebrates and invertebrates, 

 which in marine formations are usually so extremely rare as to be 

 unavailable. 



On the other hand, marine faunas embrace such a wide diver- 

 sity of forms as compared with the non-marine, and their 

 progressive and differential evolution from e|XKh to eixjch has 

 been so nnich greater, that they offer as faunas much more 

 abundant means for the characterisation and identification of 

 formations. It is clear, however, that the opinion which some 

 geologists have expressed or implied, that the fossil contents of 

 non-marine formations are of little value in practical geological 

 investigation, is not well founded. The following conclusions 

 sum up the case : — 



Formations being the only true units of local or regional 

 stratigraphic classification, their correct identification is the first, 

 and an indispensable, step in the practical field work of structural 

 geolog)-. 



Although formations as such have only a physical existence, 

 their biological characteristics aie always the best, and often the 

 only, means of their identification, and therefore the exhaustive 

 study of fossils is of paramount importance in connection w ith 

 all practical investigations of that kind. 



The value of fossils in this respect is as purely practical as is 

 that of any other aid to geological investigation, and it may be 

 made available without reference to their great value in other 

 respects. 



-\lthough all fossil remains are valuable for this practical use, 

 those of aquatic faunas are more valuable than any others. 



Remains of non-marine faunas are of similar value ftir this 

 purpose to those of marine origin. 



The Relation of Biology to Systf.matic and Historical 

 Geology. 



It has been made apparent in the preceding sections that each 

 case of structural classification of stratified rocks based upon 

 formations as physical units is independent of all others, and 

 that its application is necessarily of limited geographical extent, 

 because formations are themselves thus limited. It therefore 

 follows that the structural geolog)' of any district or region, 

 embracing even an extensive series of formations, may be 

 practically and thoroughly investigated, as regards both scientific 

 accuracy and economic requirements, independently of that of 

 any other district or region, especially of those regions which 

 are not adjacent. It is now to be shown how the nuiltitiule of 

 series of formations thus locally cla-ssified throughout the wiirld 

 have been grouped into a universal system of classification in 

 connection with a scale having its divisions arranged in 

 chronological order. 



When the fossil faunas and floras which characterise each of a 

 given series of sedimentary formations are compared with thirse 

 which .severally characterise the formations of the next preceding 

 and succeeding series, and the whole are systematically com« 

 pared with living faunas and floras, there is to be observed 

 among those fossil forms, when studied through an unbroken 

 vertical range of fonnations, an order of successive changes and 

 modifications indicative of a general advance in biological rank, 

 and also an indication of structural relationship. Furthermore, 

 when the faunas and floras of a given series of formations are 

 compared with those of other series in other i>arts of the world, 

 it frequently appears that there is a close similarity between those 

 of a certain portion of each series which indicates their co'rrela- 

 tion. In such cases an order of biological rank is to be obserNeci 



