KELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 277 



It was shown in the preceding essay that although formations as they 

 are there denned are not, and cannot be, the units of a universal classi- 

 fication of the stratified rocks they arc the true units of local or regional 

 classification of those rocks, and their use as such is indispensable 

 in field studies of structural geology. Therefore the accurate identifi- 

 cation of formations is indispensable, and because of this the means of 

 correctly identifying them is of prime importance. It is true, as 

 already stated, that formations have really only a physical existence, 

 but their biological characteristics become in fact a part of their iden- 

 tity, and these characteristics constitute the principal, and in most cases 

 the only criteria of identification. The criteria of identification of for- 

 mations will be specially discussed in Essay VI, but it is necessary to 

 consider them briefly in this connection. 



Of the two ways iu which formations are naturally charaterizable 

 one is physical and the other biological. Physical characterization 

 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 possession of 

 that more general or indefinite property or condition which indicates 

 homogeny. 



The physical or, more specially, thelithological, characteristics of any 

 given formation may be so different from those of an underlying or 

 overlying one that the contrast may be an efficient aid in its identifi- 

 cation, but this is too seldom the case to be generally relied upon, the 

 physical difference between them being usually no greater than that 

 which may occur between different parts of one and the same forma- 

 tion in its geographical extension. Again the physical identification 

 of an unfossiliferous formation may sometimes be satisfactorily deter- 

 mined from its position with relation to overlying or underlying forma- 

 tions whose biological characteristics are known, but such methods 

 are usually too indefinite to meet the requirements of practical field 

 studies. 



It is true that in certain regions where erosion, corrasion, and denu- 

 dation have been especially active, the field geologist may trace forma- 

 tions continuously and completely for many miles by means of their 

 lithological and other physical characteristics and without the aid of 

 fossils, but usually they have become so obscured by the overlapping of 

 one upon another, or by being overlain by glacial or other drift or the 

 debris resulting from their own erosion, that they are exposed to view 

 only at wide intervals, and then incompletely. It is also true, that as 

 a result of a long series of observations at such limited exposures of 

 formations as those just referred to, one may obtain an approximately 

 clear idea of the identity of a formation from the physical evidence 

 which it presents of its homogenesis. Although in late years it has 

 become the custom of some geologists in seeking to identify formations 

 to rely upon these indications to the exclusion of others, a careful con 

 sideration of all available relevant facts will make it plain that the 



