108 

 PALAEONTOLOGY L\ ITS RELATION TO GEOLOGY. 



(Stratigraphy.) 



Fossils may be studied in various ways, aud with several 

 ends in view. The simplest use which can be made of them 

 is that of geological indices, or "medals of creation," each 

 characteristic, to a certain extent, of the particular geologic 

 horizon in which it is found. This is the empirical method 

 of study, for it is only by experience that ^^'e learn to recog- 

 nize particular fossils as characterizing particular forma- 

 tions. It is the method most frecjuently emploA'ed, and it is 

 sufficient for the purpose of identifying a stratum over a 

 limited area, or of correlating, in a general way, formations 

 at widely separated localities. Studied in this Avay. fossils 

 become '"finger-marks" by which to recognize the ]»osition 

 of a given formation in the geological scale. 



In order that such identification and correhition ma}' be 

 successfully accomplished, intimate accpiaintance with the 

 fossils on whicli the correlation de})ends is required. It is, 

 furthermore, important that the geologist who proposes to 

 use fossils should have a clear knowledge of the relative 

 stratigraphic value of the S])ecies to be used ; in other words, 

 he must know whicli species are to be depended upon as 

 indicative of a given horizon. Species of animals or plants 

 which are thus characteristic of definite geologic horizons 

 are known as ''Index Fossils,"* and theprecision with which 

 they indicate the geologic horizon is, in general, inversely 

 proportional to the distance between the localities in which 

 the formations are to be correlated. Single strata can 

 be identified only over vei-y limited areas by their index fos- 

 sils, — usually only over the area characterized l)y a uniform 

 lithologic condition of the stratum. Thus, it is only within 

 a radius of perhaps fifteen or twenty miles from Eighteen 

 Mile Creek that the brachiopod Stropheodonta deinissa is 

 characteristic of the upper beds of the Hamilton shales, and 

 that the coral Pleiirodictvum stylopora is characteristic of 

 the lower beds of the Hamilton. But within these limits 



* German '' Leitfossilien.'^ — See further. Chapter III. 



