correlating Geological Formations. 419 



on a geologic map by a definite color, and is defined and dis- 

 cussed in literature under a definite name), and a fossil fauna 

 (which is the aggregate of fossil remains representing once 

 living animals which were associated together at the time of 

 sedimentation of the formation in which they were buried). 

 The geological column is made up of the successive forma- 

 tions, which would be exposed by a vertical section through 

 any part of the earth's crust. In common usage the geologi- 

 cal time-scale is constructed by applying chronologic terms, 

 like age, period, epoch, etc., to the several formations of the 

 geological column. The reference of each formation to its 

 proper place in the scale, is established not by lithologic like- 

 ness but by faunal likeness. So far as common usage goes, 

 the epochs or ages are the time equivalents of the formations 

 and not of the faunas by which the time relations of the 

 formations are established. 



Further, it is a generally accepted belief that each geologi- 

 cal formation occupies a definite place in such a theoretical 

 time-scale. This belief carries with it the further conclusion 

 that, if we determine the limits (below and above) of a given 

 formation in the rocks of a particular locality, the recognition 

 of the same formation in another section (fifty or a hundred 

 miles distant, for instance) carries with it the inference that 

 the second section of the formation represents the same inter- 

 val of geological time. 



These are ideas that constitute the working hypothesis of 

 field geologists everywhere ; and, although they may be applied 

 as general, and not absolutely exact rules, they are used pretty 

 commonly in what is called correlation. For instance, when 

 the statement is made that the Independence Shales of Iowa 

 are, or are not, equivalent to the Hamilton Formation,* — the 

 meaning is that the formation which outcrops at Independence, 

 Iowa, and specifically is named the " Independence shale," was, 

 or was not, formed by deposition at the same time in which 

 the Hamilton formation was forming over the area now called 

 New York State. 



Now the fact basis of that affirmation is not the recognition 

 of the same constitution or thickness of the rocks in the two 

 regions, but it is an opinion based solely upon the evidence of 

 fossils occurring in the rocks. 



The assumption, at the basis of the opinion, is that the 

 fauna present in the rocks of the Hamilton formation repre- 

 sents a period of time which is recorded by the deposits of 

 sediments called Hamilton in New York, and therefore that 

 the same species seen in different regions can be relied on as 

 evidence of the same portion of time. 



* Iowa Geol. Survey, vol. viii, p. 205, etc., 1897. 



