HISTORY OF CORRELATIVE GEOLOGY 1123 



age and occupied a place similar to that assigned to the "Old Red 

 Sandstone" by Werner. Geologists, however, were not long in find- 

 ing out that beds of the same lithic character are not all of the 

 same age, but it has taken them much longer to realize that beds of 

 the same age are not always of the same or even similar lithic char- 

 acter. 



With the detailed study of the New York strata by the five 

 geologists and palaeontologists on the survey (Mather, Emmons, 

 \anuxem, Conrad and Hall), correlation by fossils became recog- 

 nized as the most reliable known method. At first American species 

 were directly identified with European types, and such identification 

 was in many cases not far wrong. Extensive collections of fossils, 

 however, soon showed that the rocks of this country contained an 

 assemblage of organisms largely peculiar to themselves and specifi- 

 cally, if not generically, distinct from that of Europe. Correlation by 

 similarity of faunas was then substituted for correlation by species 

 and so the general correspondence of the strata in the two continents 

 was established. The need of an American standard of comparison 

 was soon felt, and such a need was supplied by the development 

 of the "New York series" of geological formations. The succes- 

 sion of New York strata and the organic remains characterizing 

 them was so thoroughly worked out that "it is and has been for 

 decades a standard of reference for all students of the older rocks 

 throughout the world." (Clarke-ii.) Professor James Hall was 

 one of the first in America to recognize the importance of naming 

 formations from localities in which they were best exposed. In 

 his report to the New York State legislature in 1839 ^^ urges that 

 neither lithic character nor characteristic fossils is a satisfactory 

 source from which to derive the name of the formation, for the 

 first may change while the second is not always ascertainable and 

 may even be absent. He holds that it "becomes a desideratum to 

 distinguish rocks by names which cannot be traduced, and which, 

 w'hen the attendant circumstances are fully understood, will never 

 prove fallacious." Such names can be derived only from localities. 

 It is most fortunate that this principle was recognized before the 

 New York series of formations was fully promulgated in the final 

 reports of the survey. As a result, the majority of formations were 

 named from typical localities, only a relatively small number re- 

 taining the lithic or palreontologic names given them by the earlier 

 investigators. ]\Iore recently these, too, have been replaced by 

 names derived from typical localities, of which the following is a 

 partial list: 



