8 HISTORICAL GEOLOGY 



has been thus established, it may be employed as a general 

 standard." 1 



The student should bear in mind that strata cannot be deter- 

 mined as precisely contemporaneous, because geologic time has 

 been very long and the evolution of organisms very slow, and al- 

 most exactly similar fossils may be expected in strata showing an 

 age difference of at least some thousands of years. Also, at any 

 given ancient time of earth history, as now, organisms were not the 

 same in all parts of the world, so that rocks formed at exactly the 

 same time in different parts of the world always show certain 

 differences in fossil content. As compared with the vast length 

 of geologic time, however, practical contemporaneity of the strata 

 can usually be determined. 



An excellent example of well-marked differences in the distri- 

 bution of organisms over a comparatively limited area during one 

 of the ancient epochs of earth history has been worked out by 

 J. M. Clarke. During the Portage epoch of the Devonian 2 period 

 an arm of the sea or gulf extended over much of the southern part 

 of New York state, and "the distinctions in the life provinces over 

 the north shore of this ancient gulf are marked with wonderful 

 clearness. No such striking illustration of distinct faunal associa- 

 tions in an area of so slight extent is elsewhere afforded by the 

 rocks of New York." 3 



For the determination of geologic chronology, certain organisms 

 are more valuable than others, the best being those which have had 

 wide geographic distribution and short geologic range. For 

 example marine organisms, which live near the ocean surface 

 (so-called pelagic forms) and are easily distributed over wide areas, 

 while, at the same time, the species are extant for only a compara- 

 tively short time, are the best chronologic indicators. The Grap- 

 tolites of the early Paleozoic era furnish excellent illustrations. 



2. Past physical geography conditions. Typical stratified rock 

 occupying any region proves the former presence of water over that 

 region. By the study of the fossils we can further usually tell 

 whether the water was ocean or lake, fresh or salt, open sea or arm 



1 W. B. Scott: An Introduction to Geology, 2nd Edition, pp. 521-522 

 and 525. 



2 The subdivisions of geologic time are tabulated near the end of chap- 

 ter 2. 



3 J. M. Clarke: N. Y. State Mus. Memoir Xo. 6, 1903, p. 209. 



