AERANGEMENT OF STRATA. 105 



after time. Trilobites become wholly extinct at a certain epoch in 

 the history. And so in multitudes of cases. 



This criterion based on fossils serves for the comparison of the con- 

 tinents with one another, as to their successions of rocks. Had we a 

 table containing a list of the complete series of rocks, and of the fam- 

 ilies, genera, and species of fossils which each contains, it would be a 

 key for use over the whole world, — South and North America as well 

 as the Orient; and, by comparing the fossils of any rock under investi- 

 gation with this key, the age would be approximately ascertained. 

 This is the method now pursued in studying the geology of the globe. 

 The key, is, in fact, already sufficiently complete to be constantly ap- 

 pealed to by the geological observer. The list which is made for the 

 Silurian and Devonian rocks in New York State is used for identifying 

 the strata of the Mississippi basin ; and that which has been prepared 

 in Europe is constantly employed to make out the equivalency of the 

 rocks of the two continents. 



By such comparison of fossils, it was discovered' that the Chalk for- 

 mation exists in the Atlantic border of the United States, although 

 the region contains no chalk ; that the coal formation of North Amer- 

 ica and that of Newcastle, England, belong in all probability to the 

 same geological age ; and so on. 



The commencement in the preparation of such a key was attended 

 with much difficulty. In New York State, it was necessary — first to 

 study all the sections in the eastern, central, and western parts, and 

 determine carefully the fossils in each stratum ; then to compare the 

 sections with one another : when any case of identity in the fossils 

 among these strata of the different sections was observed, it was set 

 down as one horizon determined. By this method, and other aid from 

 observing the continuity of beds, one horizon after another was as- 

 certained, and the strata between were arranged according to their 

 true order of succession. 



By the means explained, great progress has been made in arranging 

 the rocks of the different continents in a chronological series. North 

 America has some large blanks in the series, which in Europe are 

 filled ; and in this way various countries are contributing to its perfec- 

 tion. 



But this criterion requires precaution in its applications, for the fol 

 lowing reasons : — 



1. The difference in species attending difference of conditions in cli- 

 mate, sotl, etc. In the same regions, during any era, the species of the 

 land differ from those of the waters : those of fresh water, from those 

 of salt ; those of the surface or shallow waters, from those of deeper, 

 and in these deeper waters according to the depth ; those of warm 



