METHODS — GEOLOGICAL 11 



the containing strata. Hence, it is always a welcome con- 

 firmation of the chronological inferences drawn from the study 

 of fossils, when those inferences can be unequivocally estab- 

 lished by the succession of the beds themselves. For example, 

 in the Tertiary deposits of the West are two formations or 

 groups of strata, called respectively the Uinta and the White 

 River, which had never been known to occur in the same region 

 and whose relative age therefore could not be determined by 

 the method of superposition. Each of the formations, however, 

 has yielded a large number of well-preserved fossil mammals, 

 and the comparative study of these mammals made it clear 

 that the Uinta must be older than the White River and that 

 no very great lapse of time, geologically speaking, occurred 

 between the end of the former and the beginning of the latter. 

 Only two or three years ago an expedition from the American 

 Museum of Natural History discovered a place in Wyoming 

 where the White River beds lie directly upon those of the 

 Uinta, thus fully confirming the inference as to the relative 

 age of these two formations which had long ago been drawn 

 from the comparative study of their fossil mammals. 



The palaeontological method of determining the geological 

 date of the stratified rocks is thus an indispensable means of 

 correlating the scattered exposures of the strata in widely 

 separated regions and in different continents, it may be with 

 thousands of miles of intervening ocean. The general principle 

 employed is that close similarity of fossils in the rocks of the 

 regions compared points to an approximately contemporaneous 

 date of formation of those rocks. This principle must not, 

 however, be applied in an offhand or uncritical manner, or it 

 will lead to serious error. In the first place, the evolutionary 

 process is a very slow one and geological time is inconceivably 

 long, so that deposits which differ by some thousands of years 

 may yet have the same or nearly the same fossils. The method 

 is not one of sufficient refinement to detect such relatively 

 small differences. To recur to the illustration of the develop- 



