"Only two or three years ago an expedition from the discovered a place in Wyoming where the lie directly upon those of the , thus fully confirming the inference as to the relative age of the two s which had long ago been drawn from the comparative study of their fossil mammals. The palæontological 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. The principal must not, however, be applied in an offhand or uncritical manner, or it will lead to serious error."