MAJOR DIVISIONS OV THE PRE-CAMBRIAN 21 



basis of fauna, that we have in reference to the Archean and Algonkian. 

 It is not to be supposed that the characteristic Mesozoic fauna appeared 

 in all parts of the world at the same time. Indeed, it is probable that 

 after this first appeared in some favored portion of the globe a long time 

 was required before the fauna had opportunity to migrate to distant parts 

 of the world. In consequence the rocks of the remote district deposited 

 during the time of migration would be called Paleozoic, although of the 

 same actual age as the lower part of the Mesozoic, in the region where 

 the fauna of this era originated. Indeed, Huxley many years ago recog- 

 nized this principle to be of such importance that in 1862 he suggested 

 a modification of the word homologous (homotaxis) to represent the 

 rocks that have the same fauna, and thus escape the implication given by 

 the word contemporaneous.® 



Applications of the proposed Classification to Eegions other 



THAN THE GrEAT NORTHERN ArEA 

 REFERENCE TO STUDIES IN OTHER AREAS 



Aside from the great Canadian area, the largest areas of pre-Cambrian 

 rocks which have been closely studied are those of western America and 

 the northern part of the eastern continent, especially Scotland, Finland, 

 and China. In each of the areas the dual divisions here advocated obtain. 



THE CORDILLERAN REGION 



In most of the early work in the western part of the United States the 

 term Archean was applied to the pre-Cambrian rocks, no attempt being- 

 made to divide them on any other basis than lithology. The more careful 

 work of recent years shows that there are in numerous regions, illustrated 

 by the Uncompahgre mountains, the Grand Canyon district, Utah, and 

 Montana, very extensive and thick series of rocks which belong in the 

 Algonkian ; also in this region there are extensive areas of Archean rocks. 

 Indeed, it is certain that in the Cordilleran region is a vast geological 

 province in which the Algonkian rocks have a great development, and 

 that these rocks rest unconformably upon a basement complex having all 

 tlie characteristics of the Archean. 



SCOTLAND 



For Scotland, it is fortunate for this discussion that the final results of 

 the many years' work of Peach, Home, and others on the Highlands are 

 at last available. The splendid memoir of the Geological Survey of 



» Discourses, biological and geological essays, by Thomas Huxley, vol. 8, p. 27C. 



