500 THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



duced into geology as the equivalent of the Azoic of Foster and Whit- 

 ney. By them this latter term was used to designate the rocks which 

 had assumed their present position prior to the deposition of the low- 

 est member of the Lower Silurian.* Thus, for instance, in accordance 

 with this view, granite erupted after the Jurassic epoch could not be 

 called Archaean. It does nut appear, however, that Mr. King had this 

 idea clearly present in his mind while writing the volume in portions ; 

 at least, this is the result to which an examination of the work leads us. 

 In fact, at the beginning of the chapter headed "Archajan," he says : — 

 " At intervals over tlio, whole mountainous area west of the 100th meridian, 

 masses of gneiss or crystalline schists, with their associated marbles, duloniites, 

 and quartzites, and eruptive bodies of granite, porphyries, gabbros, &c., are 

 found to underlie more recent strata." 



The rocks here mentioned he then immediately proceeds to desig- 

 nate as "these Archaean bodies," without any limitation as to their 

 geological age. 



In order, therefore, that we may ascertain how ftxr there is evidence 

 justifying the calling of these various "bodies" Archsean, according to 

 the original geological meaning of that term, it is desirable that we 

 should take up some of the more important regions where this " Ar- 

 chsean " occurs, and examine briefly the evidence by which its geological 

 age has been established. 



The first supposed Azoic (Archcean) rocks with which we have to do 

 are those of the Colorado (Laramie) Range. The rocks of this range 

 are granites and gneisses, with a very little mica schist (I., p. 22). It is 

 evident that Messrs. King, Emmons, and Hague in general regard as 

 a gneiss any granitoid rock which possesses in the slightest degree a 

 parallel arrangement of its constituent minerals. It is also evident 

 that in most cases they call this condition stratification. 



Regarding this range we find no proof advanced that the granites and 

 gneisses are sti-atified, and not eruptive, except the foliation above men- 

 tioned. Since it may be claimed by some that Professor Zirkel by his 

 microscopic observations proved these and other granites to be of meta- 

 morphic sedimentary origin, it is perhaps well to look at his evidence. 

 He distinctly states (VI., p. 59) that the diagnostic characters tliat he 

 has given as distinguishing metamorphic, older eruptive, and younger 

 eruptive granites from one another, "are valid only for the examined 

 rocks of the Fortieth Parallel, and that it is not allowable to generalize 

 from them for other countries." From this it is evident that these 

 * Sec Dana's Manual of Geology, 2J ed., 1875, p. 148. 



