600 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



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

 ney. By them this latter term was used to designate the rooks 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 eruptedafter the Jurassic epoch could not bo 

 called Archaean. It docs not 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 *' Archxan," he says : 



*' At intervals over, the whole mountainous area west of the 100th meridian, 

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

 and quartzitea, and eruptive bodies of granite, porphyriesj gabbros, &c., are 

 found to underlie more recent strata." 



The rocks here mentioned he then immediately proceeds to desig- 

 nate as "these Archaian bodies,'' without any limitation as to their 



geological age. 



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

 justifying the calling of these various "bodies" Archaean, 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- 

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



age has been established. 



The first supposed Azoic (Archaean) rocks with which wo 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 (L, 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 ^his range we find no proof advanced that the granites and 

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

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

 ■microscopic observation's proved these and other granites to be of meta- 

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

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

 has given as distinguishing motamorphic, older eruptive, and younger 

 eruptive .granites from one anotlicr, "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 



* See Dana's Manual of Geology, 2d ed., 1875, p. M8. 



