46 GEOLOGY AND MINING INDUSTRY OF LEADVILLE. 



and amphibolitic schists of singular composition, are also closely allied by 

 their mineralogical character to this latter group. 



To the former of these classes belong the mass of the Archean rocks 

 so largely developed throughout the whole Colorado Range of the Rocky 

 Mountains, of which excellent sections are afforded by all the streams which 

 flow out upon the Great Plains. Here in a very general way they seem to 

 consist principally of gneisses resting on a central core of red, friable, 

 coarse-grained granite. 



Although no opportunity has been had of making a study of the other 

 Archean bodies of the Rocky Mountains, it would seem, from what has 

 been seen in traveling across them, that the Archean of the Mosquito Range 

 is distinguished from that of the Colorado Range by a greater prevalence 

 of granite over schists, and in a very general way that the more schistoid 

 rocks of the Mosquito Range are resting upon the almost entirely granitic 

 mass of the Sawatch, which should therefore be considered the older. 



As shown in the section afforded by the cafions of the Mosquito Range, 

 and hence in comparative nearness to the overlying sedimentary rocks, 

 the Archean formation consists essentially of granite, gneiss, and amphib- 

 olite. The granites are in many cases undoubtedly metamorphic and form 

 bedded masses. In other cases there seems little doubt that they are erup- 

 tive, but probably of Archean age, since they have not been found to intrude, 

 into or contain fragments of the Paleozoic rocks. In the majority of cases 

 the structural evidence was not decisive either way, but the texture, of the 

 granite was decidedly that which is found characteristic of the metaphor- 

 phi c types. 



GRANITE. 



The granites are prevailingly very coarse-grained, especially those in 

 which evidences of bedding are found. If the classification given by Rosen- 

 busch 1 be here adopted the greater part will belong to his class of granite 

 in the narrower sense of the word, or granite proper, consisting, namely, of 

 quartz, two feldspars, biotite, and muscovite. These granites always con- 

 tain muscovite and variable biotite, but rarely if ever hornblende; where 



1 Mik. Physiog. tier mass. Gesteino. H. Rosenbnsch. Stuttgart, 1877, p. 18. 



