322 GEOLOGY AND MInInG INDUSTRY OF LEADVILLE. 



Classification of Mosquito Range eruptives — The eruptive rocks of the MosquitO Rauge 



are classified as belongiug to the following groups : 



/ Qnarfz porphyry. 

 Older . . } Diorite. 



' PorpLyrite. 



xr ( Rhvolite. 



\ ouuger { •' . 



( Audesite. 



For the reasons gjvea in the chapter on rock formations, the possible eruptive 

 granites of the Archean areas are not inclnded in this discussion. It is at once noticed 

 that basic eruptives, such as diabase or basalt, do not occur in this region, and even the 

 andesites above mentioned are only found outside the area mapped. 



Nearly all the important rocks of the district are described as quartz-porphyry 

 or as porphyrite. Of these two classes there are several marked types, and they are 

 so connected by intermediate or transition forms as to build an almost complete series, 

 uniting the dissimilar extremes. The treatment of these rocks in the present chapter, 

 which has been revised in the light of experience gained in adjoining districts, is some- 

 what diiferent from that at first adopted ; hence a few minor discrepancies may be noted 

 between the classification here given and that indicated by the coloring of the map 

 and the text of the main work. The map was engraved and colored before this infor- 

 mation from other districts was obtained, and could not, therefore, be changed ; the 

 general text is, however, consistent with the divisions of the map. The inconsisten- 

 cies alluded to are really of but little moment, as they relate to certain more or less 

 questionable forms near the line between quartz-i)orphyry and porphyrite, which, taken 

 by themselves, might readily be differently classed by different persons. The changes 

 are introduced here for the sake of preserving, as f;\r as possible, a uniform system la 

 this and in forthcoming reports on adjoining districts. 



