TEN-MILE AND CLINTON AMPHITHEATERS. 201 



Ten-Mile and Clinton amphitheaters. The Archean types represented in the 

 area embracing these basins are as varied as those which have already been 

 described from the adjacent region to the east and south. In Clinton am- 

 phitheater there is an unusually large amount of the rudely porphyritic 

 granite referred to earlier in this chapter, and in both are numerous dikes 

 of eruptive rocks, among which the Lincoln Porphyry and a dark horn- 

 blendic porphyrite are the most frequent. These bodies cannot be indicated 

 upon the map with satisfactory accuracy, and are for the most part omitted. 

 A rhyolitic rock of coarse grain in Ten-Mile amphitheater seems more closely 

 related to the Chalk Mountain Nevadite than to any other. 



Bartlett Mountain, which separates the two amphitheaters, has a dike 

 of dark-green porphyrite running through its summit and down into the 

 Ten-Mile basin. Upon the western slopes of this mountain are large masses 

 of white quartz which might at first glance be considered as derived from 

 a remnant of the Cambrian strata left on the east side of the Mosquito 

 fault, but they are quite homogeneous and have no trace of the granular or 

 sandy structure which is found even in the most glassy quartzites; they 

 were probably derived from some of the vein-like masses of quartz which 

 are found developed on a large scale in the Archean of other parts of the 

 Rocky Mountains, and which have been removed by erosion. Abundant 

 evidences of glaciation exist in these as in other amphitheaters which have 

 been described. 



