SUMMARY. 383 



by andesites, of which there are three varieties distinguishable both htho- 

 log-ically and geologically. These are a younger and a later hornblende- 

 andesite, the latter of which has hitherto been considered a trachyte, and 

 an augite-audesite intermediate in age. The older hornblende-andesite has 

 in part long been recognized as such, and is deceptive only when highly 

 decomposed. It occupies a belt immediately east of the older diabase (see 

 Atlas-sheet VII.), a large area on the heights immediately west of the 

 diorites, and a considerable area at and north of Silver City. The latter 

 occurrence is noteworthy for the unusual size of the hornblendes, which 

 are sometimes several inches in length. The augite-andesite occupies a 

 second belt of country east of the Lode and beyond the earlier hornblende- 

 andesite, and is also extensively developed to the north and south of the 

 diorite. The Forman shaft penetrates 1,200 feet of this rock before pass- 

 ing into the hornblende-andesite. 



The reasons are given elsewhere for considering the rock heretofore 

 regarded as trachyte to be an andesite. Its roughness and softness, its red 

 and purple colors and large glassy feldspars made the mistake an easy one. 

 The Flowery Range, the Sugar Loaf, Mount Emma, and Mount Rose, are 

 all of this rock, which also occurs in two little jjatches close to the Sierra 

 Nevada mine. These latter have been cut otf from the quarry above the 

 Utah by the erosion of Seven-Mile Canon. The patches of rock near the 

 Comhination shaft and the new TeUow Jacket which have sometimes been 

 regarded as trachyte are merely decomposed older hornblende-andesite. 



Basalt. — The occurrence of basalt is exceedingly limited, and is confined 

 within the area of the map to two small localities, one at Silver City and 

 the other a mile west of it. It is a fine, fresh, and typical rock. 



Area of decomposition. — The area of Hiost profouud decomposition is shown 

 as nearly as may be on the sketch map. Fig. ] . The amount of decomposi- 

 tion increases with depth. The pei'iod at which it was produced is almost 

 certainly the same as that of the faulting action and the deposition of ore. 

 It cannot have been earlier than the eruption of the later hornblende-ande- 

 site, and was more probably posterior to it. There is no indication of a 

 connection between the basalt eruption and the solfataric action, and it is 

 not improbable that the latter, though of volcanic origin, was independent 

 of any eruption of lava. 



