196 GEOLOGY OF THE COMSTOCK LODE. 



the metamorphism is due. to the action of tiie porphyry on the strata over 

 which it flowed. Metamorphic diorite occurs on the Comstock only at the 

 extreme south end, in the Silver Hill and Justice mines. Mines have been 

 sunk in it south of Silver City — for example, the Amazon — and have struck 

 ore which was calcareous and carried mixed sulphurets. The Justice ore 

 associated with this rock was of a similar charactei\ 



Quartz-porphyry. — The CjUartz-porpliyry which appears on the map is merely 

 the northeasterly corner of an extensive area of this rock. A noticeable 

 peculiarity is that it is everywhere decomposed, and everywhere to almost 

 precisely the same degree, while it is fissui'ed only to a very slight extent. 

 It seems scarcely possible that this decomposition should have taken place 

 from below, for the underlying granite and metamorphic diorite are for the 

 most part very fresh. The decomposition would seem rather the result of 

 the action of surface waters, favored by a porous structure. This structure 

 is perhaps due to the unequal contraction of quartz and feldspar in cooling. 

 Before later eruptions covered it, the porphyry occupied the surface for a 

 considerable distance farther to the northeast than at present, for it appears 

 in the Belcher ground and in the Forman shaft. In both these cases it under- 

 lies hornblende-andesite, while in the Belcher 1648, and in the Overman, it 

 also seems to underlie diabase. The accessible points at which these two 

 rocks come in contact, however, are so few that the order of their succession 

 is less satisfactoril}^ made out than that of tiny other important members of 

 the series of rocks found in the Washoe District. The quartz-porphyry 

 does not appear to be intimately associated with the ore bodies of the Com- 

 stock, though occurring near to some of those in the Gold Hill mines ; nor 

 have any considerable quantities of ore been discovered in this rock in out- 

 lying mines. It also assa5-s little or nothing. It is worthy of note that 

 quartz-porphyries in some mining districts have almost certainly supplied 

 the deposits with their chai-ge of precious metals, though the Washoe 

 occurrence is so barren. 



The felsitic modification of the quartz-porphyry is confined to a limited 

 area near the granite. To what cause tlie difference between its structure 

 and that of the ordinary variety may be due I cannot suggest. 



