210 GEOLOGY OF THE COMSTOCK LODE. 



little progress can be made until definite criteria are discovered by which 

 the state of combination of the elements in fresh minerals can be decided. 

 Formation of pyrite. — Pcrliaps the iiiost Striking characteristic of the decom- 

 posed rocks of Washoe is the presence of innumerable bright crystals of 

 pyrite disseminated through the mass. The unaltered rocks do not appear 

 to carry this mineral; if it occurs in them at all it is certainly a very rare 

 ingredient. In the altered rocks pyrite, when present, is abundant nearly in 

 proportion to the degree of decomposition, and, except where it is exposed 

 to tlie direct action of the atmosphere, it is almost invariably perfectly fresh. 

 All the circumstances thus indicate that it is a product of decomposition 

 The massive rocks contain iron, chiefl)' aS magnetite and as a component of 

 the bisilicates; and the pyrite must have been formed by the action of soluble 

 sulphurets on one or both of these compounds. The slides of the pyritous 

 rocks, however, frequently show large quantities of sharply defined mag- 

 netite, while the bisilicates are in a majority of cases Avholly decomposed. 

 There is certainly nothing in the association of pyrite and magnetite to sug- 

 gest a relation ; but the pseudomorphs of decomposition products after the 

 bisihcates are very frequently studded with small pyrite crystals, and occa- 

 sionally real pseudomorphs of pyrite after augite or hornblende appear to 

 occur. Of these it is difiicult to be certain, however; for the size of the 

 pyrite individuals is usually considerable, relatively to that of their hosts; the 

 original crystal form is consequently never unmodified, and is commonly 

 altered beyond recognition. The distribution of the pyrite in the rock also 

 reminds the familiar observer of the distribution of the bisilicates in the 

 same rock, and macroscopical comparison of suites of specimens from the 

 same localities shows that the pyrite to all appearances is associated with 

 the bisilicates, and in extreme cases replaces them. It is easy to lay too 

 much stress on an impression of this sort, yet Avhen such an impression is 

 derived from the examination of many thousand instances it deserves some 

 weight. All tlie evidence thus tends to the supposition that the pyrite is 

 mainly a decomposition product of the bisilicates and of mica. Such an 

 alteration is quite possible in the presence of alkaline sulphides, or of 

 hydrosulphuric acid ; and, as has been seen, the waters even now entering 

 the mines three thousand feet from the surface are charged with the latter 



