78 GEOLOGY OF THE (JOMSTOCK LODE. 



Insolubility of epidote. — Epidotc IS usuallj clflssed as an insoluble mineral, and 

 the evidence to the contrary in the Washoe District is slight. Epidote, it 

 is true, frequently crystallizes in vugs, but since chlorite certainly possesses 

 some degree of solubility, their growth might be accounted for by supposing 

 them to form in a solution of the mineral from which they are derived. 

 There seems, liowever, to be a relation between the size of epidote masses 

 and of the crystalline grains of which they are composed, which is most 

 easily accounted for by supposing the mineral to be somewhat soluble. In 

 very small patches epidote is frequently so fine-grained as to reflect almost 

 all the light, and under low powers appears opaque. In larger masses, 

 formed apparently under similar conditions, the epidote often shows crystal- 

 line grains of considerable size, and transmits light readily. In a few cases 

 among the diabases epidote appears to be replaced by opaque mixtures of 

 iron oxide and other substances, but no certain instance of this sort was 

 made out, and there is usually no indication of any tendency to decompose. 



Decomposition of feldspars. — Thc study of thc process of decompositiou which 

 the feldspars of the Washoe rocks undergo is much less satisfactory. They 

 have offered a far greater resistance than the bisilicates, and no great con- 

 tinuous area exists in which they are not sufficiently fresh to be readily 

 determinable. Incipient decomposition is marked by the appearance of 

 specks of calcite, readily recognizable in polarized light. At a later stage 

 quartz grains make their appearance, accompanied by particles of a white 

 opaque substance, of a nature unknown to me. In the last stages of decom- 

 position nothing further than these three substances is recognizable. Kaolin, 

 according to Mr. H. Fischer, is an isotropic substance, accompanied in the 

 slides he studied by polarizing grains and scales.^ Nacrite is crystalline and 

 consists of an aggregate of six-sided scales of fibrous texture, each composed 

 of six triangular sectors. Nothing corresponding to the description of 

 either was observed in any of the slides, a fact which seems to prove that 

 kaolinization, if it has taken place at all, is a very subordinate phenomenon. 

 The analyses of the '-clays," too, show that they are not concentrations of 

 kaolin washed out of the surrounding rocks, but represent so much rock 

 crushed and degenerated in place. The water contents of some of them is 



'Rosenbnsch: Phys. derMin. u. Gesteine, Vol. I., p. 374. 



