CHEMISTRY. 213 



best developed, as in Crown Point and Ophir ravines, the accompanying 

 pyrite is usually decomposed either wholly or in part; and in localities at a 

 small distance beneath the surface, hke the McKihhen tunnel, it is in those 

 belts of rock which are evidently most highly decomposed that epidote is 

 found replacing chlorite. 



Probable course of the alteration of chlorite to epidote. StrOHg Ulineralogical CvidcnCe 



has already been offered to show that epidote at Washoe is an alteration 

 product of chlorite. The indications of relative solubility are worth con- 

 sidering in this connection. Chlorite is manifestly rather easily soluble, and 

 soon after its formation becomes diffused through the groundmass and any 

 porous crystals which may be present, settling, too, in veins when cracks offer 

 an opportunity for such a concentration. Epidote appears to be soluble only 

 in a greatly inferior degree ; indeed, its faggot-like masses of crystals seldom 

 show anything which can be interpreted as attack by a solvent. If, there- 

 fore, chlorite and solutions of calcium carbonate containing free oxygen are 

 brought together under physical conditions compatible with the formation 

 of epidote, it seems inevitable that epidote should be precipitated, unless 

 still more insoluble substances may also be thrown down under the same 

 conditions. 



It is well known that chlorite is frequently altered to a mass of quartz, 

 ferric hydrate, and carbonates. When this change takes place it is prob- 

 able that at least a portion of the alumina is mingled in some form with the 

 iron oxide, and the carbonates most likely contain magnesium as well as 

 calcium. In the numerous cases of this change which have been observed 

 at Washoe, the carbonates form a large portion of the resulting mixture, a 

 fact which appears to prove that the active solutions were but slightly charged 

 with carbonic acid, since, had it been otherwise, calcite and magnesite, if 

 separated out at all, would have been redissolved. Cases of the conversion 

 of chlorite to epidote and to carbonates, etc., often occur in the same slide, 

 and presumably under nearly the same physical conditions. It may be 

 that the decisive point is the quantity of carbonic acid present. If the two 

 processes went on at different times such a difference would be readily expli- 

 cable, and if simultaneously it is not difficult to understand how the quantity 

 of carbonic acid might vary. Though rocks are permeable, the aqueous cui-- 



