LINDGRKN.] ALTERATION OF THE COUNTRY ROCK. 639 



doubt directly due to the chemical action of the solutions from which 

 the mineral content of the vein was deposited. The process consists in 

 a sericitization. or replacement of the ferromagnesian silicates, feld- 

 spar, partly also the quartz, by sericite, a fine-fibrous or felted variety 

 of white mica. In composition it is a hydrous silicate of aluminium 

 and potassium, probably identical with muscovite. In many places a 

 carbonatization, or replacement by carbonate of lime and magnesia, 

 goes on at the same time, and sulphides, chiefly pyrite and arseno- 

 pyrite, rarely other minerals, are usually also formed in the rock 

 as minute and perfect crystals. It is certain that this metasomatic 

 process is a common one in fissure veins, 1 audits chemical character 

 is very different from kaolinization. Kaolin, in fact, is a product 

 not ordinarily found on the mineral veins, and talc occurs still more 

 rarely. 



This altered granite, together with the ore and gangue occurring in 

 seams or veins through it, constitutes what miners term a "ledge" and 

 ' ' ledge matter. " This may be many feet wide, and the paying ore may, 

 and in fact does usually, form only a small portion in width of the 

 ledge matter. The altered country rock, though often well filled with 

 pyrite and arsenopyrite, is ordinarily very poor, containing at most 

 one or two dollars in gold. Exceptionally it contains enough gold to 

 be considered an ore, but generally only when adjoining rich vein 

 filling, and large ore shoots; even then it constitutes only second-class 

 ore, and its sulphides are much poorer than those in the vein proper. 

 It rarely contains any free gold. As examples a few typical rocks of 

 this kind may be described. 



The altered dioritic granite from the Checkmate mine, Willow Creek 

 (88 Boise sheet collection), is a granular, white, soft rock, consisting 

 of quartz grains, white earthy grains replacing the feldspar, a few 

 foils of pale-yellowish mica and abundant small and perfect crystals 

 of pyrite, showing the combination GO GO, GO 2, and a few small prisms 

 of arsenopyrite. A few narrow seams, l nnn wide, traverse the rock and 

 carry only blende and galena. The microscope shows quartz grains 

 with undulous extinction, which contain in places a few shreds of 

 sericite, but are on the whole hardly attacked by any metasomatic 

 process. There are a few larger muscovite foils, which evidently rep- 

 resent the biotite of the fresh rock. The space between the quartz 

 grains is filled by a fine-felted sericite mass, which incloses nearly all 

 of the idiomorphic pyrite. Intergrown with and inclosed by the pyrite 

 is a little brown zinc blende and galena in small anhedral grains ; one 

 crystal of arsenopyrite was also noted. No calcite was found. This 

 rock occurs close to a rich ore body, but an assay of it gave only 0. 1 

 ounce of gold and 0.5 ounce of silver per ton. The total replacement 



1 For studies of the metasomatic processes of fissure veins by the author, see Fourteenth Ann. 

 Kept. U. S. Geol. Survey, Part II, pp. 243-284, and Seventeenth Ann. Rept.U. S. Geol. Survey, 

 Part II, p. 144; also Bull. Geol. Soc. Am., Vol. VI, p. 221. 



