118 ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE, [bull. 182. 
country rock to within a few inches of the productive lodes. Fre- 
quently, but not always, the rock in contact with the ore, or occurring 
as small horses in the veins, is free from chlorite and calcite and 
consists mainly of quartz and sericite, with some disseminated ore 
minerals. The latter, however, are not limited to the immediate 
walls of the fissure, but specks of galena and crystals of pyrite may 
occur 25 feet or more from the nearest lode, in rock carrying much 
calcite and chlorite. As an example of the extreme alteration which 
the country rock has undergone in the formation of these ore bodies 
may be described a specimen from the third level of the Silver Lake 
mine, taken from the wedge of country rock between the Stelzner and 
Royal veins at their junction. The usual breccia is here bleached 
almost white, and shows a fine, even-granular texture to the naked 
eye, but as a whole is shattered, full of quartz stringers, and contains 
flakes of galena. Under the microscope, pseudomorphs of sericite 
after feldspar and biotite preserve the only remnants of original 
structure. The groundmass is a very finely crystalline aggregate of 
quartz and sericite traversed by microscopic quartz veinlets. As 
usual, there is a little r utile present, and some crystals of pyrite. 
To sum up briefly, the rocks in which the ore deposits of Silver 
Lake Basin occur are chiefly andesitic or latitic breccias. In the 
neighborhood of the mines these breccias have been generally altered 
to an unknown depth. The alteration involves the change of feld- 
spar to sericite, calcite, and quartz; of augite to calcite and chlorite; 
and of biotite to chlorite, sericite, and r utile. Although this metal 
morphism is probably connected with the ore deposition, it is so gen- 
erally prevalent that it can not in any case be recognized as being 
connected with the deposition of ore in any given fissure. It appears 
to have been effected through the agency of water charged with car- 
bon dioxide or carbonates. The change involved in the rocks is prop- 
ylitic in nature, and, as Lindgren 1 justly maintains, should be distin- 
guished from ordinary weathering, with which it is often confused. 
Close to the veins, usually within a few inches, and in small horses 
of country rock within the veins, metamorphism of a different kind fre- 
quently occurs. Here calcite and chlorite have diminished in amount 
or are wholly absent, and quartz and sericite constitute the bulk of 
the rock. This alteration, which plainly emanates from the indi- 
vidual fissure, differs from the more general metamorphism less in 
kind than in the relative proportions of calcite and chlorite on the 
one side and of quartz and sericite on the other. This usually incon- 
spicuous and very local alteration of the wall rock to quartz and seri- 
cite is rather common within the quadrangle, especially near lodes in 
the andesitic rocks of the Silverton or San Juan series. It occurs 
in somewhat pronounced degree in the Ridgway mine, which pro- 
1 Metasomatic processes in fissure veins: Trans. Am. Inst. Min. Eng., Vol. XXX, 1900, p. 645. 
