THE GOMSTOOK LODE. 29 



quartz, up to a comparatively light trachytoid porphyry, thickly studded with 

 granules of vitreous quartz. Upon the one side it seems to be clearly allied 

 with the propylite, so that it may be classed without difficulty as a quartz-pro- 

 pylite, but at the other extreme it becomes so acidic as to approximate closely 

 to the rhyolites. Above American Flat the prevailing form is a compact, 

 slightly porphyritic, green mass, having in general a felsitic appearance, con- 

 taining a small proportion of quartz granules. - In the region of Silver City, 

 where it surrounds the overflows of metamorphic rocks, as also at the Justice 

 vein, it seems to have made considerable progress toward the rhyolitic form ; 

 and still further to the southward, upon the flanks of basalt hills, its feldspar 

 has become glassy, and is associated with a large percentage of free quartz. 

 The resemblance to the rhyolites is heightened by a film of red silicate 

 of iron, which surrounds the quartz and gives them the garnet tints. The 

 quartz-propylites of the Silver City ridge are generally of a deep, brownish- 

 purple color, becoming lighter to the southward, until, near their contact with 

 the basalt, they have attained the light chocolate and pinkish tints of the 

 rhyolite family. Mica, which was a prominent mineral north of the American 

 Flat road, has almost entirely disappeared, and the paste has changed from a 

 felsitic to a trachytic texture. The two outcrops are not connected, and it is 

 still possible that the westernmost one is a true rhyolite, but from the gradual 

 transition from the other extreme, it seems probable that they are only various 

 forms of the same rock. As will be hereafter seen the balance of probabilities 

 indicate that it is all propylite, for the solfataric fissures penetrate and alter 

 it as they probably could not have done were it so recent a rock as rhyolite. 

 West of the American Flat road the quartz-porphyries appear in bold dikes, 

 or rather longitudinal masses, for their great size entitles them to a more 

 important term than dike. It is interesting to observe that the white solfataric 

 product of the quartz-porphyry is identical with that of the feldspathic 

 propylite, with the exception of the granules of quartz, which the chemical 

 action has failed to touch. In the unaltered rock may be seen crystals of 

 oligoclase, and accumulations of hornblende, exactly resembling the original 

 propylite. Interrupting these a second quartz-porphyry, has flowed out which, 

 although still closely resembling the propylite family, begins to tend toward 



