20 MINING INDUSTEY. 



to two distinct groups ; a series of thin, imperfectly developed, highly altered 

 limestones and another group of crystalline schists. Richthofen attempts 

 a subdivision of these schists, and makes a classification w^hich does not seem 

 to hold good upon a more extended study. It is preferable here to consider 

 them as one series whose variety is due rather to metamorphism than to 

 original difference. Lowest of these, and lying in direct contact with the 

 granite and syenite, is a series of beds of mica schists, striking in general a 

 little west of north, and dipping at a high angle to the east. The lowermost 

 strata are of a coarse nodular schist, containing dense nuclei of hornblende, a 

 considerable proportion of quartz-granules, magnetic iron, and enough mag- 

 nesian mica to impart the fissile structure. In ascending the series the mica 

 schists are found to grow finer and finer, the nodules gradually diminishing 

 from the size of a bean until they entirely disappear, the rock at last becom- 

 ing a fine mica slate. The ridges to the south of American Canon are com- 

 posed of these fine-grained schists, and the traces of bedding, and even of fis- 

 sile structure, are gradually obliterated, "resulting finally in a compact black 

 crystalline rock which, in hand specimens, would unquestionably be classed as 

 basalt. A suite of specimens in the possession of the Exploration completely 

 shows this transition from the nodular schist, up to the compact crystalline 

 rock, which has no trace of aqueous origin. 



The limestones cannot be assigned definitely, either as regards super- 

 position or relations to the syenitic core. They are very limited, being acci- 

 dentally excavated from the propylites by erosion. Above the American Flat 

 road, about a mile from Gold Hill, are the only outcrops of the formation. It 

 appears here as a highly crystalline marble, having an almost granitic appear- 

 ance, and impregnated throughout its whole mass with small crystals of iron 

 pyrites. 



The third series of metamorphic products are found dispersed through- 

 out the district: first, as an intermediate zone between the mica schists 

 and hornblende ; secondly, in limited outcrops in Crown Point Eavine ; 

 thirdly, in obscure masses within the Gold Hill workings of the Cora- 

 stock ; and, fourthly, on the slopes of Cedar Hill. At the latter locality 

 they are mere outcrops, emerging from the general mass of propylite ; 

 they are of a light greenish-gray tint, and of a prevailing felsitic char- 

 acter. The fine-grained, compact mass is everywhere studded with pyrites, 



