THE COMSTOCK LODE. 27 



fracture. On Middle Hill itself, rising through masses of silicious and feld- 

 spathic metamorphic rocks, are high dikes of porphyritic propylite. On the 

 hills west of the Geiger grade there are also, lifted above the general level, 

 hard slabs from twenty to seventy feet high, which are evidently the crests 

 of dikes, whose superior hardness has prevented their weathering with the 

 surrounding rock. Half way up Crown Point Ravine are further instances of 

 the turreted form. 



There is evidence that over the country included in the map there 

 have been three successive outflows. From the series of fissures, which 

 traversed the whole surface of the older range, appeared at first a form of por- 

 phyritic rock, unvarying in composition, but having every variety of arrange- 

 ment of the crystalline particles composing it. This is the normal propylite, 

 from a typical specimen of which the analysis, given in the third section of 

 this chapter, was made. Succeeding this was ejected a vast amount of pro- 

 pylitic breccia, which slopes in gently inclined beds toward the Carson Plains. 

 Finally, traversing these two, were a great number of fissures, which were 

 filled with a highly crystalline form of the rock. The earlier porphyritic 

 form is that which rests against the slope of Davidson, and forms the east 

 wall of the Comstock lode. The second, or breccia variety, is found chiefly 

 to the north and south of the lode. A purple occurrence of this breccia 

 caps the Geiger Summit, and, south of the lode, the hills adjoining Mount 

 Butler are formed of a finely fractured mass. At several points, usually 

 near the contact with the syenite, there is observable a fine lamination of 

 compact, close-grained rock, which resembles very closely some of the meta- 

 morphic (uralitic) rocks, and still more closely the metamorphic diorites of 

 the California gold belt. 



Above the level of the Comstock the whole of the propylite is 

 unaltered, with the unimportant exception of the hill next south of Mount 

 Butler, but below that elevation, quite to the Carson Plain, the whole 

 rock is decomposed to a greater or less degree. Innumerable fissures 

 have traversed it in a north and south direction, and each appears to have 

 been the theater of an intense hot-spring action. The results of this 

 solfatarism are evident in the generally decomposed and earthy condition 

 of the formation. The series of flats are wholly formed of this altered 



