QQ MmmG mDUSTEY. 



importance and lateral extent than that of Virginia. The wedge-shaped 

 surface expansion of the lode is in this region gashed down in a series of 

 parallel fissures, dividing its material into thin vertical plates, separated from 

 each other by immense developments of clay, and several quartz veins of 

 remarkable continuity. North of the Ophir contraction the same parallel 

 system opens up to the northward, and, in the region of the Sien-a Nevada 

 and Sacramento, prominent plates of quartz traverse the front of Cedar Hill, 

 and penetrate downward as deeply as the excavations have gone. The pecu- 

 liarities of the parallel plates of quartz and horse material in the Ophir 

 Consolidated chamber are, first, their extreme thinness, and, secondly, their 

 habit of curving sharply to the east and west, producing in the horizontal 

 section a wrinkled and twisted outline. 



That part of the lode in which the fissures are best understood is the 

 Virginia division, where the solid front of syenite forms the western wall. To 

 the south of this the west wall is still somewhat problematical, although the 

 prominent clay-vein, which has been considered as limiting the lode in a 

 westerly direction, is in all probability a true wall. North of the syenite the 

 wall of propylite is a mere continuation of the syenite conditions, both of 

 inclination and undulation of surface. From the position and relation of rocks, 

 and the fact that the west walls north and south of Mount Davidson are, in all 

 larger characteristics, simply repetitions of the syenite form, it is believed that 

 this mass of Mount Davidson has been the most powerful agent in determining 

 the position and character of the Comstock lode. When the mountain chain 

 was subjected to a series of longitudinal strains, which resulted in the system 

 of andesite dikes, it was natural that this rigid mountain spur should oppose 

 an immense resistance to the dislocating force, and no place offers such condi- 

 tions of easy fracture as the contact plane, between the ancient and deeply- 

 bedded formations, and those later and less coherent -eruptive rocks which 

 have been superimposed upon their bases. Accordingly along this important 

 junction occurs the relics of a dike of andesite, which seems to have been the 

 first foreign substance to invade the contact plane and start the system of 

 intruded materials, which has finally resulted in the Comstock lode. The 

 general mode of occurrence of andesite, noticed in this district, proves conclu- 



