Vol. 4] 



Reid. — The Comstock Lode. 



179 



the genesis of the ores. Their importance to the lode lies in the 

 fact that they have most to do with the notable short length of the 

 main Assure. Their importance in regard to the ores lies in the 

 fact that they have caused the blocking out of the wall rocks 

 with the resultant opening to mineral solutions. These points will 

 be discussed later. Becker* has noted the presence of such 

 faulting, but only in a general way, and as of little importance. He 

 says (p. 181) that the topography is due chiefly to faulting, "for 

 on the Sutro tunnel section, at least, there is evidence of but slight 

 erosion.* * * The ravines which furrow the range are not there- 

 fore the results of erosion, but of faulting." And again (p. 184 

 ct seq.) "To the north and south of Mt. Davidson the evidences of 

 faulting diminish (north-south faulting is meant). From the 

 Overman far into the Sierra Nevada claim, a distance of two and 

 one-third miles, the amount of fault has been great, and the indi- 

 cations unmistakable. Beyond these points the disturbance of 

 equilibrium has been to some extent adjusted in a different man- 

 ner. This is partly indicated by the union of the andesite fields, 

 which are separated near the center of the lode by diorite. To- 

 ward the ends of the lode the dynamic action seems to have been 

 distributed in part by the forking of the fissure and in part by 

 the formation of east and west cracks." According to Becker, 

 these same east-west faults have their downthrow to the north. 

 Practically nothing more is written in the monograph of Becker 

 concerning these movements. But even now, with almost none of 

 the old workings accessible, their large development is well shown. 

 The most noteworthy examples are as follows: 



Mexican Fault. — In the Mexican ground, on the Sutro tunnel 

 level, a very strong east -west fault was followed into the diorite 

 of Mt. Davidson, and east into the hanging wall block for several 

 hundred feet. No ore was developed, and the work was aban- 

 doned. Pyrite was very abundant, however, in the crushed zone. 

 This fault was nearly vertical, with a slight inclination to the 

 north. The throw, from all appearances seen, as striations, etc., 

 was down to the north, making a normal fault. This line of mo- 



*G. F. Becker. Monograph III. U.S.G.S. ' ' Geology of the Comstock 

 Lode." 



