182 GEOLOGY OF THE COM STOCK LOBE. 



the horizontal sections with the surface map, and has long been well rec- 

 ognized among those who have had to do with the mines. 



The ravines which furrow the range are not therefore the result of 

 erosion, but of faulting. Once formed through the dislocation of the 

 country, they have, of course, received the drainage, and have been modi- 

 fied thereby to some extent. 



East vein. — It lias bceu shown that even if a fault takes place on a fissure 

 perpendicular to an original surface, the hanging wall will assume the shape 

 of a sharp wedge, and that under the conditions of pressure necessary to 

 produce a logarithmic surface, it is unlikely that this wedge would remain 

 intact. Such a fracture occurred in the faulting of the Comstock, and 

 opened the famous "east vein", from which a large part of the ore produced 

 has been extracted. Baron von Richthofen regarded this .structure as a 

 result of faulting, and as a surface phenomenon. I have simply shown in 

 addition how the east country came to assume the tapering form most 

 favorable to such a fracture. 



Origin of the sheeted structure. Theory of eruptive stratification. TllC character of the 



sheets of rock into which the walls of the Comstock are divided is an open 

 question, for one observer has maintained that the)' form a series of thin, 

 bedded, regular layers of rock, presenting a fine example of eruptive strati- 

 fication. It is true that in confined spaces in several of the rocks a 

 stratified or laminated texture is visible; but in the half-dozen such cases 

 known to me the phenomenon extends for very short distances, often only a 

 few feet, and appears to be the result of some local variation in the compo- 

 sition of the rock; for not only can I perceive no general uniformity in the 

 direction of the layers in these different spots, but I have a single hand- 

 specimen which shows portions of two sets of them at an angle of nearly 

 i)0° to one another. These occurrences, however, cannot be meant in the 

 statement referred to, for they are rare. As applied to the great mass of 

 rock I am also unable to agree with it. To me it is nearly inconceivable 

 that a granular crystalline rock like the diorite of Mount Davidson, con- 

 taining only cr3'stals of "secondary consolidation," should ever have been 

 sufficiently fluid to permit of eruptive bedding The face of Mount David- 

 son shows no lamination, though the division into parallel sheets is strikingly 



