THE LODE. 281 



fortunately preserved. The drift itself was inaccessible, and could not have 

 been opened at an)- moderate cost. 



The east-and-west fault. — There are clear evidences of a slight downward 

 movement to the north of the Sierra Nevada, or an equivalent rise of the 

 region to the south. It is impossible to state definitely that this was not 

 independent of the great fault, but after considerable study of the case it has 

 seemed to me unlikely, on the whole, that the two movements were uncon- 

 nected. Everything shows that the eruptive rocks of the District are exceed- 

 ingly rigid, and cannot be flexed perceptibly without breaking. At the 

 same time there is, as has been seen, strong proof that the faulting dimin- 

 ishes rapidly to the north and south, beyond the points atwhich the main Lode 

 ramifies. In part the strain was weakened by distribution over various 

 fissures, but this would have been insufficient to effect adjustment in the 

 absence of flexibility. This argument would therefore point to the proba- 

 bility of east-and-west fractures as a means of relief, and it is to this action 

 that the little slips in the Sierra Nevada appear to me attributable. 



Cross-section through the Utah. — III the Utah the uorth fissure again straightens, 

 so as to exhibit approximately the usual dip of the Comstock, and though 

 the fault was slight it left a trace of a secondary fracture. Diabase appears 

 in several levels, but only as an irregular dike, backed by micaceous 

 diorite, which also shows extensively on the surface in this neighborhood. 

 As nearly as can be made out, this diabase comes in on a cross-fissure from 

 the southeast and not on the branch of the Lode. The evidences of solfa- 

 taric action are not great in this mine, much of the rock being even fresher 

 than that to be found on the surface at any point in the District. In the 

 lowest levels, however, there are belts of somewhat decomposed rock. 



Horizontal section. — It was intended to make horizontal sections of the Com- 

 stock on three levels, but this proved wholly impracticable on account of 

 the inaccessibility of the older workings. Fortunately it was possible to 

 explore nearly all of the Sutro Tunnel level, 1,900 feet below the croppings. 

 The result is recorded in Atlas-sheets VIII. and IX., where the inaccessible 

 drifts are shown in hair lines; while the projection of the principal workings 

 on other levels, of which use was made in drawing inferences as to the 

 conditions existing on the 1900-foot level, is shown in dotted lines. The 



