282 GEOLOGY OF THE COMSTOCK LODE. 



care with which the determinations were made is shown by the abundance 

 of the marks indicating the points from which specimens were collected, 

 and slides ground. This very laborious collection was necessitated chiefly 

 by the extreme state of decomposition of the rocks, which here almost 

 wholly effaces their distinguishing characteristics. It was also necessary to 

 prove the presence or absence of any rock which could properly be brought 

 under the definition of propylite. 



Ore bodies occur at the diabase contact. It appCarS from this SCCtlOn that the eaSt 



wall of the Comstock, from the Overman to the Sierra Nevada, is diabase, 

 while the west wall is diorite for only a part of the distance. By comparison 

 with the vertical sections and the vertical projection of the Lode it will 

 be seen that all the ore bodies of any importance, except that in the Justice, 

 are at or close to the diabase; while the Gold Hill bonanzas rest upon met- 

 amorphic rocks. The forking of the vein at the Overman is well exhibited 

 on this level, with its cause, the divergence of the black dike from the main 

 diabase mass. To the north it is evident that the north fissure is on the 

 strike of the Lode, and that its formation was probably facilitated by the 

 presence of the limestone body in the Sierra Nevada ground. 



Faulting. — The evidence with regard to faulting offered by this level is 

 interesting. The course of the Lode is very closely the same as the line of 

 the croppings, with the exception of the undulation shown at the surface 

 opposite the Virginia group of bonanzas. The disappearance of this undu- 

 lation was discussed in connection with the vertical section through the 

 Sutro Tunnel. The effect of the compression produced by the sharp bend 

 of the diabase contact to the eastward at the north end, in conjunction with 

 the southeasterly dip, is seen in the great mass of crushed rock in the 

 northern mines. This crushed rock has been denominated vein matter, in 

 accordance with local mining usage, because it is decomposed past certain 

 lithological determination; it is not laid down as forming a part of the 

 vein, however, because it is not a loose aggregation of fragments considerably 

 removed from their original position, but consists of huge rock masses fis- 

 sured- in every direction. 



Close contact of the walls. — Considering the extent of the vein and the indu- 

 bitable evidences of an extensive fault, it is at first sight very remarkable 



