THE LODE. 275 



were very thin perpendicularly to the plane of the Lode as compared with 

 that previously described, and hence occupy a much greater space on the 

 vertical longitudinal projection. In detail the structure of these bodies was 

 excessively complicated, as may be seen from Mr. King's report. It is not 

 in my power to add anything to his description, to which the reader is 

 referred for more detailed information. 



Virginia group of ore bodies. — Tlic Virginia group of bouanzas lies in an undu- 

 lation of the west wall, the general shape of which may be clearly traced on 

 the surface map; but by inspection of the horizontal section on the Sutro 

 Tunnel level it will be perceived that this depression has flattened so as almost 

 to disappear at a vertical distance of about 1,900 feet from the datum point. 

 Before the walls were disturbed in their relative positions, a solid mass of 

 diabase lay in this local depression. The fact that the depression is limited 

 to the neighborhood of the surface must have brought an extraordinary strain 

 to bear upon the tongue of east country rock lying within it when the fault 

 took place. The lines of secondary fracture, instead of running nearly paral- 

 lel to the Lode, appear also to have crossed the continuous prismatic horse so 

 often referred to, and to have reached the foot wall at the extremities of the 

 undulation. The mass thus separated would be canted eastwards by the 

 same force which effected the separation, and between it and the main body 

 of the east country there would form a crescentic opening, the points of 

 which would lie at the croppings on the west wall, while its greatest width 

 would also be on the west wall at the bottom of the tongue of east country. 

 From the west wall vertically, or in the direction of the secondary fracture, 

 the opening would everywhere taper, ending in a mere line at the surface 

 or more probably somewhat below the surface, since the crushing stress in 

 an east-and-west direction would be powerful. This opening once formed 

 would be immediately blocked by fragments of rock, and could never close. 



Such I conceive to have been the nature of the case in the region of the 

 Virginia group, modified in detail by more or less important irregularities 

 of structure ; and it will be observed from the Tunnel section that the west 

 quartz tapers from the surface downward, while the east quartz thickens ; 

 showing that the horse has revolved slightly on a horizontal north-and-south 

 axis, remaining firmly in contact with the east wall at the top and with the 



