OCCURRENCE AND SUCCESSION OF ROCKS. 197 



Earlier diabase. — Tlie diabases are almost wholly confined to the mines, only 

 two small patches having been discovered on the surface. Of these, that 

 between the Julia and Ward shafts appears normal in character though much 

 altered, and as it occurs at the bottom of a ravine vertically above the main 

 body of the rock nothing is easier than to account for its presence. Such 

 is not the case with the mass in Ophir Ravine. This bears a very strong 

 outward resemblance to a granular diorite, and it seems impossible to make 

 out a sharp contact between the two rocks. There is also no evidence of 

 any connection between this area and the main mass east of the Lode. As 

 has been explained in Chapter III., I am by no means sure that it should 

 not be regarded as a local modification of diorite, rather than an independent 

 eruption. Apart from the interest attaching to such an occurrence it is of 

 little importance, no further consequences, so far as I know, depending on 

 its determination. As may be seen from the sections, diabase approaches 

 the surface very closely immediately below the city of Virginia, so closely 

 that at least a few croppings would be expected in the ground covered by 

 the town. It is highly probable that a considerable area might have been 

 traced before the settlement was made, but the ground is now so graded and 

 built up that a careful search failed to reveal any rock in place. 



Relations to the Lode. — The earlier diabase forms the east or hanging wall of 

 the Lode throughout its more productive portion; that is, from the Overman 

 to the Sierra Nevada, and from the surface, or very close to it, down to the 

 lowest depths yet reached. It also penetrates the west country, at the 

 north end of the Lode, in stringers, as may be seen on the horizontal sec- 

 tion on the Sidro Tiiimel level, Atlas-sheets VIII. and IX. This fact scarcely 

 requires explanation, for that a single clean fracture of the diorite mass 

 should have been effected at the time of the diabase eruption is almost 

 inconceivable. If the diabase succeeded the diorite it would be natural to 

 expect diabase in fissures within the diorite masses, and fragments of diorite 

 inclosed in diabase. It has already been pointed out that these occur. 

 There is a considerable sheet of diorite east of the bonanza of the CaJifor- 

 via and Consolidated Virginia mines, and similar masses were encountered 

 in sinking the new Yellow Jaclet shaft. In the higher levels, too, it is 

 probable, from the accounts of former examinations, that diorite horses were 



