OCCURRENCE AND SUCCESSION OP ROCKS. 199 



of the Gold Hill mines, and black rocks and clays have sometimes been 

 classed with it in the north end mines. In the upper levels it was met with 

 only in an indistinguishably decomposed form. I was not able to authen- 

 ticate its occurrence north of the Saimje, and found it, wherever struck, of 

 a very uniform width, always a few feet, never more than a couple of yards. 

 From the Savage to the Overman it generally marks the contact between the 

 older dia.base and the west wall with precision, but on one level of the Chollar 

 it is 80 feet west of the contact, and in the Yelloiv Jacket a narrow belt of 

 slate sometimes lies east of it. In the Overman the dike diverges from this 

 contact, extending towards American Flat as far as the Caledonia. The uni- 

 form thickness of the dike shows that no considerable movement between 

 the diabase and the west wall took place at or previous to its eruption, for 

 otherwise the fissure which it filled must have presented the enlargements 

 and contractions characteristic of veins the walls of which have experienced 

 a relative motion. The divergence of the dike towards American Flat ex- 

 plains the so-called forking of the vein. A certain amount of solfataric 

 action is perceptible along the dike fissure, accompanied by the deposition 

 of quartz which is not wholly barren The American Flat vein is a stringer, 

 the position of which was predetermined by this fissure. 



Earlier hornbiende-andesite. — Thc miuc workiugs show that the coutact betwecu 

 the earlier diabase and the earlier hornblende-andesite is very steep, and 

 that it must be represented by a Hne something like that indicated in the 

 section through the Sutro Tunnel, Atlas-sheet VI. The inference from this 

 section is strong that the body of older hornblende-andesite cut by the tun- 

 nel occupies a portion at least of the fissure through which it was erupted. 

 The eastern surface of the diabase is far too steep to admit of the supposi- 

 tion that it was ever exposed. Previous to the outbreak of the hornblende- 

 andesite the diabase must either have extended much farther east than now, 

 or a mass of diorite must have occupied the place now filled by hornblende- 

 andesite. In either case, the rock lying east of the present limit of the 

 diabase must have been submerged by the andesite eruption; and of the 

 two suppositions the former seems the more probable. The augite-andesite 

 stands in much the same structural relation to the earlier hornblende-ande- 

 site as the latter holds to the diabase, and the east and west surfaces of the 



