OCCURRENCE AJ^D SUCCESSION OF ROCKS. 201 



area north of Cedar Hill Canon this andesite is much less homogeneous 

 than usual, varying in texture from coarse to fine frequentl}', and almost 

 without transitions. These differences have been emphasized by decom- 

 position and erosion, which have carved out projecting dike-like sheets, 

 fantastic columns, and the like, from the heterogeneous mass. 



That this rock is younger than the quartz-porphyry and the diabase is 

 very evident from the sections, since it overlies these rocks vertically in 

 wide areas, while there is nothing in their relations suggesting laccolitic 

 masses. 



There are some east-and-west veins in the hornblende-andesite near 

 Silver City, which are said to have yielded in the aggregate considerable 

 quantities of bullion. The only mines which could have thrown any light 

 on the origin of this ore, however, were closed at the time of the examina- 

 tion. They are near the Justice mine, which shows a great complication of 

 rocks in its ore-bearing region, and the ore of the east-and-west veins is 

 very probably due to the same general causes as the Justice ore-body. The 

 andesites themselves do not give considerable assays. 



Augite-andesite. — lu its general features, the occurrence of augite-andesite 

 closely resembles that of the preceding rock. It, too, appears to have issued 

 on a fissure nearly parallel to the Lode and to have spread very extensively 

 over the country; indeed, the present surface shows a greater area of it 

 than of the earlier hornblende-andesite It is possible that its eruptions 

 were not confined to the fissure cut by the Sutro Tunnel. Basalt Hill, B 6, 

 for example, still some 300 feet high, may well be a relic of a still larger 

 eruptive cone, rather than a remnant of an overflow from a fissure at a con- 

 siderable distance. Like the older andesite the relations of the augite rock 

 to the faulted surface near Virginia seem to show that it was eroded down to 

 a level in that region before the fault occurred. Its character throughout 

 the District is, as a rule, very uniform. It is possible, however, that a 

 few localities described as hornblende-andesite are in i-eality local modi- 

 fications of this rock. Thus the rock containing the hornblendes with 

 two concentric belts of magnetite, a crystal from which is shown in Fig 17, 

 Plate III., is exposed only by a cut 1 ,000 feet east of the railroad station, in 

 C 7. It is within but very near the edge of an area of augite-andesite, 



