204 GEOLOGY OF THE COMSTOCK LODE. 



of the Flowery range with the older hornblende-andesite declivity west of 

 Ophir Hill. In the latter locality every water-way has eaten deeply into 

 the rock, and every slightest undulation in the line of cliffs has given rise 

 to an eroding streamlet during wet weather. On the Flowery range the 

 drainage channels are far apart, and very shallow, and many undulations 

 which in a deeply eroded district would be sure to be emphasized by water 

 carving show nothing of the sort. The contact line between this rock and 

 the augite-andesite seems to me unlike contacts developed by erosion. It 

 has a very different character from the other contacts in the District, and 

 reminds one strongly of the forms assumed by slag slowly oozing over the 

 floor of a smelting-works. The structure of the rock, as seen on large 

 exposures, appears to indicate subaerial rather than subterranean deposition. 

 Plate VII. shows the east flank of Mount Rose, and is accurately repro- 

 duced from a photograph. Rude, thick layers of eruptive material, mostly 

 tufa and breccia, are plainly visible in this locality, though they are trace- 

 able over no great distance. It is easy to see how such beds might form 

 in successive eruptions, or through the variations in activity of a single jiro- 

 longed eruption; but it is difficult to account for such a structure in a mass 

 which has cooled beneath the surface, and has been exposed by erosion. 

 Such a mass would be characterized by dike-structure rather than by beds. 

 The physical character of the varieties of this rock, considered with 

 reference to their occurrence, is also difficult to reconcile with the suppo- 

 sition that the range is a mere relic of erosion. As has been explained, in 

 Chapter III., some of the younger hornblende-andesite is dense and glassy, 

 and other modifications are firm enough to resist decomposition better 

 than oi'dinary augite-andesite. In an eroded district these harder rocks 

 would be looked for on the summit, and the soft tufas would be found, if 

 at all, in protected localities; but, as has been pointed out, the tufas are most 

 abundant at the summits. Deeply eroded areas of eruptive I'ocks almost 

 always show patches isolated, or patches nearly separated from the main 

 field, by the action of water. To a certain extent this is the case with the 

 younger hornblende-andesite, for the two little areas near the Sierra Nevada 

 mine were unquestionably cut off from the tongue of this rock extending 

 from the Flowery range towards the Utah, by the erosion of Seven Mile 



