THE COMSTOCK LODE. 31 



point due east fi-om Gold Hill is also of andesite. It is whoUy surrounded 

 and limited by propylite, and leaves a doubt as to whether it is a normal part 

 of the fissure system, or simply a relic of one of the overflowed fields which 

 has been spared in the general erosion. Accumulations of clay propylite soil 

 cover the surface for half a mile northward and mask all rock in situ. Near 

 the railroad tunnel, directly to the north of the Dayton toll road, appears 

 a third mass of andesite, of a long oval form, tapering to the northward in 

 a thin spine-like ridge. Earthy deposits again cover the surface from this 

 point until about opposite Taylor street, where, among the first occurrences of 

 solid rock, the andesite again makes its appearance, this time in a mere thread 

 which traverses the bottom of the Slaughter-House Canon. 



Where the road to the Odd Fellows' burying ground diverges from C 

 street, the same dike reappears. This whole zone bends slightly in conformity 

 to the foot of Mount Davidson, and seems to be a proof that the underlying 

 syenites have in great measure determined the direction of subsequent fissures 

 to the eastward. 



Zone number three, and by far the most important of all, lies concentric 

 with this last, and about two miles to the eastward. An almost continu- 

 ous overflow of andesite covers the country from near the Devil's Gate, 

 in Gold Canon, across the Divide spur, continuing northward across Six-Mile 

 Canon, covering a large part of the Silver Terrace Spur, and reappearing 

 on the Catholic burying ground. The indication of a third, but much more 

 limited group, is found in a train of small outcrops which lie directly along 

 the edge of the trachyte, and without doubt underlie it. 



Under its proper head, in the volume on Systematic Geology, will be dis- 

 cussed two important questions concerning the andesite formation. First, its 

 origin and relation to other allied rocks ; and second, its own chemical and 

 mineralogical constitution, concerning which authorities differ so widely and 

 on the basis of so few data that the whole question is at present in a form 

 unprofitable to discuss. In general terms, however, it may be said here that 

 the balance of probability points to a close alliance between this rock and 

 propylite, and it will not be at all surprising if it should finaEy prove to be 

 chemically identical and, in reality, only a different form, bearing the same rela- 

 tion to the typical propylite as the hyaline-pearhtes and obsidians do to the 



