Crosby.] 512 [March 7, 



a high degree of alteration. He described the igneous cones in 

 every case as pustular eruptions, conceiving that great volumes of 

 lava had been forced up en masse in a viscid or semi-solid state ; 

 and not merely lifting the horizontal formations into arches or 

 domes, but actually breaking through them. He evidently re- 

 garded the cones as essentially extrusive rather than intrusive, 

 and their viscidity at the time of eruption thus became a neces- 

 sary inference from the fact that the lava did not spread far from 

 the vent or form flows. This view is, however, completely nega- 

 tived by an important class of facts which Newton seems to have 

 entirely overlooked. 



The igneous rocks, which appear to be in the main of rather 

 acidic types — rhyolite and trachyte — and approximately noncrys- 

 talline, being commonly called "porphyry" by the miners, occur in 

 masses of three different forms : (1) The more or less distinct cones 

 or mountain masses, already referred to. These properly embrace, 

 besides such prominent topographic features as Bear Butte, Black 

 Butte, Crow, Terry, Custer and Warren Peaks, the Sun Dance 

 Hills and Inyan Kara, numerous broader, flatter and more obtuse 

 masses or bosses, such as cover a large part of the surface between 

 Black Butte and Terry Peak, and also southeast of Terry Peak, 

 about the Deer Mountains. The broad and apparently thick 

 masses of coarsely crystalline porphyry in the vicinity of Bear 

 Butte Creek probably belong here. From these masses, which are 

 undoubtedly, in some cases at least, thick intrusive beds from which 

 the covering strata have been worn away, we pass naturally to 

 (2) The more normal type of intrusive beds, which are exposed 

 abundantly in canon walls and by various mining operations, and 

 usually range in thickness from five to perhaps fifty feet. They are 

 found chiefly in the Potsdam sandstones and shales, with which 

 they are usually perfectly conformable. It is not uncommon to 

 find, as in the district on the east side of Terry Peak known as 

 the Ruby Basin, from four to six intrusive sheets in one continuous 

 vertical exposure ; the thickness of the Potsdam partings, in 

 some cases, scarcely exceeding that of the eruptive layers. (3) 

 True dikes of porphyry, the feeders of all these intrusive sheets 

 and cones, traverse both the horizontal sedimentary rocks and the 

 underlying Archsean schists in every direction. 



That these three structural types — cones, sheets and dikes — are 

 parts of one connected system and merge into each other must be 



