1894. PROCEEDINGS OF THE NATIONAL MUSEUM. 661 



abundant sprinklintr of a dark-greenish black hornbleudic mineral in a 

 gray feldspatliic base is readily recognized. 



In the thin sections tlie most abundant constituent is hornblende, in 

 broad plates of green color and fibrous aspect such as to at once sug- 

 gest that they are wholly secondary, that is, uralitic; accompanying 

 tliese are small, irregular flecks of brown mica, which is also secondary. 

 The groundmass of the rock is composed of badly kaolinized feldspars, 

 in part at least plagioclastic, and granular quartz. Frequent patches 

 of a dirty brownish amorphous matter, acting between crossed uicols 

 like a gum, are evidently residual products from the decomposition of 

 titaniferous iron. Mineralogically the rocks may be classed as quartz 

 diorites, but I am inclined to regard them as altered diabases. 



Forpliyrite. — On the eastern si<le of the valley, in the upper valley of 

 Bear Creek, the eruptives occur in the form of three sheets of porphy- 

 rite, a liparite, and two inconspicuous outcrops of a dense greenish 

 basaltic rock closely related to that described as occurring near Fort 

 Ellis. Between Bear and Indian Creek, to the southward, are extru- 

 sions of basalt. The most conspicuous eruptive on this side of the 

 valley is that ibrming the mass of Lone Mountain, and which is found 

 in the form of sheets and dikes in the Cretaceous and older beds of 

 the surrounding liills. As exposed in the canon of Cedar Creek (Nos. 

 72S(iO and 72880, U.S.K.M.), the mass is evidently laccolitic in Cre- 

 taceous sandstones. The entire thickness of the mass as exposed can 

 not be less than 3,000 feet. In its most conspicuous development the 

 rock is a compact light-gray hornblende porphyrite, with both horn- 

 blende and feldspars sufficiently developed to be recognizable by the 

 unaided eye. Black mica is commonly i^resent, and near the lower 

 contact this mineral prevails, to the entire exclusion of the hornblende. 

 [ISo. 72880, U.S.N.M.) This variety of the rock is fui'ther character- 

 ized by abundant rounded blebs of quartz. 



In the thin section the prevailing type shows a groundmass varying 

 from densely microlitic or felsitic in samples from near upj)er contact 

 to finely microgranular in specimens more remote. Phenocrysts of 

 striated feldspars and green hornblendes are abundant, and occasion- 

 ally rounded blebs of quartz occur. The feldspars are in most sec- 

 tions opaque through decou) position, and an abundance of secondary 

 calcite indicates that t'hey belong to a lime-rich variety. The mass, 

 as shown by specimens collected at various points, is very uniform 

 throughout in structure and mineral composition. i«[ear the lower 

 contact, as found in the canon of Cherry Creek, it becomes a dense, 

 almost porcelain-like rock, breaking with a beautiful conchoidal frac- 

 ture (No. GG928, U.S.K.M.). This variety bnovvs under the microscope 

 a dense felsitic groundmass, with many small, rounded, and wedge- 

 shape bits of quartz and feldspars. Through weathering, the upper 

 portion of the peak has become hollowed, out so as to resemble a vol- 

 canic crater broken down on the side facing the valley. The rock is 



