DESCRIPTIVE GEOLOGY. 27 
and the composition of the rock is transitional to that of the mon- 
zonitic rocks. Rocks similar in composition to those of Wolverine 
Mountain occur also in Lynx Mountain along with monzonitic rocks, 
but their outcrops were not observed. 
A porphyritic biotite granite is found in the vicinity of Baker Hot 
Springs, where it is intrusive, in black carbonaceous slates. 
MONZONITIC HOCKS. 
The most common intrusive in the Rampart region is a monzonitic 
rock that varies in color from dark brown to nearly black. It is a 
medium to fine-grained rock, and the coarser varieties show abundant 
plates of reddish-brown mica, the most striking mineral present. All 
the minerals of this rock are fresh and include about equal propor- 
tions of an orthoclase feldspar and of plagioclase, which is embedded 
in the irregular limpid mottled grains of potash feldspar, abundant 
pale-green monoclinic pyroxene, biotite, and a small proportion of 
olivine. Hypersthene occurs frequently, its small prisms often fring- 
ing the grains of olivine, at the expense of which it has probably been 
formed. There is some apatite and often much magnetite. 
This rock occurs in Lynx Mountain and in the ridge at the heads 
of Glenn, Rhode Island, and Omega creeks, where it forms a mass of 
considerable extent. The numerous small dikes of minette-like rock 
in the slates, from 1 foot to 3 feet thick, containing prominent plates 
of bleached biotite and a large proportion of nearly colorless pris- 
matic crystals of monoclinic pyroxene, are probably to be referred 
to this type. 
A portion at least of the granular rocks of intermediate composi- 
tion in the Rampart region are intrusive in Upper Cretaceous rocks 
and it is probable that all of them are of the same general period. 
GREENSTONES. 
The greenstones include serpentine, altered gabbro, diabase, basalt, 
and much tuifaceous material, and have at many places been intruded 
by fresh diabasic rocks. Some show clearly their mode of origin, 
others are indefinite aphanitic chertlike rocks. They occur abundantly 
in the area lying between the White Mountains and Rampart. Their 
dark color contrasts strongly with the associated limestone- of the 
White Mountains. They form the prominent ridges aero-- Beaver 
Valley west of these mountains and occur in the area between this 
ridge and Yukon Flats. Farther west they become prominent in 
the ridge north of Hess Creek. In the Rampart region they form 
the bed rock in the lower part of Troublesome Valley and are the 
most, widely distributed rocks in the lower valley of the Minook below 
the mouth of Florida Creek. 
