24 GEOGRAPHY AND GEOLOGY OE RAMPART REGION. 
so that in a hand specimen most intricate foldings may be observed. Brecciation 
has been common, and the interstices of the fragments have been filled with fer- 
ruginous matter. The banding resembles sedimentation, but under the microscope 
the rock is found to be composed almost entirely of minute spherulites clouded with 
a very minute development of graphic intergrowths. The rock forms a sharply 
pointed prominent hill rising about 200 feet above the valley. Its relations to the 
surrounding rocks are not known, but small dikes of a rock composed essentially of 
quartz and feldspar phenocrysts occasionally occur in a fine-grained, partly graphic 
groundmass, and it is possible these are derived from the same magma. 
MONZONITIC ROCKS. 
The most common intrusive is a monzonitic rock which 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 ine-lude about equal proportions of an ortho- 
clastic 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 
fringing the grains of olivine. There is some apatite and often much magnetite. 
This rock occurs in Lynx Mountain and in the ridge at the head of Glenn, Rhode 
Island, and Omega creeks, where it forms a mass of considerable extent. The numer- 
ous small dikes of minette-like rock in the slates, from 1 foot to 3 feet thick, contain- 
ing prominent plates of bleached biotite and a large proportion of nearly colorless 
prismatic crystals of monoclinic pyroxene, are probably to be referred to this type. 
The coarse granular type is similar in composition and texture to the rock found in 
1902 by the Brooks party at one locality on the eastern side of the Alaska Range, and 
this occurrence in the Rampart region is the only one thus far found in the Yukon- 
Tanana country. 
GREENSTONES. 
The greenstones include serpentine, altered gabbro, diabase, basalt, and much tuf- 
faceous material, and have frequently been intruded by fresh diabasic rocks. Some 
show clearly their mode of origin. Others are indefinite aphanitic chert-like rocks. 
They occur throughout the area from the White Mountains to Rampart. Their dark 
color contrasts strongly with the associated limestones of the White Mountains; they 
form the prominent ridge across Beaver Valley to the west of these mountains, and 
occur in the area between this ridge and the 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 dis- 
tributed rocks in the lower valley of the Minook below the mouth of Florida Creek. 
The greenstones are partly intrusive and partly extrusive to the rocks in which 
they occur. Those in association with the limestone are, so far as has been observed, 
parallel to the structure, and furthermore some of them are altered basalts contain- 
ing numerous amygdules filled with calcite. Diabasic intrusives occur cutting the 
serpentine, and in the Rampart region intruding the Rampart slates. 
BASALT. 
A fresh olivine basalt occurs on Minook Creek about 1 mile above the mouth. On 
Hunter Creek, a short distance above the mouth, and apparently related to the basalt 
in their occurrence, are volcanic glasses containing basic feldspar phenocrysts. 
SUMMARY. 
The greater part of the area is occupied by closely folded sedimentary rocks, and 
most of these are regarded as of Devonian age. Folding and metamorphism appear 
