collier.] REPORTED LODE DEPOSITS. 27 
2,308 feet above the sea. It stands on a well-marked plateau surface 
that has an elevation of 1,000 feet. This plateau has been correlated 
with the Kugruk Plateau, and is due to an earlier era of erosion than 
that which produced the York Plateau. a 
The sedimentary rocks surrounding Ear Mountain consist mainly 
of quartzites and dark slates, which resemble the slates near York and 
have been correlated with them. The core of the mountain is a gran- 
ite boss or stock intruded in these slates. Radiating from the main 
granite mass there is a fringe of intrusive quartz-porphyry and rhyolite 
dikes which are regarded as offshoots from the main intrusion. b 
The granites of the main mass are coarsely crystalline and consist 
essentially of quartz, orthoclase, and biotite. A specimen from one of 
the smaller bodies, examined microscopically, is made up essentially 
of quartz and of orthoclase and plagioclase feldspars. A narrow dike 
from the same region was found to consist essentially of quartz and 
feldspar, with muscovite, largely secondary, and a secondary growth 
of feldspar surrounding the larger orthoclase ciystals. In Ear Moun- 
tain a platy structure brought out by the weathering gives the rock a 
stratified appearance. 
Tin ore has been reported to occur in this region, and it is probably 
true that some cassiterite has been brought out by prospectors. The 
specimens of supposed ore which were submitted to the writer con- 
tained, however, only traces of tin, though some of the minerals often 
associated with its ores were present. On the north side of the moun- 
tain quartz-porphyry dikes can be traced for considerable distances. 
Several specimens of these rocks have been carefully examined in the 
laboratories of the U. S. Geological Survey. Apparently they were 
originally rhyolites or quartz-porphyries, but in thin sections they 
show considerable alteration. In one case the porphyritic texture of 
rhyolite remains, but the minerals, especially the feldspar phenocrysts, 
are partly replaced by tourmaline and pyrrhotite or magnetic pyrite, 
as shown on PI. VI. In this case the tourmaline was probably first 
introduced and was followed by the pyrrhotite. No cassiterite has 
been identified in the section. In another section the original texture 
is completely obliterated and the rock consists essentially of tourma- 
line in radiating groups of crystals surrounded by a groundmass made 
up principally of calcite with some quartz (PI. YII^ A). Magnetite 
and biotite seem to be present in small amounts, and probably also 
cassiterite, though it has not been detected in the thin sections. This 
specimen resembles in texture the luxullianite c from Cornwall, but 
differs from it in composition, since the groundmass of the typical 
a Collier, A. J., Prof. Paper U. S. Geol. Survey No. 2, p. 35. 
bCollier, A. J., op. cit., p. 30. 
cHarker, Alfred, Quart. Jour. Geol. Soc. London, 1895, vol. 51, p. 141. Rosenbush, H., Mikroskop- 
ische Physiographie der Massigen Gesteine, vol. 2, pt. 1, p. 50. Kemp, James Furman, Handbook of 
Rocks, p. 32. 
