collier.] GENERAL GEOLOGY. 15 
IGNEOUS ROCKS. 
Two distinct types of igneous rocks are present, one of which is basic 
while the other is acidic. The first group includes basic dikes and sills, 
all more or less altered and sometimes schistose, which may be grouped 
together under the general name of greenstones. The greenstones and 
greenstone-schists include a number of more or less altered intrusive 
masses, and occur most frequently in the slates near the contact with the 
limestone which forms the York Mountains. Bowlders of this rock 
are widely distributed in the gravels of the region. Under the micro- 
scope they appear, for the most part, to be altered gabbros. They 
are often called granite by the miners, but can readily be distinguished 
from the true granite by a general green color and the absence of 
quartz. This distinction is of importance, for, so far as known, no 
tin deposits have been found in association with the greenstone. 
The second group consists of more acid rocks and includes a number 
of large masses of granite together with dikes of a fine-grained, por- 
phyritic rock containing prominent quartz crystals. These dikes often 
form a fringe surrounding the larger granite masses, of which they are 
probably offshoots. Granite masses of the same type occur in occa- 
sional outcrops from Cape Prince of Wales northeastward for over 
100 miles, and form a zone which also finds a western extension in the 
Diomede Islands and possilnV in the granites on the Siberian coast. 
In the York region these rocks find their greatest development in 
Cape Mountain, where a great stock of granite is intruded into the 
limestone. The Cape Mountain granite is coarsely crystalline, some- 
what porphyritic, and consists essentially of quartz, microcline, and 
biotite, but contains as accessory minerals, albite, muscovite, zircon, 
apatite, tourmaline, pyrite, and lluorite. 
At Brooks Mountain, which is largely made up of slates, a number 
of dikes of granitic and rhyolitic rocks were observed, but these have 
not yet been studied microscopically. A few miles to the south, near 
Lost River, a number of granite and rhyolite intrusions in the lime- 
stone have been examined and will be described in some detail in con- 
nection with the Lost River tin deposits. The granites of this region, 
and especially those at Lost River, have been considerably altered and 
have taken various forms to which the name "greisen" has been 
applied because of their similarity to the vein rocks of the tin deposits 
of Cornwall and Saxony. The typical greisen of Saxony is a granite 
made up of quartz and lepidolite, or lithia mica, with fluorite, tourma- 
line, topaz, and cassiterite in small amounts. 
The distribution of the granite intrusives is of the greatest economic 
importance, since many of the known lode deposits of tin occur in 
granite dikes. The prospectors of the region have readily recognized 
this and have made careful search along these contacts. 
