YORK TIN REGION. 151 
GEOLOGY. 
As has been stated, Cape Mountain is a large mass or boss of granite, thrust up through 
Carboniferous limestone. It is 2,300 feet high, about 5 miles across in an east- west 
direction, and something less north and south. The upper 400 to 450 feet of the mountain 
is a dark quartz-sericite-schist, separated from the granite by 10 or 12 feet of thin-bedded 
limestone in alternating dark and white bands an inch or two thick. The white strata are 
due to the limestone having been changed to wollastonite (a lime silicate), probably by 
siliceous solutions accompanying the intrusion of the granite. In the darker strata the 
color is probably due to the segregation of carbon from the more metamorphosed portions, 
as it is much darker than the unaltered limestone. 
From the main mass of the granite, dikes and sills varying greatly in size cut the lime- 
stone in all directions. The limestone is often altered to wollastonite for a distance of sev- 
eral feet in each direction from sills and dikes but a few inches thick. There is such an 
occurrence on the shore about a mile west of Tin City, where a sill of alaskite (quartz and 
feldspar) about 18 inches thick has altered the limestone in this way both above and below 
through a distance of about 3 feet. Radiating crystals one-eighth inch in thickness and 4 
inches long run directly across the bedding, and the washing of the waves has made them 
stand out in distinct relief. At other places the wollastonite forms delicate radial bunches 
of crystals lying in the bedding planes of the limestone. There are often thin layers of mar- 
ble in the wollastonite of a faint blue color. At one place on the northeast side of the 
mountain the limestone above a granite dike has been replaced by silica over an area that 
was crossed in one direction for perhaps 200 feet, though its other dimensions are unknown. 
This occurrence will be referred to again. There is occasionally a little fluorspar in some of 
the dikes, but it is comparatively rare in this district. There are also occasional small segre- 
gations of glistening white muscovite mica, which has been taken for lithia mica, but no 
lithia could be detected by Schaller in specimens collected by the writer (specimen T5AH20). 
Basaltic dikes, from 3 or 4 feet to 30 feet in width and composed almost wholly of olivine, 
plagioclase, and magnetite (specimens and slides T5AH4 and T5AH19), cut both granite 
and limestone. They are occasionally amygdaloidal with the amygdules filled by zeolites. 
The rock is fresh, black, and hard, and is accompanied by little contact phenomena. The 
basalt seems to b:> latex than the tin deposits, and so it is not likely that any tin will bo 
found in connection with it except as it may happen to cut across tin veins or tin-bearing 
rocks. « 
JOINTING AND FAULTING. 
The granite is very much jointed, the two most prominent series of vertical joints run- 
ning N. 60° E. and N. 50° W., while many lesser joints run in directions between these. A 
system of almost horizontal parallel joints gives the granite a platy structure in places. 
Other joint planes are inclined at various angles. The two main series of joints cause the 
granite to weather into numerous columns left standing upon the shoulders of the mountain, 
giving them an appearance of being capped by rows of ruined factory chimneys. These 
columns vary considerably in height and thickness, reaching a height of 30 feet with a base 
of about 4 by 8 feet, while others are much shorter and thicker. Between the columns the 
jointing seems to have been so close that the granite weathered readily and crumbled out, 
leaving the less jointed, more solid portion standing. The general level of the shoulders is 
remarkably even and furnishes an interesting example of how evenly ridges may erode 
under ordinary subaenal agencies. The height to which the columns reach shows probably 
only a part of the rock removed. 
Faulting is very frequent, and although the amount or direction of throw can not be deter- 
mined the accompanying crushing is sometimes considerable. One mile west of Tin City, 
at the contact of the limestone and granite, there is a crushed zone 4 feet in width along 
which a tunnel has been driven about 80 feet to prospect a sill of alaskite that carried some 
pyrite and tourmaline. Assays are said to have given values of from $3 to $180 per ton in 
gold, but these values did not last as the tunnel advanced. In other places in the area 
Bull. 284—06 11 
