hansome] DEPOSITS OF RED MOUNTAIN REGION. 247 
fined within the mass of the lode, the relation of the latter to the wall 
rock could not be studied. 
Silver Led <j< mine. — This property, situated about half a mile north 
of Chattanooga, and operated by the Ledge Mining and Milling 
Company, is worked through a shaft about 400 feet in depth, sunk 
in the bed of Mineral Creek. Most of the work at present is on the 
fourth level, at a depth of 3G0 feet, and in the stopes immediately 
above it. The workings are all in rhyolite. This rhyolite forms a 
considerable area extending to the westward of the mine with a 
rather irregular boundary. Its eastern limit is very close to the mine 
workings. For more detailed accounts of this rhyolite mass the 
reader is referred to the forthcoming work of Mr. Cross. The eon- 
elusion reached by him at the present writing is that it represents an 
area of the Potosi rhyolite 1 which has been faulted down into the 
surrounding rocks. If this view is correct the rhyolite is limited in 
depth. There are no data now available for even a rough estimate 
of the thickness of this rhyolitie mass, but if mining be pushed deep 
enough it will be passed through. If, on the other hand, the rock is 
intrusive, it may extend to practically unlimited depth. 
There are certain features about the rock itself which sustain Mr. 
Cross's hypothesis. It has many of the characteristics of the sur- 
ficial rhyolite flows of the Potosi series, and, as far as could be seen 
in its altered condil ion near 1 he mine, has not t he crystalline st ructure 
that might naturally be looked for in an intrusive mass <>r such size. 
Moreover, on the 4oo level of the Silver Ledge occur masses of a 
compact, slightly pinkish rock, which are sometimes fairly distinct 
from the white altered rhyolite and sometimes appear to grade into 
it. This pinkish rock occurs as isolated patches or nodules in the 
rhyolite, and al one point forms a considerable mass, of which the 
bottom is not yet exposed. Under the microscope this rock shows 
numerous small grains of quartz in a \^vy minutely crystalline seri- 
citicgroundmass. It is. withoul xrvy much doubt, a devil rified glassy 
rhyolite or rhyolitie tuff. Such material could hardly occur in such 
a place if the rhyolite were intruded. It strongly suggests volcanic 
or effusive conditions. It i> eveu possible that the appearance of 
this material indicates an approach to the bottom of the rhyolitie 
mass. 
The work of Mr. Cross has established tin 1 existence along the 
eastern border of the rhyolite mass of a fault which, just southeast 
of the mill, brings the rhyolite against andesite on the east. The 
line of this fault would carry it not far east of the main shaft, and it 
was thought that a crosscut which had been driven 100 feet to the 
eastward from the 400 (360 feet) level might be found passing through 
this fault fissure into andesite. About 70 feet east of the shaft the 
crosscut does pass through a strong gouge-filled fissure, but the 
1 See the Telluride folio. No. 57. 
