collier.] LODES ON LOST RIVER. 19 
The Lost River tin deposits are located on the east side of the north 
fork of Lost River. (See fig. 3.) The ore has been found on Cassiter- 
ite Creek and on another eastern tributary, known as Tin Creek, which 
enters Lost River about a mile below the mouth of Cassiterite Creek. 
The latter stream has a length of about 3 miles; its head is within 1 
mile of Cassiterite Creek, and after flowing parallel with Cassiterite 
Creek for about 1 mile it turns westward and enters Lost River from 
a deep canyon cut in the limestone of the York Mountains. At its 
mouth Cassiterite Creek is about 100 feet above the sea. In the latter 
part of July, 1903,- Lost River carried approximately 1,000 miner's 
inches of water. 
The York Mountains, in which the Lost River Basin lies, are com- 
posed almost wholly of ash-gray limestone of Silurian age, the Port 
Clarence limestone. Along Lost River the limestone shows little gen- 
eral metamorphism, and as a rule dips at low angles. From the 
coast to Tin Creek the strata generally dip to the north, and unless 
there are faults, which were not detected, a thickness of over 5,000 
feet of limestone must be exposed. Near the mouth of Lost River a 
section of these limestones lying nearly horizontal is exposed in a 
mountain, called by prospectors Saddleback, which has an elevation 
of more than 2,000 feet above sea level. Dikes of igneous rock cut 
this limestone at several places along Lost River, and a number of 
these were readily traced across the limestone by a growth of moss 
and other vegetation which formed over them, the limestone itself 
being utterly devoid of vegetation. Microscopic examination shows 
that these dikes are of rhyolitic nature. 
On Tin Creek, which enters Lost River from the east about li miles 
from the coast, a large body of granite was found intruded in the lime- 
stone. This granite outcrop is believed to be nearly circular in out- 
line and probably one-half mile in diameter. Around its margin the 
limestone was found to be considerably altered, and some small dikes 
of fine-grained pegmatite, probably apophyses from the main mass, 
were found cutting the limestone, apparently parallel with the contact 
of the limestone and granite. 
Under the microscope the granite from the main mass is found to 
consist essential^ of quartz, biotite, hornblende, orthoclase and acidic 
plagioelase feldspars with fluorspar, either accessory or secondary, 
and a few small grains of a mineral resembling zircon and believed to 
be cassiterite. Apparently the rock has been slightly crushed or 
sheared, producing streaks of fine-grained fragmental material of the 
same character as the original grains. 
In Tin Creek, which flows for some distance along this contact, man} 7 
bowlders and pebbles, some of considerable size, were found to con- 
tain minerals, which are the result of contact metamorphism. 
The main tin-bearing ledge outcrops nearly half a mile north of this 
