TIN DEPOSITS OF THE YORK REGION, ALASKA. [no. 229. 
granite boss. It is a white, porphyritic dike, cutting the Port Clar- 
ence 1 limestone, and striking nearly east and west. It has been traced 
from Tin Creek westward across the mountain to Cassiterite Creek, a 
distance of about 1 mile, but has not been found beyond these streams 
in either direction. All of this rock has been more or less altered, so 
that it is practically a greisen having crystals of cassiterite dissemi- 
nated through it. Specimens collected near Tin Creek appear, in the 
hand specimen, to be a white aplite or porphyry with some small spots 
and large patches of purple. Under the microscope many of the origi- 
nal minerals are seen to have been replaced by fluorite, to which the 
purple color is due. Pseudomorphs of fluorite take the place of most 
of the feldspar crystals and of some of the quartz grains. (See PL V.) 
In specimens which are still more altered, collected from the same dike, 
near Cassiterite Creek, probably very few of the original minerals 
remain. The rock here is found to consist of calcite, fluorite, lithia 
mica, and quartz, proportioned in the order named. The limestone, 
on the south side of the dike, is altered for several hundred feet, and 
contains man y greenish minerals, amongwhich epidote and garnet have 
been identified. The limestones north of this dike are reported to 
contain many small stringers of tin ore for several hundred feet. The 
ore obtained from the main ledge varies considerably in general appear- 
ance and character. Some of the weathered ore from the croppingsis 
highly siliceous, and has the appearance of weathered, iron-stained vein 
quartz with small black cassiterite crystals disseminated through it, 
while other specimens show clearly their granitic origin and contain 
comparatively little vein quartz. In the ore of the latter type the 
cassiterite occurs both as disseminated crystals varying in size from 
that of a pin head to that of a walnut and as veinlets and irregular 
masses. (See PL YI1,B.) The granitic ore consists principally of calcite, 
fluorite, quartz, and large crystals of lithia mica; and in addition to the 
cassiterite, tourmaline, topaz, pyrite, garnet, and galena were observed 
in small amounts. Quantitative analyses of the lithia mica present 
made by W. T. Schaller, of the United States Geological Survey, show 
that it has the composition of zinnwaldite. In the float of this dike 
large specimens of galena, wolframite, and some malachite were col- 
lected, and in the altered limestone near the contact some large speci- 
mens of garnet were obtained. The siliceous ore mentioned above, 
when examined with the hand lens, sometimes showed spangles of 
free gold. A sample of this ore assayed a for gold and silver gave 
0.36 ounce of gold per ton and a trace of silver. The piece assayed 
was a picked specimen, and not a commercial sample. Assays made 
for other parties are reported to show smaller amounts of gold in all 
cases. The occurrence of so much gold associated with the cassiterite 
seems to be unusual in tin ores, and merits further investigation. 
a Assay by E. E. Burlingame & Co., Denver, Colo. 
