Hawortli on the Archcean Geology of Missouri. 395 
tions were carried on in an almost fruitless search for silver. 
At the time of my visit to the place the works were abandoned, 
the shafts and tunnels filled with water, so that almost the only 
available material was in the " dump pile. " A fourth of a mile 
back from the river a shaft had been sunk upon the vein and 
named the "Apex shaft." The vein itself was filled with quartz, 
argentiferous galena, fluorite, lepidolite, wolframite, and prob- 
ably other minerals. The wall-rock was originally granite. At 
present, however, it consists of quartz imbedded in a fine-grained 
mixture of mica scales with traces of iron oxide, leucoxene, 
beautiful little zircon crystals, and probably other materials. 
Scattered through this mass in varying proportions are the 
minerals fluorite and topaz. 
We evidently have here the results of a fumerole action, 
recalling in some respects the conditions around the tin mines 
of Cornwall, Eng., of Zinnwald in Bohemia, and those of other 
places. The resemblances consist in the granite wall-rock 
being decomposed for a few feet on each side of the vein, and 
the occurrence of topaz, wolframite, lepidolite and fluorite, 
minerals which are associated with tin ores in other places. 
But so far as now known there is a total absence of cassiterite, 
as well as of tourmaline, and other minerals — excepting those 
just given — which always accompan}'^ tin deposits. 
Topaz. — In all the thin sections examined from the wall- 
rock of the vein just described a considerable quantity of a 
mineral thought to be topaz was found. It does not have a 
regular cTystalline form, but in many cases the direction of the 
crystallographic axes could be determined. Its index of refrac- 
tion, as shown by its apparent thickness, its parallel extinction, 
and its polarization colors strongly implied that the mineral in 
question was topaz. Portions of it were isolated by first mak- 
ing a separation with the Thoulet solution, sp. gr. 3.12, and then 
treating the powder thus obtained with the strong, hot acids. 
This dissolved everything but the mineral in question. Each 
of the three acids was tried in turn, and finally aqua regia, but 
after boiling no less than an hour the little grains were un- 
affected. A blow-pipe examination with the salt of phosphorus 
proved it to be a silicate. There can be little doubt, therefore, 
that the mineral is topaz. 
