44 GOLD AND TIN DEPOSITS OF SOUTHERN APPALACHIANS. 
In former descriptions of the Carolina tin deposits reference has often been made to the 
fact that the strike of the tin belt, if prolonged, would just about pass through the tin 
locality of Rockbridge County, Va., and from this single fact some have gone as far as to state 
that the two deposits are probably related. Largely with the idea of testing the correctness 
of this view, the writer visited the Virginia locality. Owing to lack of time and the fact that 
no work has been done at this place for a number of years, a thoroughly satisfactory idea of 
the deposits at the Cash mine was not obtained. But this much may be said. Cassiterite 
occurs there in quartz veins cutting a coarse granite. The veins are well banded and con- 
tain, besides cassiterite, bands of pyrite and arsenopyrite. The granite walls are converted 
into greisen, being composed of quartz, fine scaly muscovite, and a small amount of cas- 
siterite. 
From the standpoint of mode of occurrence and genesis, therefore, the Virginia and Caro- 
lina deposits have almost nothing in common. Moreover, the geologic instead Of geometric 
continuation of the Carolina tin belt passes far to the east of the Virginia deposits. 
AGE OF THE DEPOSITS. 
Since the granite and pegmatite are considered to be of pre-Cambrian age, the tin deposits 
are likewise pre-Cambrian. They have therefore been subjected to the various agencies of 
disintegration and decomposition through an enormous length of time. 
The first cassiterite discovered in the Carolina region occurred as loose pieces unattached 
to any rock in place, and during the first five years that the existence of tin was known 
nothing hut this float tin was found. Since the discovery of cassiterite in the rock in place 
nearly all prospecting and primary development work has been guided by the occurrence 
of pieces of float. In a few places, as at the Ross mine and the Jones mine, the amount of 
cassiterite scattered through the soil has been sufficient to encourage efforts for its recovery, 
and a comparatively small return has been obtained. A little panning has been done in 
the beds and flood plains of some of the streams, but the results have not been promising. 
All this float or placer tin is directly traceable to the original deposits in dikes. As the 
rocks are decomposed and disintegrated, the cassiterite of the dikes, being very resistant 
to alteration, is freed from the rock and on removal of the other materials is left behind 
because of its high specific gravity, being in this way gradually concentrated. 
It is known that since the formation of the tin deposits this region has been subjected to 
extreme degradation. All the phenomena of intrusion, contact metamorphism, and vein 
formation which have here been observed point to the fact that these porcesses went on 
under great pressure and therefore far below what was then the surface. The physiographic 
evidence in the planation of this much-folded region is in the same direction. It thus 
appears that extensive portions of the tin-bearing dikes have been removed. Much 
cassiterite must thus have been liberated from the rock and remained close to the original 
ore bodies as placer deposits. It is probable that more thorough prospecting of stream 
gravels along the tin belt will result in the discovery of small amounts of the mineral; but 
such search as has been conducted leads one not to expect the presence of extensive placers. 
The question then arises as to what has become of the large amount of tin which must have 
resulted from the disintegration and degradation of the tin-bearing rocks. It is believed 
that the answer to this question lies mainly in the fact that the cassiterite has been dis- 
solved and thus carried away. 
While from the customary standpoint of the laboratory chemist cassiterite is pronounced 
insoluble, it nevertheless does dissolve very slowly, even in pure water.a .The oft-cited 
case of pseudomorphic replacement by cassiterite of the material of deer horns & shows 
that in a comparatively short interval of geologic time marked solution will take place. It 
aDoelter, C, Tschermaks Min. u. petrogr. Mittheilungen, vol. 11, p. 325. 
b Collins, J. H., Trans. Roy. Geog. Soc. Cornwall, vol. 10, pp. 98-100. 
