ORE DEPOSIT THEORIES. 245 
typical greissen, or a mixture of quartz and mica, the 
feldspars having been completely replaced. Both in the 
vein-stuff and in the altered wall-rock a number of minerals 
are present which are typical of tin veins, namely, lithia- 
mica, topaz, fluorspar, wolframite, muscovite, &c. The 
veins themselves are composed largely of quartz carrying 
argentiferous galena and pyrites and the sulphides of iron 
and copper. Picked samples of the ore have gone as high 
as 300 ozs. to the ton, while the average of 50 assays was 46 
ozs. to the ton, according to Hawarth, the Missouri State 
Geologist. The nature of the alteration of the granite walls 
leaves no doubt that there was genuine fumarole action, 
during the operation of which vapours from below not only 
corroded the wall-rock, but also brought up in the volatile 
form different elements necessary for the production of the 
several minerals. 
In many mining districts tin veins have been shown to 
pass over into mineral veins of various types, and in some 
districts whole groups of varieties have been shown to have 
an intimate connection with tin veins and with one another. 
Thus, in the Freiberg district, in the Erzgebirge, there are 
two distinct groups of veins which have been proved to have 
been formed during separate periods of ore-deposition 
corresponding to two periods of eruptive activity. In each 
group there are several varieties of totally different mineral- 
ogical composition; but, whereas the two prime groups 
always keep separate, the different varieties of each group 
graduate into each other. Thus, in the older of the two 
prime groups the veins of the tin formation graduate into 
both the veins of the pyritic copper formation and of the 
pyritic lead formation, 7.e., cases have been recorded in 
which the tin veins change their character and gradually 
assume the character of the pyritic copper formation or the 
pyritic lead formation. In the same way the pyritic copper 
formation graduates into the pyritic lead formation, and the 
latter into the noble quartz formation, and this again into 
the veins of the brown spar formation. But no case has 
ever been recorded of one of the varieties belonging to the 
older prime group graduating into one of the varieties of 
the younger prime group. Perfectly analogous relations 
exist between the varieties of the older and the younger 
prime groups in other mining districts of the Erzgebirge, 
such as Schneeberg, Annaberg, and Altenberg, so that it is 
evident that throughout the whole of the Erzegebirge there 
were two distinct periods of ore-deposition, in each of which 
a number of varieties of veins were formed. 
A direct genetic connection between the varieties of each 
period has been proved, and among the varieties of the older 
