156 M. Daubrée on the Deposition, Composition, and 
rock ; and when the latter is schistose, as is the case with the 
killas of Cornwall, we perceive the veins cutting across the 
lamina of the slate in a very obvious manner; distinct sal- 
bandes, besides, separate the ore, so that, to the least expe- 
rienced eye, it is evident that the tin ore is more modern than 
the formation in which it occurs; that the latter, after being 
formed, was rent, and the fissure afterwards filled with the 
tin and its matrix. But a contrary origin has often been ad- 
mitted for the massive deposits, and some geologists still be- 
lieve that the tin is separated from the mass of the rock by 
simple crystallization, or that it has, so to speak, percolated 
across. 
M. Daubrée proves that, in the masses, as in the veins, the 
formation of the oxide of tin is more modern than the en- 
closing rock, even when this mineral is found disseminated 
throughout the very mass of the rock in invisible portions, as 
in the granite of Geyer. This arrangement takes place only 
in certain places, which together form a determinate zone, a 
kind of cap which envelopes the rock on all sides. There is 
therefore a difference of origin in the granite and tin; and 
what proves this difference is, “‘ that when the granite is stan- 
niferous it loses its ordinary nature, its felspar disappears, it 
passes into a rock chiefly quartzose, with a little mica arranged 
like small veins ; there is even a connection between the hyalo- 
micte (granite without felspar) and the presence of tin, as if the 
penetration of the oxide of tin in a granite had been followed 
by the removal of the felspar.””. The difference of origin be- 
comes still more obvious when we study the small veins of the 
oxide of tin which always exist in the best characterized masses. 
That of Geyer, which we have mentioned as an example of the 
intimate penetration of tin in granite, likewise presents nume- 
rous little veins, which, although distinct at first, diminish in 
thickness by degrees, and at last become confounded with the 
mass; but in such places, when they vary from 1 to 5 centi- 
metres in thickness, they offer all the distinctive characters of 
veins. “ They are then composed principally of quartz and 
mica, and fully formed salbandes are observed in them.” 
These details, which might be much extended, therefore 
prove, that, notwithstanding apparent differences, there is still 
