and on the Origin of Granites. 33 
what more the action of chemical agents, such as hydrcfluoric acid 
or caustic potash, it is easy to see that this slight difference depends 
only upon the great cohesion which a more perfect crystallization 
determines. In fact, rock-erystal and silica, both reduced to an 
extremely fine powder, do not present any difference. It is 
almost beyond doubt that the varieties of compact and crystal- 
line silica, such as flint, agate, and siliceous woods, have been 
formed in the humid way. The preservation of the ligneous 
structure in these last, the presence of infusoria in flint, signal- 
ized by Ehrenberg, the transformation of a great number of 
fossils into flint, are sufficient proofs. A great number of facts 
also prove that rock-crystal and ordinary quartz, which have the 
same density, can only have been formed in the humid way, 
or at least by the influence of water. 
We have succeeded by various modes of treatment, but only 
in the humid way, in obtaining crystallized silicic acid in the 
form of quartz. M. de Sénarmont has obtained this result by 
heating, in a closed vessel at 200° or 300°, a solution of silicic 
acid in water acidulated by carbonic or hydrochloric acid. 
M. Daubrée has obtained silica in a crystalline state by the 
action of the vapour of water on chloride or fluoride of silicium at 
a red heat ; afterwards he obtained it in distinct crystals by the 
action of water upon glass, under the influence of an elevated 
temperature and high pressure. 
The frequent association, in several formations, of crystallized 
quartz with silica in a compact crystalline state, also shows that 
it must have been formed in the humid way. 
On the contrary, notwithstanding several attempts, we have 
never succeeded in obtaining by means of heat crystallized or 
compact crystalline silica. 
But the strongest argument against the supposition that quartz 
has passed through the state of igneous fusion before its crystal- 
lization, is found in the fact that quartz passes by fusion into 
the amorphous condition whose density is 2:2. 
. The fusion of quartz has often been effected, by Davy, Clarke, 
Stromeyer, and Marcet, more recently also by Gaudin and De- 
ville. In every case, after this fusion, silica is completely amor- 
phous and vitreous, its density being 2°2. If the objection is 
raised, that fused quartz might have passed into a crystalline 
state by an extremely slow cooling, one may reply that it is im- 
possible that a granitic mass can have cooled with the same slow- 
ness throughout its whole extent ; there must necessarily have 
been portions exposed to a more rapid cooling ; so that here and 
there the amorphous modification of quartz ought to be found, 
But its presence has never been detected in rocks. It has some- 
times been supposed that the crystallization might have been 
Phil. Mag. 8. 4. Vol. 19. No, 124. Jan. 1860, D 
