38 Prof. H. Rose on the different States of Silicic Acid. 
The remarkable purity of the quartz of granites is also in op- 
position to this theory, which would make of this element, in a 
state of surfusion, a mother-liquor from which the other ery- 
stallized minerals of granites had by degrees separated them- 
selves. 
If the external appearance of granites is not that of a fused 
mass which has erystallized by slow cooling, like that of devitri- 
fied glass, neither have we ever succeeded in producing, by the 
slow cooling of a mass of fused granite, a mass of crystalline 
structure. In this manner vitreous masses similar to obsidian 
have alone been obtained. 
The author mentions on this subject an interesting experi- 
ment by his brother, M. G. Rose. When a granite rich in 
quartz is subjected to fusion, felspar and mica fuse and gradu- 
ally dissolve a part of the quartz, but a portion of the latter 
remains in the form of grains or nuclei m the middle of the 
vitreous mass. A granite very rich in quartz, from Warmbrunn 
in Silesia, having been subjected to the heat of a porcelain fur- 
nace, was transformed in this manner into a mass of obsidian, still 
containing white nuclei of quartz. These nuclei were carefull 
separated from the obsidian, when they were found to be formed 
of amorphous silica, whose density was from 2°34 to 2°35, and 
which was acted on by hydrofluoric acid with the energy peculiar 
to this variety. 
Lastly, M. Rose regards as a strong objection against the 
igneous origin of granite, the presence of minerals, such as 
Gadolinite, Allanite, &c., which at a temperature more or less 
elevated, suddenly present the phenomenon of incandescence, at 
the end of which they experience a permanent modification of 
their properties, and become less capable, or even incapable of 
being acted upon by acids. These minerals are found exclusively 
in granite rocks ; and their presence proves that these rocks have 
not been exposed to a temperature sufficient to determine their 
metamorphosis, which nevertheless generally requires only a dark 
red heat. 
M. Scheerer having remarked in some of these minerals an 
increase of density after their ignition, endeavoured to explain 
their presence in granite rocks, to which he attributes an 
igneous origin, by supposing that during the very slow cooling 
of these rocks, giving rise to a contraction to which the mass 
did not completely yield, the elements of these minerals assumed 
a peculiar state of separation and tension. This explanation 
falls before the fact verified by M. Rose, that all these minerals 
do not present this increase of density ; samarskite, for example, 
has a less density after its ignition. In order to meet the ob- 
jection which geologists might raise, that pressure might oppose 
