LITHOLOGY. 193 
with brilliancy, and are then found to have suffered marked physical 
changes in color, specific gravity, deportment with acids, &c., to be proof 
that the granite must have formed at a temperature not above that indi- 
cated by a dull red. 
Now, it has already been explained that the size of the bubble in a 
fluidal cavity is dependént on two factors, temperature and pressure, 
either of which being known the other can be calculated. Assuming 
the stated temperature as being correct, Mr. Sorby has calculated the 
pressure in a variety of cases, and finds it, though variable, to be always 
enormous. Mr. Ward* followed with more extensive calculations, using 
the same formule. Comparing the results with those obtained in the 
field, it was usually found that the pressures deduced from calculation 
are much larger than those which would result from the pressure of the 
superincumbent strata that can be proved to have rested upon the rocks 
investigated. If, for example, the calculations indicate a pressure equal 
to 50,000 feet of rock, but 30,000 can be found, etc. This indicates that 
granites are rocks which were formed deep down in the earth, and crys- 
tallized from a plastic condition, under pressure, which resulted in part 
from the weight of superincumbent strata, and in part from the lateral 
pressure which elevated the mountains. Although accuracy can scarcely 
be claimed for such calculations, their general agreement, when regarded 
as approximations, renders them valuable. Much more, which is very 
interesting, has been done to substantiate the accepted theory ; and espe- 
cially in this connection may be mentioned the experimental researches 
of Daubree, who has reproduced some of the circumstances in sealed 
tubes, and crystallized the minerals. Enough has been said, however, 
to define the method by which the members of our most important 
group of rocks were formed, and to explain the variety of circumstances 
in their mode of occurrence which is met with. 
Muscovite Granite. This is an uncommon rock, and the one specimen 
that I have to examine comes from Newcastle, where it occurs sparingly. 
It is therefore a rock of little importance to us. It would seem as if the 
entire removal of iron from sediments was usually accompanied by the 
removal of so many other constituents as rarely to leave the material for 
* Quart. Four. Geol. Soc., Nos. 124 and 125. 
VOL. Iv. 25 
