48 
POPULAR SCIENCE REVIEW. 
when this fluid is evaporated off. Thus^ in each cubic foot of 
such quartz there is more than half a pint of water. The 
water is not generally pure, but holds in solution common 
salt, or the sulphates of potash, soda, and lime. It should 
be added that no two granites are alike in the number or 
magnitude, nor in the nature of the fluid contents in the 
cavities of their quartzy portions. 
In contradistinction to this condition of granite, it has been 
found that rocks melted and cooled near the earth^s surface 
away from the presence of water, or without pressure (such as 
lava), have cavities ; but that cavities in such rocks are filled 
with gas and not fluid. The water would seem to be caught 
up in the act of crystallization just as now the same result is 
produced when we crystallize salt, and thus the temperature at 
which the solidification took place may be estimated. 
Granite rock, however, is often seen not only in mountain 
masses, but occup3nng narrow and tortuous cracks in various 
rocks, and even injected, as it were, into other varieties of 
granite. Examples of this kind have been quoted again and 
again in proof of the identity of granite with lava, or at least 
of a common origin to the two. It need hardly be said that 
these appearances, however striking, must not be explained by 
any hypothesis that does not meet all known facts, and among 
them the presence of water in the cavities of the quartz. It 
must be admitted that whatever be the case with lava and with 
rocks containing no water cavities, the rocks that do contain 
water must have crystallized in contact with water. By test- 
ing the temperature at which the water of the cavities becomes 
steam, we learn the limits of temperature during the formation 
of the rock. A possible pressure of miles of water and rock 
and a high equable temperature must not be omitted in esti- 
mating the circumstances of formation of such rocks. 
But whatever may have been its origin, granite has often 
been altered and metamorphosed since its original formation. 
Its crevices and veins have not only been filled with crystals, 
but these in many cases have been entirely changed more than 
once, the new mineral sometimes assuming the form of some 
more ancient crystal, just as the hermit crab adapts itself to 
the old shell of a whelk. 
There are then no rocks that are not metamorphosed. All 
are changed, and in all the process of change is gradual, but 
incessant. The chalky mud has become converted by slow de- 
grees into soft chalk or limestone. This, at one time friable and 
stratified, has become harder and more compact, and has gra- 
dually lost its fossils and its stratified character. It has chang’ed 
into an imperfect marble, and sometimes into a perfect granular 
marble fit for statuary purposes. Its cracks and cavities have 
