PROCEEDINGS OF SOCINTIES. - 191 
6. Crystals contaiming few cavities were formed slowly, in 
comparison with those of the same material that contain 
many. 
7. Crystals that contain no cavities were formed very 
slowly, or by the cooling from fusion of a pure, homogeneous 
substance. 
Applying these general principles to the study of natural 
crystalline minerals and rocks, it was shown that the fluid- 
cavities m rock-salt—in the calcareous spar of modern tufa- 
ceous deposits, of veins, and of ordinary limestone—and in 
the gypsum of gypseous marls, indicate that these minerals 
were formed by deposition from solution in water at a tem- 
perature not materially different from the ordinary. The 
same conclusions apply to a number of other minerals in 
veins in various rocks, and to many zeolites. The constituent 
minerals of mica-schist and the associated rocks contain 
many fluid cavities, indicating that they were metamorphosed 
by the action of heated water, and not by mere dry heat and 
partial fusion. 
The structure of the minerals in erupted lava proves that 
they were deposited from a mass in the state of igneous 
fusion like the crystals in the slags of furnaces; but, in some 
of those found in blocks ejected from volcanos (for example, 
in nepheline and meionite), there are, besides stone- and 
glass-cavities, many containing water, the relative amount of 
which indicates that they were formed, under great pressure, 
at a dull red heat, when both liquid water and melted rock 
were present. The fluid cavities in these aqueo-igneous 
minerals very generally contain minute crystals, as if they 
had been deposited on cooling from solution in the highly 
heated water. The minerals in trappean rocks have also 
such a structure as proves them to be of genuine igneous 
origin, but they have been much altered by the subsequent 
action of water, and many minerals formed in the minute 
cavities by deposition from solution in water. 
The quartz of quartz-veins has a structure proving that it 
has been rapidly deposited from solution in water; and in 
some instances the relative amount of water in the fluid- 
cavities indicates that the heat was considerable. In one 
good case the temperature thus deduced was 165° C. (829° F.) ; 
and apparently, when the heat was still greater, mica and 
tinstone were deposited, and in some cases probably even 
felspar. There is, then, as has been argued by M. Elie de 
Beaumont, a gradual passage from quartz-veins to those of 
granite, and to granite itself; and there is no such distinct 
line of division between them as might be expected if one was 
