ORIGIN OF GRANITE. 15 
scopic cavities in the quartz of granite is alone evidence that the 
rock was formed under pressure. The amount of contraction that 
this inclosed liquid has suffered in cooling has afforded a basis for 
estimating not only the amount of heat under which the rock began 
to form, but also the pressure under which it solidified. That the 
temperature at which granite solidified was comparatively low lias 
been inferred from the fact that it contains minerals which lose their 
physical properties at temperatures higher than dull-red heat. The 
relations of the mineral constituents of granite to one another show 
the order in which they must have crystallized. This order differs 
from that in which they would crystallize if molten in a dry state, 
but laboratory experiments have shown that the presence of even a 
small quantity of water suffices to change that order of crystalliza- 
tion; The presence of superheated water in the formation of granite, 
inferred from the arrangement of its minerals, and the pressure indi- 
cated from a study of the vacuities in the microsc^ pic cavities of its 
quartz show that the conditions requisite to its formation included 
not only the pressure of a great overlying mass of rock but also 
powerful expansive pressure from below. Had this molten matter 
been extruded at the surface it would have cooled so rapidly that 
but few of its constituent molecules would have had time to arrange 
themselves in geometric order. The process of crystallization would 
have been arrested by the sudden passage of the material into the 
solid state, and the product would have been a volcanic glass some- 
what resembling that which forms cliffs in Yellowstone National 
Park. In granite, however, the mass has cooled slowly enough to 
permit the complete crystallization of the originally molten glass- 
like matter, and no unarranged molecules remain. 
The overlying rock mass which furnished so large a part of the 
pressure required to form granite has at many places been removed 
from it by erosive processes that operated through great stretches of 
time. Indeed, it is only by the removal of this mass that granite is 
anywhere naturally exposed. Although this mass may have measured 
thousands of feet in thickness, its former presence is at some places 
attested only by a thin capping on the granite or by fragments which 
the lacerating action of the intruding granite has incorporated into 
itself. A remnant of this capping -occurs at the Waldoboro quarry 
and an inclusion of it at the Freeport quarrv. (See Pis. IX, A/ 
VII, B.) 
The lacerating effect of an intrusive eruption and the subsequent 
erosion of some of the overlying strata have been reproduced experi- 
mentally." The conversion of granite itself back into a material 
which upon cooling under ordinary conditions has proved to be a 
° Howe, Ernest Twenty-first Ann. Rept. U. S. Geol. Survey, pt. 3, pp. 294-296," PI. XLIII. 
