Microscopical Light in Geological Darkness. — Clay pole. 219 
ture than 88° F. for the globe at the time of the crystallization 
of the minerals, we must also assume a higher pressure than 
seventy-three atmospheres as one of the conditions prevailing 
during crystallization. 
This, however, is doubtless far too low for both. Instead 
of 88° Mr. Sorby and others consider the temperature of con- 
solidation to have been nearer 700° F. Mr. J. C. Ward, a few 
years ago, following in the footprints of Mr. Sorby, carried his 
work a little farther. Assuming his datum of 680° F. as the 
temperature of crystallization of the minerals, he shows that the 
corresponding pressure was not less than twenty-six tons, or 
3,500 atmospheres on the square inch, and that these micro- 
scopic flasks must have been charged with their effervescing 
contents under that enormous compression. 
This is equal to the weight of a mass of overlying strata 
52,000 feet thick. It is not right, however, to attribute the 
whole of this to the weight of overlying strata. There is no 
doubt that it is the resultant of this and the violent lateral com- 
pression to which the contortion and folding of the gneiss is 
due. The latter is probably the larger of the two components. 
Mr. Ward's conclusion is that the granite of Skiddaw, in Eng- 
land, was formed at least six miles below the surface, a depth 
at which the temperature is normally very near Mr. Sorby's 
datum of 680° F. 
This is surely a vast deduction from data, microscopically 
minute, and seemingly insignificant. But insignificant as one 
of these "crystal flasks," as they have been aptly called, may 
be, we are not dealing with one alone but with vast numbers, 
for investigation has revealed them by myriads and by mil- 
lions, and not in gems only, but in other crystalline minerals. 
In size they range between the one-thousandth and the fifty- 
thousandth of an inch, but they are so multitudinous as often 
to impart a white tint to the crystal, and many specimens of 
milky quartz owe their whiteness solely to the presence of 
these innumerable bubbles. In some of the Cornish granites 
the cavities make five per cent of the volume, and yield four 
pounds of the liquid to every ton of the rock. ■ 
Mr. Ward says: "Such is the minuteness of these cavities 
and their number in many cases, that more than a thousand 
millions might be contained easily within a cubic inch of 
