Prof. C. Lloyd Morgan—Elevation and Subsidence. 293 
May we assume the existence of a fluid underlayer beneath the 
superficial crust of the earth? I think we may. From this source 
volcanic rock would seem to be derived ; and in the plutonic rocks 
we seem to see the solidified crust of the once deeply-buried under- 
layer. Whether the fluid underlayer be continuous or in more or 
less isolated reservoirs we cannot say. I shall suggest in the sequel 
a reason why this substratum should assume the fluid or viscous state. 
At what depth beneath the surface of the earth may the plutonic 
rocks have solidified ? It is well known that Mr. Sorby from the micro- 
scopical study of the contained erystals concluded that the granites 
of Cornwall solidified under a pressure equivalent to that of about 
50,000 feet (94 miles) of rock, and that those of the Highlands 
indicate one of about 76,000 feet; while Mr. Clifton Ward suggested 
for the granite and granitoid rocks of Westmoreland and Cumberland 
a mean pressure of 44,000 feet of overload. Commenting upon these 
results Mr. Prestwich asks, “Are we warranted in supposing that 
there has been denudation to the extent of removing such enormous 
masses of rock as this would imply?” and Mr. A. Geikie answers, 
“Tt is not probable that any such thick overlying mass ever did cover 
the granite.” (Prestwich, Geology, vol. i. p. 432; Geikie, Text Book, 
p. 297.) 
What was the condition of the potential granite previous to 
solidification? Nearly all agree that it was rather from hydro- 
thermal solution (at a temperature of, say, 500° C.) than from 
igneous fusion that the rock solidified. As was long ago pointed 
out by Bischof, the thinness of the granite veins which branch out 
into clay slate, the sharpness of the line of junction between the 
material of the vein and that of the country, and the slight alteration 
the slate has undergone, point to the mobility of the fluid and render 
the possibility of gradual solidification from igneous fusion out of 
the question. (Chem. and Phys. Geol., Paul’s Translation, 1859, 
vol. iii. p. 52.) 
What is the amount of expansion under solidification ? Bischof’s 
experiments led him to believe that the formation of granite from 
a liquid magma involved a contraction of 25 per cent. Delesse 
gave from 8 to 7 per cent. as a more probable estimate. Mr. Mallet 
found a smaller amount of contraction. In the solidification of 
plate-glass the contraction was 1:59 per cent. In the case of iron- 
slag the diminution of volume was 6-7 per cent. Now the sp. gr. of 
obsidian is from 2:4 to 2°5, while that of granite is (say) 2°65, and 
that of syenite (say) 2-8. This would seem to point to a diminution 
of volume by 6 or 8 per cent. on passing from the vitreous to the 
erystallized condition, Probably therefore a diminution of volume 
by from 6 to 10 per cent. on passing from the molten to the crystal- 
lized condition may fairly be assumed to occur. 
It is well known that water expands when it passes into the con- 
dition of ice: that ice, say, at —1° C., subjected to pressure melts or 
is squeezed into the liquid condition: that at high pressures water 
may remain liquid though the temperature fall several degrees below 
