164 
MR. ROBERT MALLET ON VOLCANIC ENERGY. 
globe, while a large portion (of its surface even) may have been still red-hot, and 
communications partially open with the viscous interior; this accompanied by 
severe local tensions and compressions. 
3rd, The increase of rigidity in the thickened crust, when it became able to 
transmit tangential thrusts due to contraction ; and these (resolved as has been 
explained) elevated the mountain-ranges and originated the hypsometric configura- 
tion of the land, the establishment in regimen of the ocean and of the water- 
courses of the world, and with these the beginnings of climates fitted for successive 
forms of life. 
4th. The epoch of a greatly thickened and stiffened crust with a comparatively 
slow rate of cooling of the globe, being the regimen which now exists, and of the 
play of the forces of contraction by cooling still going on, in and from which we 
now hope to show that volcanic action proper originated, and is preserved 
apparently uniform, though with a constantly decreasing energy. 
These divisions in the progress of secular cooling are not merely arbitrary, for each is 
marked by a different way in which the effects of the contraction show themselves. 
The rate of contraction, enormous at first, because the rate of cooling was then most 
rapid and the coefficient of contraction the greatest, is met by deformation, afterwards 
by splitting up of the thin crust under tension (accompanied by welling up of the 
liquid matter from beneath), then, with a thicker crust, by folding over and elevation of 
ridges, primary tensions having given place to compressions, until at length the last and 
existing state of things is reached in which the crust has become of great thickness, and 
covers the still hot nucleus all over as a comparatively rigid spheroidal shell or dome; 
these stages more or less overlapped each other in their successive development. 
61. How, then, is the contraction in volume of our globe Fig. o. 
which is now going on met \ for if admitted to be cooling 
it must be still contracting. 
We have a globe (A C) subject to the laws of gravita- 
tion, composed of a relatively thick crust, and enclosing a 
hotter nucleus, which is losing its heat by convection to the A 
crust, through conduction and radiation from the surface. 
The coefficient of contraction of the matter of the nucleus, 
which is at a temperature much higher than the crust, is 
greater than that of the material of the latter. 
If the shell covering the nucleus were still thin and flexible it would give way, as 
formerly, by plication, and fall downwards by gravity as the diameter of the nucleus 
gradually diminishes by contraction. But the crust* is now a thick rigid covering dome ; 
its dimensions, owing to its existent small coefficient of contraction, are diminishing 
slowly as compared with those of the nucleus, whose coefficient, owing to its higher tempe- 
rature, is much greater. Hence in fig. 2, taking the nucleus as b d, the thickness of the 
crust being A o — do (i. e. the integral thickness of all the crust, whether absolutely solid 
