585 
of Edinburgh, Session 1868 - 69 . 
think this possible. To use a familiar illustration : — When an 
iron casting is cooling in the sand, and, of course, like the earth, 
cooling from the surface ; when once the cold hard crust begins to 
form, the greater heat of the interior has no tendency whatever to 
break through the crust. Were there no other force to upheave, 
or break through the earth’s crust, I do not believe the internal 
heat could ever do it. I have now to state how I believe such 
actions are brought about. 
The earth is cooling, and as it cools it must contract. But this 
is true only of the deeper strata: the surface is cooling and con- 
tracting no longer. But the surface, by reason of its weight, is 
clinging round the centre. 
The earth is a sphere, whereof the centre is contracting, while 
the surface clings to the centre and yet cannot contract. What, 
then, will necessarily follow? The same that happens when an 
apple contracts, while its skin clings to it and yet cannot contract — 
the surface will form wrinkles , rising in some places and sinking in 
others. This, I believe, is the explanation of those gradual move- 
ments of elevation and subsidence, which, as we know, have taken 
place everywhere, and are now going on at least over extensive 
areas. But the formation of mountain chains having axes of 
igneous rocks, and of volcanoes, needs further explanation. 
Over areas of elevation the superficial strata are, in a slight 
degree, being lifted up ; over areas of subsidence they are, in an 
equal degree, being forced down. When they are lifted up, there 
will be a diminution of pressure on the lower strata. (This may 
be made evident by considering that if the upper strata were lifted 
up into an arch, which, however, is probably impossible, they would 
not press on the lower strata at all.) A diminution of pressure on 
the lower heated strata will cause them to melt into lava, expand- 
ing at the same time (igneous rocks, in the act of melting, expand 
by from a ninth to a third of their volume when unmelted — 
Bischoff, quoted in Scrope on Volcanoes, p. 44), and the same 
cause will make their imprisoned vapours expand. (For the exist- 
ence of these vapours, see Scrope on Volcanoes, p. 56.) This 
action probably begins suddenly, as soon as the diminution of 
pressure is sufficient to permit the rocks to begin to melt. In this 
way, as I believe, the igneous axes of mountain chains have been 
