CarrutHErs.—Retardation of Earth’s Rotation by Volcanic Action. 859 
travel 20 to 80 degrees north and south, would quite obliterate this distine- 
tion. 
Lines of volcanic action would tend to be of long continuance, as the 
same causes which had at first decided the position of the line of fracture 
would be apt to make the next fracture occur in the same place; it is more 
than likely, however, with the enormous pressures which obtain in these 
movements of the earth, accompanied as they are by great heat, that rock 
when fractured would be welded together again, and it would not follow that 
a line of former fracture was weaker than other parts of the crust, because 
it had been once fractured. 
We will now return to the earth masses at A, Fig. 1. They have just 
fallen in on to the support below, and the crust on each side of the fracture 
is in a state of violent molecular agitation. 
The heat generated by the compression of the film nearest the surface, 
at once expands the matter of the crust, and is, by the expansion, imprisoned ; 
it can afterwards escape only by conduction or radiation. From each point 
of the fracture, heat is rushing in every direction, some of it along the 
earth horizontally, but some vertically towards the surface. By the time 
equilibrium had been established the surface near the line of fracture would 
be raised by continual additions of heat from below to a high temperature, 
but not sufficient, except in rare cases, to cause volcanic eruptions. Where, 
as in the Andes, the elevation is very rapid, we may expect voleanoes on 
the summit of a range, but generally they are due to quite a local cause, 
and occur, not on the summit, but somewhat to one side of the main line of 
elevation. 
Where a bend occurs in the line of fracture, and still more, where two 
lines of elevation intersect each other, a cause of volcanic eruption arises, to 
which I believe is due by far the greater part of the active volcanoes in the 
world. 
If, for instance, the line of fracture be semi-circular, the points along 
the circumference would be elevated more than the points near the centre. 
The surface crust of rigid and inelastic rock would be left unsupported in 
the middle, and as it is far too weak to bear its own weight, it would at once 
crush together and fall in. 
If the bend be on a scale of considerable magnitude, the sheet of rock 
may have to crush together several feet before it can fall in. Every foot of 
compression, at the surface, developes enough heat to raise the temperature 
of one mile of rock by 40 degrees, as a compression of 1-40th of a foot 
would raise the temperature one degree; ata depth of 33 miles, owing to the 
increased elasticity, each foot of compression would raise the temperature 
of the same mass of rock by 8,800 degrees. 
