THE ORIGIN OF MOUNTAINS. 
53 
fore, at comparatively moderate depths the heat must 
be very great. Many geologists in consequence, have 
been, and are, of opinion that the main mass of the 
Earth consists of molten matter. We know, however, 
that the temperature at which fusion takes place is 
raised by pressure, and it must not, of course, be 
assumed that the temperature continues to increase 
so rapidly beyond a certain depth. Other great 
authorities,* therefore, are of opinion that the mass 
of the Earth, though intensely hot, is solid, with, no 
doubt, lakes of molten matter. In either case the 
central mass continues slowly to cool and conse- 
quently to contract. The crust, however, remains at 
the same temperature and consequently of the same 
dimensions. This being so, under the overwhelming 
force of gravity one of the two things must happen. 
Either (i), parts of the crust must break off and sink 
below the rest; or (2), the surface must throw itself 
into folds. 
Table Mountains. 
Where the first alternative has happened we find 
more or less numerous faults. 
Those parts which have not sunk, or which have 
* See, for instance, Lord Kelvin, Lectures and Addresses, 
vol. II. 
