^Hap. II 
THE INTERIOR OF THE EARTH 
I 
surface, fornieiiy led to the belief that our planet consists of a central mass 
of molten rock enclosed within a comparatively thin solid crust. Physical 
arguments, however, have since demonstrated that the earth, with such a 
structure, would have mrdergone great tidal deformation, hut that in actual 
tact it has a greater rigidity than if it were made of solid glass or steel. 
From all the evidence obtainable it is certain that the temperature 
t't the earth’s interior must l)e high. The rate of increase of this tem- 
perature downward from the surface differs from place to place ; but an 
increase is always observed. At a depth of a few miles, every known sub- 
stance must be much hotter than its melting point at the surface. Hut at 
the great pressures within the earth, actual liquefaction is no doubt pre- 
sented, and the nucleus remains solid, though at a temperature at which, 
Fut for the pressure, it would be like so much molten iron. 
Any cause which will diminish the pressure may allow the intensely 
hot material within the globe to pass into the licprid state. There is one 
known cause which will bring about this result. The downward increment 
of temperature proves that our planet is continually losing lieat. As the 
owter crust is comparatively cool, and does not become sensibly hotter by 
the uprise of heat from within, the hot nucleus must cool faster than the 
orust is doing. Now cooling involves contraction. The hot interior is con- 
tracting faster than the cooler shell which encloses it, and that shell is tlius 
forced to subside. In its descent it has to adjust itself to a constantly 
'fiminishing diameter. It can do so only by plication or by rupture. 
’When the terrestrial crust, under the strain of contraction, is compressed 
into folds, the relief thus obtained is not distributed imiformly over the 
whole surface of the planet. From an early geological period it appears to 
have followed certain lines. How these came to be at first determined we 
cannot tell. But it is certain that they have served again and again, during 
®'mcessive periods of terrestrial readjustment. These lines of relief coincide, 
cn the whole, with the axes of our continents. The land-areas of the globe 
'‘^ay be regarded as owing their existence above sea-level to this result of 
ferrestrial contraction. The crust underneath them has been repeatedly 
Wrinkled, fractured and thrust upward by the vast oceanic subsidence around 
fhein. Xpe long mountain-chains are thus, so to speak, the crests of the 
Waves into which the crust has from time to time been thrown. 
Again, the great lines of fracture in the crust of the earth probably lie 
111 large measure within the land-areas, or at least parallel with their axes 
niid close to their borders. Where the disposition of the chief ruptures and 
cf the predominant plications can be examined, these leading structural 
matures are found to be, on the whole, coincident. In the British Islands, 
°r instance, the prevalent trend of the axes of folding from early Paheozoic 
kertiary time has been from south-west to north-east. How profoundly 
is direction of earth-movement has affected the structure of the region is 
shown by any ordinary map, in the long hill-ranges of the land and in the 
I'l'g inlets of the sea. A geological map makes the dependence of the 
Scenery upon the building of tlie rocks still more striking. Not only have 
