1878.] Physiography. s677 
sand and clay and chalk which form our cliff were laid down be- 
neath the sea; how come they now to form dry land? Now it is 
clear that one of two things must have taken place: either the- 
level of the sea has been depressed or the land has been 
raised. Geologists do not hesitate to say that it is the land which 
has undergone the change in level, while the sea has remained 
stationary. The sea is, in fact, more stable, more constant, more 
ancient than our oldest continents. All land is, on the other 
hand, subject to changes of level. In the Himalaya mountains 
shells, which once lived in the sea, are found at an elevation of 
16,000 feet above the level of the ocean. The northern part of 
Scandinavia is even now slowly rising, while the southern portion 
is undergoing depression. But how? There lies the question. 
It is now well known that the earth is, in the interior, in an in- 
tensely heated condition. In deep wells and mines the tempera- 
_ture rises about 1° Fah. for every sixty feet we descend. The 
melted lava poured forth during volcanic eruptions gives us some 
idea of the temperature comparatively near the surface. The cen- 
tre of the earth must then be hot beyond conception. But it is 
gradually cooling. Heat is flowing outwards through the crust 
into space: the cooling of the earth is accompanied by contrac- 
tion of the mass of the earth: and unequal contraction produces 
areas of depression and elevation. 
_ Is this clear? Perhaps a comparison of great things with small 
will make it clearer. The human mind seems at times to fail to 
grasp facts which are, in truth, simple, but which from their mag- 
nitude are hard of conception. If, for instance, we stand on a high 
peak and look out over a portion of a great mountain chain, and 
see the grand summits standing out along the central ridge, it is 
difficult to conceive how this grand upheaval could have been pro- 
duced; and perhaps the mind, wearied with the attempt to grap- 
ple with a subject almost too great for its powers, finds relief in 
the thought, that the mighty elevation was due to some great 
cataclysm or convulsion of nature, concerning the cause of which 
—as a matter beyond our ken—it would be rash to speculate. 
And if it were then suggested that mountain chains, such as that 
in the midst of which we were standing, must be the inevitable ` 
result of the contraction of a cooling globe, it may be that our 
understanding would reject a conclusion which it could not at — 
once grasp, | : 
