154 
THE GEOLOGIST. 
also to be able to give charts of the teeming oceans during each of 
those past wonderful ages severally characterised as the stages of 
progress and development of organic beings. If we regard tlie out- 
lines of those primitive land-domes and crests when laid down upon a 
map of the world, we are struck with their simplicity. We may 
remark, too, their frequent concurrence with those lines of greatest 
heights which abut against the oceans of greatest extent. The highest 
mountains of the land face the heaviest waves of the sea. The lines of 
igneous-and volcanic products in all ages have been, and still are, the 
barriers to the sea's most powerful labours ; the lavas and granites 
fused in the subterranean depths have been evolved to form an un- 
conquerable wall against the most destructive powers of the foaming 
waves. A towering chain borders the Pacific from Russian America 
to Tierra del Fuego : lower hills face the narrower Atlantic ; but 
against the smaller Arctic Sea no special mountain -land is presented. 
While those lines of primeval uplift determined the directions of the 
mountain-ranges and thus established the basis of the subsequent con- 
tinental areas in the accumulated sediments successively deposited on 
their protected flanks, so the great parallel lines of subsidence and 
depression gave the form and direction to the profound abysses of the 
deeps and ocean-basins. 
America thus presents almost the simplicity of a single continuous 
result compared with Europe, which is full of complexities. 
We have said against the gi-eatest oceans there is the highest land. 
Throughout all known time this has been the rule : for wherever the 
sediments have been most thickly deposited, there has taken place the 
greatest uplifts. Nature always works by positive laws, and there is 
some reason for this. We are not very partial to the doctrine of a 
central incandescence ; we admit, however, most entirely, the existence 
of a deep-seated internal heat, of, even now, very great intensity. 
There are certain lines of equal temperature in the subterranean 
portions of the earth's crust. There is such a line, for instance, of 
temperature equal to that of boiling water. Now this subterranean 
isothermal line would not be continuous at one even depth all round 
the circumference of this planet ; but it would vary in its depth from, 
and in its approach towards, the surface, according to the density of the 
rock-materials, the free circulation of water, and piany other natural 
