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 the 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 Kussian America 

 to Tierra del Fuego : lower hills face the narrow^er 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 greatest oceans there is the highest land. 

 Throughout all known time this has been the rule : for wherever the 

 sediments have been most thickly deposited, tlm-e 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-nuiterials, the free circulation of water, and many other natural 



