Of the other inequalities of the land, the more remarkable are the cavities forming lakes, and the grooves occupied by the beds of rivers. The former are of all sizes, from several hundred miles in circumference down to very small dimensions, and occur in all situations,--between mountain-chains, like the Caspian,--in plains, like Onega,--and along the course of rivers, like those of Canada. The streams necessarily flow in the line which marks the greatest depression of the valleys; although, in some instances, towards their mouths, they occupy a higher level, their beds having been raised by the deposition of the debris carried down by the torrent. The bottom of the ocean, being merely the continuation of the surface of the land, may be supposed to present inequalities of a similar nature, although, owing to the action of currents, they are probably not so distinctly marked. The transition from what is above to that which is under the water is not in general denoted by any striking phenomenon, excepting the not unfrequent occurrence of long ranges of cliffs, pebbly beaches, and accumulations of sand. When the coast is low and flat, the depth of the sea in its vicinity is usually small; whilst along a rocky and abrupt shore it generally presents a depression in some measure corresponding to the height of the land. The existence of submarine chains of mountains is established by the numerous shoals and rocks which are to be considered as their summits. On these, coral reefs and islands have been gradually raised by myriads of zoophytes. The mighty mass of waters, which is collectively termed the sea, occupies, as has been already mentioned, more than two-thirds of the surface of the globe. Its chemical composition, its tides, its currents, and all the varied phenomena which it presents, afford subjects of highly-interesting research. The atmosphere, in like manner, which envelopes the earth, supplies, in its ever-varying aspects, its motions, its electrical phenomena, and the influence which it exercises on animal and vegetable life, an object of investigation pregnant with curious and useful knowledge. The mysterious agency of subterranean fire has elevated great masses of rocky matter in various parts of the globe. Earthquakes have effected extensive and remarkable changes upon its surface; the waters of the ocean have alternately worn away the shores and eked them out by depositions of sand and mud; the rivers have furrowed the land, and carried the debris of the higher regions to the valleys and plains; while air and moisture have exercised their decomposing influence upon the hardest substances. By the action of these powers the earth has become a fit receptacle for the varied forms of animal and vegetable existence with which we see it so profusely stored.