400 IDIOPHYSICS. 



as of the elephant, rhinoceros, and numerous other genera, such as 

 now exist. Lastly, the deposites formed by the wash of rivers, the 

 action of currents, the labors of coral animals, and similar causes, 

 are called alluvial; and of course contain numerous remains, chiefly 

 of animals and plants still living. 



3. Under the head of Physical Geology, we are to treat of the 

 causes which have been, or now are in operation, producing changes 

 in the structure, or aspect of the globe. These may all, it is be- 

 lieved, be referred to the action of wind, water, heat, or organic life. 

 The wind, by transporting sand, forming hills, and burying cities or 

 forests, becomes a geological agent worthy of notice. The action 

 of water, requires to be studied both on the land, and in the ocean. 

 On the land, it has an abrasive action, wearing away the solid earth, 

 and transporting it to some lower region, or to the sea. This action 

 is increased by the effect of frost ; as the freezing of water in porous 

 rocks, by its expansion, causes them to crack or scale off, and thus 

 assists in their disintegration. The earth, on mountain sides, be- 

 coming soft by thawing, is then more easily carried away by torrents ; 

 forming avalanches, or slides of earth, like those of ice. Rocks are 

 also corroded by vegetation ; and this partly by the action of acids, 

 produced from the mosses or lichens which cover them. 



By such means, the loose superficial earth is believed to have been 

 derived from solid rocks : and the detritus, carried away by streams, 

 is still forming alluvial land ; especially in the deltas, or islands and 

 shoals, at the mouths of rivers ; which are continually encroaching 

 upon the sea. Islands may also be formed by deposition, from cur- 

 rents in the ocean ; or by coral animals, shells, or submarine plants ; 

 and in some instances by subterranean forces, as in modern volcanoes, 

 raising the bed of the ocean above the level of its surface. If this 

 were done upon an extensive scale, it would account for the existence 

 of marine shells in elevated positions ; and the currents which such 

 upheaving would produce, may account for the boulders, or rounded 

 rocks, which we find scattered over the land, dragged from their pri- 

 mitive beds, and leaving stratches on the rocks over which they have 

 passed ; while beds of gravel and sand were deposited in the eddies. 



The cause adequate to produce such a rise of the bed of the ocean, 

 must be sought in the internal fires of the earth ; whose existence 

 is proved by more than two hundred volcanoes, still burning, as well 

 as by the numerous hot springs, in various parts of the globe. An- 

 other proof is found in the fact, that the earth grows warmer, as we 

 descend in caves or mines, at the rate of 1 Fahrenheit, for every 50 

 feet, nearly ; commencing with the average temperature not far below 

 the surface. At this rate of increase, the earth, at the depth of ten 

 miles, would be at a red heat ; at the depth of twenty miles it would 

 be at a white heat ; and at a depth of fifty miles, the hardest rocks 

 would be in a liquid state, like melted iron or lava. Nor is this 

 statement incredible, when we consider how cool may be the exterior 

 of a large furnace, while the iron is melting within it. The tempera- 

 ture of the bottom of the ocean, is probably, nearly that of its sur- 

 face ; while, beneath it, the heat may increase according to the ratio 

 above given. 



