XCVIU PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



earth's centre, while hot springs and lava-currents indicate stiU 

 greater heat below the mines thus reached. Thus between attain- 

 able limits we find cold outside and heat inside the earth's crust 

 composed of ponderable materials, all of which, either separately or 

 combined, can, and do, exist in three conditions — Damely, gaseous, 

 fluid, and solid. Many of thcbc materials are gaseous when heated, 

 solid when cooled, and fluid at intermediate temperatures. Each 

 melts and freezes, or sohdifies at- definite degrees of heat, and they 

 vary in dimension and specific gravity according to temperature. 

 Heat is force, according to modern views ; and rays from the sun, 

 from luminous substances thrown out from the earth, and from 

 artificial sources of light, include heat-rays, and are consequently 

 mechanical forces. Thus force radiates with light and heat from 

 heated centres, causing expansion, and separating particles from 

 each other. 



Many examples of this action are given by the author in illustra- 

 tion of these views, drawn partly from natural phenomena, and 

 partly from experiments ingeniously contrived by himself ; a full 

 description of the cooHng of silver is given at p. 346, vol. ii., as one of 

 the neatest experiments to illustrate the cooling of a molten mass. 

 The same forms which he has observed grov^dng on cooling slag, 

 iron, silver, &c., he finds repeated on a larger scale in cold lavas, in 

 hot and cold mountains, and in old igneous rocks in Iceland. He 

 also alludes to the experiments recently made respecting the sun's 

 atmosphere and the substances supposed to exist there, and to the 

 gradual cooling of the difl'erent bodies which form our planetary 

 system ; and assumes that the earth, which is intermediate in size, is 

 also in an intermediate state, partly fluid, partly solid, cooling, but 

 still hot within. 



Thus he assumes from the facts brought forward that an igneous 

 foundation is the base on which the sedimentary rocks were de- 

 posited and now rest ; he also assumes that air and water, steam and 

 ice, moved by the two opposing forces, heat within and cold without, 

 levitation and gravitation, have worn down the outer crust of the 

 globe, and have sorted the debris, while movements in the igneous 

 foundation, and heat radiating and transmitted from it have dis- 

 turbed and altered the sedimentary deposits ; he also considers 

 that these causes have greatly diminished in intensity since they 

 first began to act on the crust of the globe. He has illustrated by 

 numerous drawings and descriptions the marks made by rivers, 

 waves, currents, glaciers, &c., as well as those made by other de- 

 nuding agents stiU at work. 



He also assumes that in late geological periods the earth has 

 cooled so far as to freeze water everywhere at the surface, were it not 

 for heat radiating to it from the sun. "We have reached a partial 

 glacial period, and the position and extent of ice on the earth now 

 depend on the amount of heat absorbed from the sun, and on move- 

 ments in the igneous foundation or centre of the earth. 



Some interesting experiments are also shown towards the end of 

 the second volume to illustrate the eff'ects of the earth's rotation on 



