120 



THE WONDERS OF GEOLOGY. Lect. L 



luminous vapour, to opaque solid globes like our own. All 

 the materials of which the earth is composed may exist 

 either in a solid, fluid, or gaseous state ; and simply by a 

 change of temperature, or by electro-chemical agency, 

 every substance may undergo a transition from one state 

 to the other. Water existing as ice, fluid, or vapour, and 

 separable into two invisible gases, offers a familiar example 

 of a body constantly exhibiting these transmutations ; and 

 mercury, of a metal which, though generally in a fluid 

 state, becomes, when exposed to a very low temperature, 

 a solid mass like silver. The relative position of land 

 and water, and the inequalities on the surface of the 

 earth, are subject to constant changes, which are regulated 

 by certain fixed laws. The principal causes of the degra- 

 dation of the land are atmospheric agencies, variations of 

 temperature, the action of running water, and of icebergs, 

 glaciers, &c, by which the disintegrated materials of the 

 land are transported into the bed of the ocean. The mud, 

 sand, and other detritus thus produced, are consolidated by 

 certain chemical changes which are in constant activity, both 

 on the land and in the depths of the ocean, and new rocks 

 are thus in the progress of formation. But the conjoint 

 effect of these operations is unremitting destruction of the 

 surface of the land ; and were there no conservative pro- 

 cess, the whole of the dry land would disappear, and the 

 earth be covered by one vast sheet of water. The globe, 

 however, possesses an internal source of heat, and whether 

 this exists as a central nucleus of high temperature, 

 or as medial foci, — whether it be dependent on the 

 assumed original nebulous state of the earth, or pro- 

 duced by chemical or electro-magnetic forces acting 

 on the mineral substances contained in the interior 

 of our planet, — does not affect the present inquiry. 

 This internal heat, however induced, must occasion con- 

 stant changes in the relative level of the land and water ; 



