342 THE LAW OF EVOLUTION CONTINUED. 



It is now generally agreed among geologists that the 

 Earth was once a mass of molten matter; and that its inner 

 parts are still fluid and incandescent. Originally, then, it 

 was comparatively homogeneous in consistence; and, be- 

 cause of the circulation that takes place in heated fluids, 

 must have been comparatively homogeneous in temperature. 

 It must, too, have been surrounded by an atmosphere con- 

 sisting partly of the elements of air and water, and partly of 

 those various other elements which assume gaseous forms at 

 high temperatures. That cooling by radiation which, 

 though originally far more rapid than now, necessarily re- 

 quired an immense time to produce decided change, must at 

 length have resulted in differentiating the portion most able 

 to part with its heat; namely, the surface. A further cool- 

 ing, leading to deposition of all solidifiable elements con- 

 tained in the atmosphere, and finally to precipitation of the 

 water and separation of it from the air, must thus have 

 caused a second marked differentiation; and as the condensa- 

 tion must have commenced on the coolest parts of the sur- 

 face^ — namely, about the poles — there must so have resulted 

 the first geographical distinctions. 



To these illustrations of growing heterogeneity, which, 

 though deduced from the known laws of matter, may be re- 

 garded as hypothetical, Geology adds an extensive series 

 that have been inductively established. The Earth's struc- 

 ture has been age after age further involved by the multi- 

 plication of the strata which form its crust ; and it has been 

 age after age further involved by the increasing composi- 

 tion of these strata, the more recent of which, formed 

 from the detritus of the more ancient, are many of them 

 rendered highly complex by the mixtures of materials they 

 contain. This heterogeneity has been vastly in- 



creased by the action of the Earth's still molten nucleus 

 on its envelope; whence have resulted not only a great 

 variety of igneous rocks, but the tilting up of sedimentary 

 strata at all angles, the formation of faults and metallic 



