GEOLOGICAL PROGRESS OF THE EARTH. 5 



Passing from this hypothetical illustration, which must 

 be taken for what it is worth, without prejudice to the 

 general argument, let us descend to a more certain order 

 of evidence. It is now generally agreed among geologists 

 that the Earth was at first a mass of molten matter ; and 

 that it is still fluid and incandescent at the distance of a few 

 miles beneath its surface. Originally, then, it was homo- 

 geneous in consistence, and, in virtue of the circulation 

 that takes place in heated fluids, must have been compara- 

 tively homogeneous in temperature ; and it must have been 

 surrounded by an atmosphere consisting partly of the ele- 

 ments of air and water, and partly of those various other 

 elements which assume a gaseous form at high tempera- 

 tures. That slow cooling by radiation which is still going 

 on at an inappreciable rate, and which, though originally 

 far more rapid than now, necessarily required an immense 

 time to produce any decided change, must ultimately have 

 resulted in the solidification of the portion most able to 

 part with its heat — ^namely, the surface. In the thin crust 

 thus formed we have the first marked difierentiation. A still 

 further cooling, a consequent thickening of this crust, and an 

 accompanying deposition of all solidifiable elements con 

 tained in the atmosphere, must finally have been followed 

 by the condensation of the water previously existing as 

 vapour. A second marked differentiation must thus have 

 arisen : and as the condensation must have taken place on 

 the coolest parts of the surface — namely, about the poles — 

 there must thus have resulted the first geographical dis- 

 tinction of parts. To these illustrations of growing hete- 

 rogeneity, which, though deduced from the known laws of 

 matter, may be regarded as more or less hypothetical, 

 Geology adds an extensive series that have been inductively 

 established. Its investigations show that the Earth has 

 been continually becoming more heterogeneous in virtue 

 of the multij^lication of the strata which form its crust ; 



