402 cos:jig ruiLOsorn v. [n. u 



vapour, must have characterized the surface of our planet at 

 this primeval epoch. But as the ever thickening crust slowly 

 collapsed about its contracting contents, mountain ridges of 

 considerable height could be gradually formed, islands could 

 cohere over wider and wider spaces, and deeper basins would 

 permit the accumulation of large bodies of water. Numerous 

 integrations of islands into continents, and of lakes into 

 oceans, would thus occur, making the differentiation of land 

 and sea more distinct and definite. The integration of conti- 

 nents and the rise of mountain chains in different directions 

 must have enlarged the areas of denudation, and thus rendered 

 possible the integration of masses of detritus into extensive 

 sedimentary strata. Differences of watershed and river- 

 drainage thus caused added variety to the resulting geologic 

 formations ; and these, crumbling into soil of more or less 

 richness, afterwards impressed differences upon vegetation, 

 and thus indirectly upon animal life. Yet again, the thick- 

 ening of the crust must have added to the definite hetero- 

 geneity of the surface by its effect upon volcanic phenomena. 

 While the crust was still thin, the angry waves of liquid 

 matter imprisoned beneath must have continually burst 

 through volcanic vents, suddenly vaporizing large quantities 

 of surface-water, and causing phenomena similar to those 

 now witnessed upon Saturn and Jupiter. As the crust thick- 

 ened, these volcanic agencies were more and more restrained : 

 craters became restricted to certain localities where the crust 

 was less thick than elsewhere, and earthquake waves began to 

 run, as at present, along definite lines. Those well-regulated 

 earthquake pulses which raise continents and ocean-floors at 

 the rate of a few inches or feet per century, now began to in- 

 crease the definite heterogeneity of the surface. To the long 

 rhythms of elevation and subsidence thus produced have 

 been due countless differentiations in the directions of ocean- 

 currents and continent-axes, in watershed, in the composition 

 of sedimentary strata, and in climate. And to all these may 



