ch. vi.] THE EVOLUTWN OF THE EARTH. 403 



be added the metamorphosis of sedimentary rocks by volcanic 

 heat, and the seismic shoving up of strata at various angles. 



All these geologic phenomena are thus seen to be classifi- 

 able as differentiations and integrations of the earth's 

 superficial matter, caused by the continuous integration of 

 the earth's mass with its attendant dissipation of molecular 

 motion. We may next note that meteorologic phenomena 

 are similarly classifiable. Before the solidification of its 

 crust, our planet must have been comparatively homogeneous 

 in temperature, owing to the circulation which is always 

 maintained in masses of heated fluid. The surface-portions 

 must, however, have been somewhat cooler than the interior, 

 and this difference would be rendered more definite by the 

 formation of the crust, and by the subsequent separation of 

 the ocean from the gaseous atmosphere. As the contour of 

 land and sea became more definite and more permanent, 

 differences in temperature between different parts of the 

 surface must likewise have become more decided. Neverthe- 

 less the chief cause of climatic differentiations — the inclina- 

 tion of the earth's axis — did not begin to produce its most 

 conspicuous effects until a later period. As long as our 

 planet retained a great proportion of its primitive heat, there 

 could have been little difference between winter and summer, 

 or between the temperature at the poles and at the equator. 

 But when the earth had lost so much heat that its external 

 temperature began to depend chiefly upon the supply of 

 solar radiance, then there commenced a gradual differentia- 

 tion of climates. There began to be a marked difference 

 between summer and winter, and between arctic, temperate, 

 and tropical zones. And now also the distribution of land 

 and sea began to produce climatic effects, owing to the fact 

 that solar radiance is both absorbed and given out more 

 rapidly by land than by water. Areas of the earth's surface 

 where sea predominated began now to be distinguished from 

 areas where land predominated, by their more equable 



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