AND ITS INHABITANTS 37 



At last the rapid generation of heat by impact lessened, and 

 the fluid sphere, seething with slow convection currents, began 

 to cool. Certain compounds in the mutual solution of rock 

 elements became insoluble and fractional crystallization was 

 initiated. The heavy basic crystals were the first to form: 

 crystals of metallic sulphides, magnetite, hornblende or 

 pyroxene, and olivine. These crystals, because of their high 

 specific gravity, tended to work downward in the convective 

 movement. At first they were dissolved again in the abyss, 

 but as time went on they remained undissolved and accumu- 

 lated in the deeper parts of the fluid zone. The remaining 

 magma was more siliceous, of lighter gravity, and in crystalli- 

 zation gave to the upper shell a higher proportion of feldspar 

 and quartz. The original crust of the earth was consequently 

 a granite. The process of fractional crystallization may, 

 however, not be a sufficient explanation. An immiscibility of 

 the complex mineral solutions may have developed upon the 

 lowering of the temperature. In one way or the other or by a 

 combination of several causes a density stratification is sug- 

 gested by a number of lines of evidence as existing in the earth. 

 The conclusion, then, is more than a mere inference from 

 theories of crystallization. 



The gathering of the ocean waters. At last the ocean of 

 molten rock had shallowed, crystallization went forward in 

 separate basins, convection became hindered, the surface 

 froze as in a lava caldron. Then rain, ever descending from 

 the shield of perpetual cloud, but never heretofore reaching 

 the bottom of the atmosphere, at last began to splash on the 

 hot surface of the earth. The raindrops at first were dis- 

 sipated by contact and sent flying back as scattered molecules 

 of gas. But, owing to the low conductivity of rocks, the tran- 

 sition stage was very brief, and perhaps even in a few thousand 

 years from the time when the crustal congelation of the earth 

 had taken place a permanent ocean of acid water began to 



