GROWTH OF GEOLOGY. 237 



gases under a pressure of 5,000 Ibs. to the square 

 inch; but as the satellite wandered away from the 

 parent planet this pressure continuously diminished. 

 Under these circumstances the moon would become 

 as explosive as a charged bomb, steam would burst 

 forth from numberless volcanoes, and while the face 

 of the moon might thus have acquired its existing 

 features, the ejected material might possibly have 

 been shot so far away from its origin as to have ac- 

 quired an independent orbit," * and some of the 

 meteorites which now descend upon the earth may be 

 returned portions of the early envelope. 



Soon after the birth of the moon, the earth became 

 consolidated (with a surface temperature of about 

 1170C.), and the moon may have been influential 

 in determining high-pressure areas where the crust 

 was depressed, and low-pressure areas where it was 

 lowered. This, as Sollas says, was the second critical 

 period in the history of the earth, the stage of the 

 " consistentior status." Since this epoch, on Lord 

 Kelvin's estimate, twenty to forty millions of years 

 may have elapsed. 



Below the surface the temperature increased, as it 

 still does; at a depth of twenty-five miles, it would 

 be (according to Lord Kelvin's calculations) about 

 1430 C., or 260C. above the fusion point of the 

 matter forming the crust. But the great pressure at 

 this depth would counteract the heightened temper- 

 ature, and still keep the crust solidified even at a 

 depth of twenty-five miles. 



When, with continued cooling, the temperature 

 of the surface fell to 370 C., the steam in the atmos- 

 phere would begin to liquefy, and this was the first 

 step in the origin of the oceans. Supposing, as 

 * Sollas, loc. cit. 



