Earth as an Abode fitted for Life. 83 



must recede from them ; and their bounding coast-lines must 

 become enlarged. And just as water runs out of a sandbank, 

 drying when the sea recedes from it on a falling tide, so 

 rivulets of the mother liquor must run out from the edges of 

 the continents into the receding lava ocean. But, unlike 

 sandbanks of incoherent sand permeated by water remaining 

 liquid, our uncovered banks of white-hot solid crystals, with 

 interstices full of the mother liquor, will, within a few hours 

 of being uncovered, become crusted into hard rock by cooling 

 at the surface, and freezing of the liquor, at a temperature 

 somewhat lower than the melting temperatures of any of the 

 crystals previously formed. The thickness of the wholly 

 solidified crust grows at first with extreme rapidity, so that 

 in the course of three or four days it may come to be as 

 much as a metre. At the end of a year it may be as much 

 as 10 metres ; with, a surface, almost, or quite, cool enough 

 for some kinds of vegetation. In the course of the first few 

 weeks the regime of conduction of heat outwards becomes 

 such that the thickness of the wholly solid crust, as long as 

 it remains undisturbed, increases as the square root of the 

 time ; so that in 100 years it becomes 10 times, in 25 million 

 years 5000 times, as thick as it was at the end of one year : 

 thus, from one year to 25 million years after the time of 

 surface freezing, the thickness of the wholly solid crust might 

 grow from 10 metres to 50 kilometres. These definite num- 

 bers are given merely as an illustration ; but it is probable 

 they are not enormously far from the truth in respect to what 

 has happened under some of the least disturbed parts of the 

 earth's surface. 



§ 35. We have now reached the condition described above 

 in § 30, with only this difference, that instead of the upper 

 surface of the whole solidified crust being level we have 

 in virtue of the assumptions of §§ 33, 34, inequalities of 

 6 kilometres from highest to lowest levels, or as much more 

 than 6 kilometres as we please to assume it. 



§ 36. There must still be a small, but important, proportion 

 of mother liquor in the interstices between the closely packed 

 uncooled crystals below the wholly solidified crust. This 

 liquor, differing in chemical constitution from the crystals, has 

 its freezing-point somewhat lower, perhaps very largely lower, 

 than the lowest of their melting-points. But, when we con- 

 sider the mode of formation (§ 25) of the crystals from the 

 mother liquor, we must regard it as still always a solvent 

 ready to dissolve, and to redeposit, portions of the crystalline 

 matter, when slight variations of temperature or pressure 

 tend to cause such actions. Now as the specific gravity of 



G2 



