THE AGE OF THE EARTF. 29' 



tide, SO rivulets of the mother Hquor must run out from 

 the edges of the continents into the receding hiva ocean. 

 But, unlike sandbanks of incoherent sand permeated hj 

 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 fom- 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 sm-face, 

 almost, or quite, cool enough for some kinds of vegetation. 

 In the course of the first few Aveeks 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 5,000 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 Avholly solid crust might grow from 10 

 metres to 50 kilometres. These definite numbers 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. 



§ o5. 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 G kilometres as we please to assume it. 



§ 36. There nmst still be a small, but important, pro])ortion 

 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 sucli actions. Now as the specific gravity of 



