352 AGE OF THE EARTH AS AN ABODE FITTED FOR LIFE. 



sand banks of incoherent sand permeated by water remaining liquid 

 our uncovered banks of wbite-liot 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 meter. At 

 the end of a year it may be as much as 10 meters, 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 outward 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 one 

 hundred years it becomes ten times, in twenty-five million years five 

 thousand times, as thick as it was at the end of one year. Thus, from 

 one year to twenty-five million years after the time of surface freezing, 

 the thickness of the wholly solid crust might grow from 10 meters to 50 

 kilometers. These definite numbers are given merely as an illustra- 

 tion, 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. 



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 kilometers from highest to lowest levels, or as much 

 more than 6 kilometers 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 consider 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. 

 Kow as the specific gravity of the liquor is less, by something like 15 

 per cent, than the specific gravity of the solid crystals, it must tend to 

 find its way upward, and will actually do so, however slowly, until 

 stopped by the already solidified impermeable crust, or until itself 

 becomes sohd on account of loss of heat by conduction outward. If 

 the upper crust were everywhere continuous and perfectly rigid the 

 mother liquor must inevitably, if sufficient time be given, find its way 

 to the highest places of the lower boundary of the crust, and there 

 form gigantic pockets of liquid lava, tending to break the crust above 

 it and burst up through it. 



37. But in reality the upper crust can not have been infinitely strong; 



