May 19, 1899.] 



SCIENCE. 



707 



whole lava ocean becomes silted up to its 

 surface. 



§ 34. We have not time enough at pres- 

 ent to think out all the complicated ac- 

 tions, hydrostatic and thermodynamic, 

 which must accompany, and follow after, 

 the cooling of the lava ocean surrounding 

 our ideal primitive continent. By a hur- 

 ried view, however, of the affair we see 

 that in virtue of, let us say, 15 per cent, 

 shrinkage by freezing, the level of the 

 liquid must, at its greatest supposed depth, 

 sink six kilometers relatively to the con- 

 tinents, and thus the liquid 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 sand- 

 banks of incoherent sand permeated by 

 water remaining liquid, our uncovered 

 banks of white-hot solid crystals, with in- 

 terstices full of the mother liquor, will, 

 within a few hours of being uncovered, be- 

 come 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 ten meters ; with a sur- 

 face, 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 thick- 

 ness 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 freez- 

 ing, the thickness of the wholly solid crust 

 might grow from 10 meters to 50 kilo-r 

 meters. These definite numbers are given 

 merely as an illustration, but it is probable 

 that 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 dif- 

 ference, 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 im- 

 portant, proportion of mother liquor in the 

 interstices between the closely packed un- 

 cooled crystals below the wholly solidified 

 crust. This liquor, differing in chemical 

 constitution from the crystals, has its freez- 

 ing-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 tempera- 

 ture or pressure tend to cause such actions. 

 ISTow 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 upwards, and will 

 actually do so, however slowly, until stopped 

 by the already solidified impermeable crust, 

 or until itself becomes solid on account of 

 loss of heat by conduction outwards. 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 



