Earth as an Abode fitted for Life. 79 



becomes the freezing-point as raised by the increased 

 pressure ; or, perhaps more correctly stated, a temperature 

 at which some of its ingredients crystallize out of it. Hence, 

 beginning a few kilometres above the bottom, we have a 

 snow shower of solidified lava or of crystalline flakes, or 

 prisms, or granules of felspar, mica, hornblende, quartz, and 

 other ingredients : each little crystal gaining mass and falling 

 somewhat faster than the descending liquid around it, till it 

 reaches the bottom. This process goes on until, by the 

 heaping of granules and crystals on the bottom, our lava 

 ocean becomes silted up to the surface. 



Probable Origin of Granite. (§§ 26, 27.) 



§ 26. Upon the suppositions we have hitherto made we 

 have, at the stage now reached, all round the earth at the 

 same time a red hot or white hot surface of solid granules or 

 crystals with interstices filled by the mother liquor still 

 liquid, but ready to freeze with the slightest cooling. The 

 thermal conductivity of this heterogeneous mass, even 

 before the freezing of the liquid part, is probably nearly 

 the same as that of ordinary solid granite or basalt at a 

 red heat, which is almost certainly * somewhat less than the 

 thermal conductivity of igneous rocks at ordinary tempera- 

 tures. If you wish to see for yourselves how quickly it 

 would cool when wholly solidified take a large macadamising 

 stone, and heat it red hot in an ordinary coal fire. Take it 

 out with a pair of tongs and leave it on the hearth, or on a 

 stone slab at a distance from the fire, and you will see that 

 in a minute or two, or perhaps in less than a minute, it cools 

 to below red heat. 



§ 27. Half an hour f after solidification reached up to the 

 surface in any part of the earth, the mother liquor among the 

 granules must have frozen to a depth of several centimetres 

 below the surface and must have cemented together the 

 granules and crystals, and so formed a crust of primeval 

 granite, comparatively cool at its upper surface, and red hot 

 to white hot, but still all solid, a little distance down ; 

 becoming thicker and thicker very rapidly at first ; and 

 after a few weeks certainly cold enough at its outer surface 

 to be touched by the hand. 



* Proc. K. S., May 30, 1895. 



t Witness the rapid cooling of lava running red hot or white hot from 

 a volcano, and after a few days or weeks presenting a black hard crust 

 strong enough and cool enough to be walked over with impunity. 



