Geology and Natural History. 163 



(thermal capacity -36 per gramme, according to Barus) and in 

 consequence great downward rushes of this cooled liquid, and 

 upwards of hot liquid, spreading out horizontally in all direc- 

 tions when it reaches the surface. When the sinking liquid gets 

 within perhaps 20 or 1 or 5 kilometers of the bottom, its tem- 

 perature* 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, be- 

 ginning a few kilometers above the bottom, we have a snow 

 shower of solidified lava or of crystalline flakes, or prisms, or 

 granules of feldspar, mica, hornblende, quartz, and other ingre- 

 dients: each little crystal gaining mass and falling somewhat 

 faster than the descending liquid around it, till it reaches the bot- 

 tom. This process goes on until, by the heaping of the granules 

 and crystals on the bottom, our lava ocean becomes silted up to 

 the surface." 



"Probable Origin of Granite. — §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 f somewhat less than the thermal conductivity of igneous 

 rocks at ordinary temperatures. If you wish to see for yourselves 

 how quickly it would cool when wholly solidified, take a large 

 macadamizing 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| after solidification reached up to the sur- 

 face in any part of the earth, the mother liquor among the gran- 

 ules must have frozen to a depth of several centimeters below the 

 surface and must have cemented together the granules and crys- 

 tals, 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." 



* The temperature of the sinking liquid rock rises in virtue of the increasing 

 pressure : but much less than does the freezing point of the liquid or of some of 

 its ingredients. (See Kelvin, Math, and Phys. Papers, vol. iii, pp. 69, 70.) 



f Proc. Koy. Soc, May 30, 1895. 



\ Witness the rapid cooling of lava running red hot or white hot from a vol- 

 cano, and after a few days or weeks presenting a black hard crust strong enough 

 and cool enough to be walked over with impunity. 



