THE AGE OF THE EABTH. 25 



reaches the bottom. This process goes on until, bytlie heaping 

 of grannies and crystals on the bottom, onrlava ocean becomes 

 silted np to the sm'face 



Probable Origix of Granite. (§§ 2(5, 27.) 

 § 2(). 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 Avhite hot surface of solid 

 grannies or crystals with interstices filled by the mother 

 liquor still liquid, but ready to freeze Avith the slightest 

 cooling. The thermal conductivity of this heterogeneous 

 mass, even before the freezing of the liquid part, is pro- 

 bably 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 

 oidinary temperatures. If you Avish to see for yourselves 

 how quickly it would cool Avhen 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 j)ei"haps in less than a 

 minute, it cools to below red heat. 



§ 27. Half an hourf 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 

 beloAv 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 Avhite hot, but still all solid, a little distance down ; 

 becoming thicker and thicker very rapidly at first ; and after 

 a few Aveeks certainly cold enough at its outer surface to be 

 touched by the hand. 



Probable Origin of Basaltic Rock. (§§ 28, 29.) 



§ 2S. We have hitherto left, Avithout much consideration, 

 the mother liquor among the crystalline granules at all 

 depths below the bottom of our shoaling lava ocean. It 

 Avas ]>rob;ibly this intersritial mother liquor that was destined 

 to form the basaltic rock of future geological time. Wliat- 



* Proc. U.S., May ,30, LS95. 



J Witness the I'ajjid c-ooling 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 iin]mnity. 



