the Earth's Contraction from Cooling, 369 



ritic rocks should exist but sparingly in the Archaean instead of 

 being the prevailing kinds. 



4. Change of Density and Volume in Solidification. 



All the rocks above mentioned have a higher density in the 

 solid state than when in fusion. According to Delesse*, gra- 

 nite decreases in density in passing from the stone to the glass 

 condition f 9 to 11 per cent. ; syenite, syenitic granite 8 to 9 

 per cent. ; diorite 6 to 8 per cent. ; dolerite and melaphyre 5 to 

 7 per cent. ; basalt and trachyte 3 to 5 per cent. The difference 

 in volume is thus large between a rock in the solid and glass 

 states. As to the difference between the glass and liquid states, 

 we have no precise observations, and only know that it is ex- 

 ceedingly small J. 



From the above facts, sustaining the nearly total absence of 

 quartz-bearing rocks from the crust, and therefore of granite or 

 syenite, and the other considerations presented, we may take 8 per 

 cent, as the probable average change of density for the earth's 

 crust between the stony and the liquid states ; which is equivalent 

 to a change of volume from 100 to 92 per cent, in passing from 

 the liquid to the stone condition. 



5. Process of Solidification and Continent-making, 



The crust over the areas of solidification, after attaining a 

 thickness that would enable it to overcome by its gravity the 

 cohesion in the liquid rock beneath, would sink in masses 

 and then be remelted by the heat beneath, and this remelt- 

 ing would cool somewhat the liquid layer. So this process 

 of crusting and sinking with an overflow from either side, re- 

 melting, and cooling, would go forward until the masses could 

 sink without much remelting, to bring up at the level where 

 the density of the liquid layer was that of the solid rock, if 

 this liquid layer had not become so stiffly viscid by the cool- 

 ing as to offer too great resistance to their reaching quite 

 to this level. The sinking rock-masses may have had their 

 density somewhat increased by the pressure to which they were 

 subjected on descending; but whatever density they acquired, 



* Bull. Soc. Giol. de France, part 2, vol. iv. p. 1380 (1847). Delesse's 

 results agree nearly with those of St.-Claire Deville. Bischof in 1841 

 found the volume of basalt in the vitreous and crystalline states as 1 to 

 0*9298, the same in the fluid and crystalline states as 1 to 0*8960, and 

 for granite the corresponding ratios 1 to 0*8420 and 1 '07481. 



t It is to be here noted that the glass and stone conditions are distinct 

 molecular states of the same substance — the former produced under rapid 

 cooling, the latter under slow, — and that common glass will become stone 

 if solidified under a prolonged cooling-process. 



X I am informed that at the Lenox Glass Furnace in Berkshire, Mass., 

 no contraction is noticed in the cooling of the glass. 



Phil. Mag. S. 4. Vol. 46. No. 307. Nov. 1873. 2 C 



