254 POPULAR LECTURES AND ADDRESSES. 



confidence, that whatever may be the relative den- 

 sities of rock, solid and melted, at or about the 

 temperature of liquefaction, it is, I think, probable 

 that cold solid rock is denser than hot melted rock ; 

 and no possible degree of rigidity in the crust could 

 pr<-\ent it from breaking in pieces and sinking 

 wholly below the liquid lava. Something like this 

 may have gone on, and probably did go on, for 

 thousands of years after solidification commenced 

 surface-portions of the melted material losing 

 freezing, sinking immediately, or growing to 

 thicknesses of a few metres, when the surface 

 would be eool and the whole solid dense enough to 

 sink. " This process must go on until the sunk 

 portions of crust build up from the bottom a 

 suffieiently close-ribbed skeleton or frame to allow 

 fivsh incrustations to remain, bridging across the 

 now small areas of lava pools or lakes. 



" In the honeycombed solid and liquid mass thus 

 formed there must be a continual tendency for the 

 liquid, in consequence of its less specific gravity, to 

 .\\-ork its way up ; whether by masses of solid 

 lallim; from the roofs of vesicles or tunnel', and 



