78 Prof. Bischof on the Natural History of 



rocks. These phenomena, then, belong entirely to the same 

 class as the elevations of lava in volcanic craters. When, on 

 the other hand, no changes are perceived in the contiguous 

 rocks, when these have been simply broken through and up- 

 raised, when the broken masses consist of acute-angled fragments 

 of all dimensions heaped one upon another, then wc cannot as- 

 sume that the elevations took place in a fluid or softened state. 

 AVere elevations of this kind the work of a short space of 

 time, or did they proceed slowly ? In vain do we look around 

 us for some clew to the solution of this question. From phy- 

 sical gi'ounds we are led to the following conclusions. If fused 

 rock come in contact with water in the interior of the earth, 

 the watery vapour disengaged will operate, with the whole ex- 

 pansive force which it can acquire from the heat of the rock, in 

 a short time ; provided that the continued formation of vapour 

 be not limited by want of water. It is the same process as that 

 which takes place in the glass-blower's blow-pipe when he forms 

 large globes. If, then, water acts on fused masses in a confined 

 space, we have the conditions requisite for producing a rapid ele- 

 vation, and therefore, as a general rule, we may regard elevations 

 of fused masses and rapid elevations as co-ordinate phenomena. 

 If, on the other hand, we imagine a solid rock deep under the 

 surface, whose temperature is far below a red heat, then its 

 elevation can only take place when a considerable source of 

 heat exists under the rock, which gives rise to the formation of 

 vapour. But the more the heat of the vapour exceeds that of 

 the rock, which is to be raised and supported by it, the more 

 will it become condensed, and thus a great part of the effect is 

 lost. If the condensed vapour return to the source of heat, it 

 will again assume the form of vapour, and thus a constant cir- 

 culation will ensue. It is actually a process of heating by 

 steam. If the solid rock be a very bad conductor of heat, then 

 that surface which is in contact with the vapour, may gradually 

 acquire its temperature, and the vapour thus attain its maxi- 

 mum of operative force. It naturally depends on the weight 

 of the solid mass, whether the vapour can effect its elevation, 

 and by what elasticity. Although we must suppose that the 

 elastic force of the vapour progressively increases, in proportion 

 as the temperature of the surface of rock in contact with it rises ; 



