THEORY OF THE EARTH. 453 



Such is a brief summary of the involved points in 

 the theory of the rocks of the Earth, in as far as they 

 are in their original condition : but that theory, as 

 they are now visible, is not perfect, till the posterior 

 action of water, and apparently of air also, on them, 

 is considered. 



I have shown that water, so loosely united as to b e 

 separable by evaporation, exists in every rock, though 

 of igneous origin, as it does even in quartz. Holding 

 certain earths in solution, it thus deposits them in cavi- 

 ties: and thence the theory of calcareous and quartz 

 veins, under whatever form ; as also of the amygdaloi- 

 dal nodules, however complicated: these including the 

 agates and the septaria. The crystals and stalactites 

 of cavities are but the same things, uncompleted: and, 

 in this latter case, I have shown that chalcedony en- 

 tangles living vegetables and animal remains. I have 

 however left a* possible exception as to quartz in the 

 amygdaloids ; having proved that it can be crystallized 

 by sublimation. 



If deep-seated rocks thus possess a different character 

 from superficial ones, I have also proved that the in- 

 cluded water sometimes decomposes them within the 

 earth, without access of air; selecting even a single 

 rock, stratified or igneous, while superior ones escape: 

 thus explaining many former difficulties in geology, 

 and ending many disputes. I have proved a still more 

 important fact respecting some of the igneous rocks; 

 though yet uncertain how far exposure to air also is 

 necessary. This is, that crystalline traps acquire 

 an earthy aspect, even to enormous depths, in this 

 manner; producing those claystones, the character of 

 which had, not unnaturally, caused many to deny this 

 origin; thus relieving the theory of trap from a 

 great former difficulty. And I desire here to sug- 



