§ 52. EFFECTS OF HIGH TEMPERATURE. 103 



suffice to remark, that in silicified wood the most minute 

 vegetable structure may often be detected, although the 

 specimens will strike fire with steel; and that the most 

 delicate and perishable animal tissues are often preserved 

 in flint. 



52. Effects of high temperature. — The phenomena 

 presented to our notice in this investigation of the Geysers 

 of Iceland, and other natural solutions of silex in thermal 

 waters, lead to the consideration of another agent in the 

 transmutations that take place in the crust of the globe. 

 It must be obvious to every intelligent mind, that beds of 

 loose and porous materials can have acquired hardness and 

 solidity only by one of the following processes; namely, either 

 by matter dissolved in a fluid, and afterwards deposited 

 among the porous masses in the manner just described; or 

 by the reduction of the materials by heat into a state of 

 softness or fusion, and their subsequent conversion, by cool- 

 ing, into a solid mass.* Fire, or to speak more correctly, a 

 high temperature, however induced — whether by electro- 

 magnetic influence, or by central or medial sources of heat — 

 and water, are therefore the chief agents by which the mine- 

 ral masses composing the crust of our planet have been 

 and are still being modified. We have already seen how 

 vast are the changes which result from the effects of water ; 

 we must now take a rapid survey of the influence which 

 caloric is capable of exerting ; an influence far more uni- 

 versal and varied than we may at first be prepared to expect. 



The expansive power which heat exerts on most sub- 

 stances, and its conversion of the most solid and durable 

 bodies, first into a fluid, and lastly into a gaseous state, are 

 phenomena so familiar as to require no lengthened comment. 

 But the effects of heat are found to vary according to the 

 circumstances under which bodies are submitted to its 

 influence ; hence the changes induced by high temperature 

 * Plavfair. 



