OUTLINES OF GEOLOGY. 



81 



and, assuming their form, generate petrifactions. The che- 

 mical formations also enclose and envelop the organic mat- 

 ter, which gives its impression to them. It may thus be 

 preserved from diminution, but as its elements are combined 

 under an action very different from that of chemical affinity, 

 they gradually enter new combinations, and while the form 

 and structure may remain unchanged, compounds unknown 

 either to the organic or mineral kingdoms are generated. 

 Such compounds also occur when the organized bodies are 

 enclosed only by mechanical deposits. 



188. The foregoing phenomena require close investigation 

 in order to estimate their importance. It is not so with those 

 which go by the name of volcanic ; these, from their violent, 

 although intermitting action, more powerfully arrest the at- 

 tention. The most obvious phenomenon of volcanic agency, 

 is the ejection of quantities of earthy matter liquified by heat. 

 These are thrown out of cylindroidal orifices, and running 

 over their edges, raise them, on consolidating by cooling, into 

 the form of conoidal eminences. The succession of such 

 eruptions gives to the mass of the cones a structure closely 

 resembling the stratification produced by deposit from water. 

 From this it differs in the variable extent of the several lay- 

 ers. The matter thus ejected often differs in its chemical 

 character, but a more obvious distinction arises from the 

 varying rate at which the cooling is effected. In contact 

 with the air, or in masses of small thickness, the cooling is 

 sudden, and the rock formed is compact, and of uniform com- 

 position ; but when the cooling is more gradual, a tendency 

 to crystalization becomes apparent, and finally rocks of un- 

 questionable volcanic origin assume a granitoid structure ; 

 the several elements contained in the liquid mass having 

 obeyed their chemical affinities, and the compounds thus 

 formed, arranged themselves according to the laws of crys- 

 taline aggregation. 



