70 



ON GEOLOGY. 



stones, which chiefly differ from each other in the coarseness or fineness of 

 their grains, or in the cement which connects them. And if the water be 

 loaded, as it ctten is, with minutely divided particles of quartz, it will pro- 

 ceed to crystallize whenever it becomes quiescent ; and will form stalac- 

 tites, agates, cornelians, rock-crystals, plain or coloured, according as it is 

 destitute of or combined with any colouring material : and if the material 

 with which the water be impregnated be lime instead of quartz, the crys- 

 talhzation will be calcareous alabaster, or marble. 



Many of the earths are now known to be metallic oxydes, and all of 

 them are suspected to be so : and hence a degree of heat capable of fusing 

 them, and depriving them of the oxygene which gives them their oxyde 

 form, will necessarily convert them into their metallic state. That such 

 currents of heat, from electricity and other causes, are occasionally, and 

 perhaps in different places perpetually, existing beneath the surface of the 

 earth, the Neptunian is as ready to admit as the Plutonic geologist ; and 

 hence the origin of metallic minerals, of mines, ores, ochres, and pyrites. 



The decomposition of animal and vegetable matter contributes largely, 

 moreover, in the view of the system now before us, to the changes which 

 the globe is perpetually sustaining. The exuviae of shell and coral animals 

 is perpetually adding to the mass of its earths, and laying a foundation for 

 new islands and numerous beds of limestone in which we very often per- 

 ceive impressions of the shells from which the soil has originated. Gn the 

 other hand, we observe numerous quantities of vegetables, both submarine 

 and superficial, heaped and deposited together by currents or other causes, 

 constituting distinct strata, which progressively become decomposed, lose 

 their organization, and confound their own principles with those of the 

 earth's. Hence the origin of pitcoal, and secondary schists or slates ; to 

 which, however, the decomposition of animal substances has also largely 

 contributed. Hence, too, the formation and extrication of a variety of 

 acids and alkalis, which have essentially administered to the actual phse- 

 nomenaof the face of the earth. 



The action of volcanoes has contributed much in all ages, and is still 

 contributing in our own, to the present state of the earth's surface. We 

 have daily proofs of the mountains which it has elevated, and have already 

 noticed it as one source of the numerous islands that stud the face of the 

 ocean ; and we have just adverted to the subterranean agencies of electricity, 

 heat, water, and other gases and fluids which form its fuel. But the ope- 

 ration of volcanoes is more limited and local than that of the preceding 

 agents. " They accumulate substances," says M. Cuvier, " on the sur- 

 face that v/ere formerly buried deep in the bowels of the earth, afler having 

 changed or modified their nature or appearances, and raise them into 

 mountains ; but they have never raised up nor overturned the strata through 

 which their apertures pass, and have in no degree contributed to the ele- 

 vation of the great mountains, which are not volcanic." 



Inundations of seas and rivers have also, from time to time, added their 

 tremendous force ; but there is no ground for concluding that any catas- 

 trophe of this kind has been universal for the last four thousand years ; 

 nor, in fact, that such an event has ever occurred more than once since 

 the earth has been rendered habitable. 



In examining, then, the merits of the antagonist systems of geology 

 before us, the Plutonic is perhaps best entitled to the praise of boldness 

 of conception and unhmited extent of view. It aspires, in many of its 

 im)difications, not only to accountfor the present appearances of the earth. 



