350 SPECIAL GEOLOGY. 



place, in the abrading force of running water, by which the 

 solid materials of the earth are continually worn off and 

 transferred to the beds of adjacent seas ; and, secondly, in 

 the fissuring and cleaving of rocks, by the expansive power 

 of frost. 



As the agencies above mentioned are chiefly referable to 

 destructive causes, and are attributed to the action of water, 

 those of a renovative nature are assigned to elevating forces, 

 and are principally due to the agency of fire. By these 

 antagonistic powers, the solid crust of the earth is changed 

 and renewed. The agency of heat is manifested, either by 

 the rapid effects of the volcano, or the slower process of 

 gradual upheaval; the former being exemplified by the 

 raising of Monte Nuovo in a night, the latter by the uplifting 

 of Sweden and part of Scandinavia by gradual continental 

 elevation. 



The conservative principle is also exemplified in the 

 tendency of the materials derived from the destruction of 

 pre-existing rocks, to become consolidated into new strata, 

 by means of cement, either of a siliceous, calcareous, or ferru- 

 ginous nature. The production of silex, which is far more 

 abundant in the older formations than in those of recent 

 date, constitutes a problem which the present state of our 

 knowledge does not enable us to solve. The siliceous depo- 

 sits of the springs of Carlsbad, and, in particular, the Geysers 

 of Iceland, are due to the action of thermal agents pro- 

 duced by the internal heat of the earth, which, it is sup- 

 posed, was more active during the earlier periods of its 

 history than at present. The nature of calcareous cement 

 is more easily explained. Most fresh water holds a certain 

 quantity of carbonic acid gas and carbonate of lime in solu- 

 tion. A change of temperature liberates the gas, and the lime 

 is, in consequence, precipitated. Streams thus impregnated 

 flowing over loose materials, as sand, pebbles, &c., have the 

 effect of cementing them into rock, and producing limestone, 

 calcareous sandstone, or conglomerate ; while waters hold- 

 ing iron in solution, cement similar substances into con- 

 glomerations of a ferruginous nature. 



Rivers exemplify the conservative principle, in the forma- 

 tion of deltas at their junction with the sea, by which the 

 materials they have abraded and swept away are thrown 



