FORMATION OF CALCAREOUS STRATA. 



207 



the beautiful forms of many shells covered with spines, prove that they 

 could not have been drifted from a distance, or deposited in an ag- 

 itated ocean. 



I have never been able to comprehend, why any peculiar difficulties 

 should be supposed to attend the enquiries respecting the origin of 

 calcareous or magnesian earths; or, of the carbon and sulphur oc- 

 curring in rocks, in the state of carbonic or sulphuric acids. It 

 would be equally proper to institute an enquiry into the origin of si- 

 lex or alumine. I hold the earth itself, and its ancient atmosphere, 

 to have been the great chemical laboratories, in which all the solid 

 and fluid parts of the surface were originally prepared and formed. 

 This opinion I stated at some length in Chap. XVI. of the second 

 edition of this work in 1815, and also in the third Edition, in a chap- 

 ter on the agency of subterranean fire in the formation of rocks, and 

 on igneous and aqueous eruptions of earthy matter. It has been too 

 much the fashion to consider all the secondary strata as mechan- 

 ical depositions ; but the siliceous strata in the Paris basin, the lay- 

 ers of flint in chalk, and the beds of chert or hornstone in transi- 

 tion limestone, are certainly as much original formations as granite 

 itself. 



In referring to the vast magnitude of ancient volcanoes, I have 

 stated that they had doubtless an important office to perform in na- 

 ture : and can it be unreasonable to believe, that the earth itself is 

 the great laboratory and storehouse, where the materials that form its 

 surface were prepared, and from which they were thrown out upon 

 the surface in an igneous, aqueous, or gaseous state, either as melted 

 lava, or in aqueous solution, or in mechanical admixture with water 

 in the form of mud, or in the comminuted state of powder or sand ?' 

 Inflammable and more volatile substances may have been emitted in 

 a gaseous state, and become concrete on the surface. 



These primeval eruptions, judging from the size of the ancient fis^ 

 sures and craters, may have been sufficient to cover a large portion 

 of the globe. Nor can it be deemed improbable, that still larger 

 and more ancient craters have been entirely covered by succeeding 

 eruptions. In proportion as the formation of the surface advanced, 

 these eruptions might decline, and be, more and more, limited in 

 their operation. 



It is not necessary to suppose, that these subterranean eruptions 

 consisted only of lava in a state of fusion. The largest active vol- 

 canoes at present existing, throw out the different earths intermixed 

 with water in the form of mud. Nor should we limit the eruptions 

 of earthy matter in solution or suspension, to volcanic craters : the 

 vast fissures or rents which intersect the different rocks, may have 

 served for the passage of siliceous solutions to the surface. We 

 know no instances in nature of siliceous earth being held in aqueous 

 solution, except in the waters of hot or boiling springs ; and hence 

 it seems reasonable to infer, that many siliceous rocks and veins have 



