Page:Memoirs of the Geological Survey of Great Britain, Volume 1.djvu/30

Rh AND SOUTH-WESTERN ENGLAND. 17

giren depths with the zoological evidences as to those depths afforded by the organic remains contained in such deposits.

The mode in which calcareous matter may be formed is one of great geological interest, and it may be desirable briefly to advert to the con- ditions under which this may be effected. Stalagmitical incrustations may, perhaps, be regarded as the commonest form in which the produc- tion of limestone is made apparent to us. Waters containing an abun- dance of bicarbonate of lime in solution, losing the carbonic acid which kept the lime thus dissolved, common carbonate of lime is thrown down, friable, compact, or crystalline, according to the quickness with which this is eflected. In many situations considerable masses of limestone are tfaos formed. Many rivers, fed by springs charged with bicarbonate of lime flowing into lakes, may be the means of limestone accumulations in them. In like manner rivers of the same character, discharged into tideless seas, may be considered as affording materials in the quiet depths of such seas for the production of limestone. SimUarly charged waters would, however, be differentiy circumstanced in tidal seas, for the movements produced by the tides would prevent that quiet deposit which we can readily conceive to be affected in nearly stiU waters, merely agitated on the surface, or slightiy moved in some situations by currents formed by the friction of the winds upon the sea. It is not difficult to conceive that on some coasts of tideless seas a large amount of calcareous matter may become accumulated after the lapse of ages, from the con- stant additions of water, containing much bicarbonate of lime, falling into a limited area, more particularly in situations where the agitation of the surface waves can be littie felt.

The enormous mass of carbonate of lime forming or entering into the composition of the harder parts of so many animals, and which geology teaches us it has so done from the earliest periods, probably since Hfe was first called into existence upon our planet, must have been in solution before it could have been appropriated to the purposes for which it was required. Land animals obtain it, with otiier requi- sites for their harder parts, either from plants or from water, the former baring extracted it from the soil, and the latter in its passage through or among calcareous matter, the strictly carnivorous creatures getting it through tiie herbivorous. The same modes of appropriating the carbonate of lime prevail probably among fresh-water and marine animals, but among these classes there is reason to conclude that much of it is directly obtained from the aqueous medium by which they are surrounded, constant additions of calcareous matter being made to the lakes and seas by rivers which bring it down in solution.

Coral reefs afford striking instances of masses of carbonate of lime obtained through the medium of animal life, this substance baring been

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