Ch. XXXVIL] scaecity of lime in metamorphic rocks. 617 



associated sometimes with gneiss, sometimes with mica-schist, and in 

 other places with other members of the metamorphic series. But where 

 limestone occurs abundantly, as at Carrara, and in parts of the Alps, in 

 connection with hypogene rocks, it usually forms one of the superior 

 members of the crystalline group. 



The scarcity, then, of carbonate of lime in the plutonic and meta- 

 morphic rocks generally, seems to be the result of some general cause. 

 So long as the hypogene rocks were believed to have originated antece- 

 dently to the creation of organic beings, it was easy to impute the 

 absence of lime to the non-existence of those mollusca and zoophytes by 

 which shells and corals are secreted ; but when we ascribe the crystalline 

 formations to plutonic action, it is natural to inquire whether this action 

 itself may not tend to expel carbonic acid and lime from the materials 

 which it reduces to fusion or semi-fusion. Although we cannot descend 

 into the subterranean regions where volcanic heat is developed, we can 

 observe in regions of spent volcanos, such as Auvergne and Tuscany, 

 hundreds of springs, both cold and thermal, flowing out from granite 

 and other rocks, and having their waters plentifully charged with carbo- 

 nate of lime. The quantity of calcareous matter which these springs 

 transfer, in the course of ages, from the lower parts of the earth's crust 

 to the superior or newly formed parts of the same, must be considerable.* 



If the quantity of siliceous and aluminous ingredients brought up by 

 such springs were great, instead of being utterly insignificant, it might 

 be contended that the mineral matter thus expelled implies simply the 

 decomposition of ordinary subterranean rocks ; but the prodigious excess 

 of carbonate of lime over every other element must, in the course of 

 time, cause the crust of the earth below to be almost entirely deprived of 

 its calcareous constituents, while we know that the same action imparts 

 to newer deposits, ever forming in seas and lakes, an excess of carbonate 

 of lime. Calcareous matter is poured into these lakes, and the ocean, 

 by a thousand springs and rivers ; so that part of almost every new cal- 

 careous rock chemically precipitated, and of many reefs of shelly and 

 coralline stone, must be derived from mineral matter subtracted by plu- 

 tonic agency, and driven up by gas and steam from fused and heated 

 rocks in the bowels of the earth. 



Not only carbonate of lime, but also free carbonic acid gas is given 

 off plentifully from the soil and crevices of rocks in regions of active 

 and spent volcanos, as near Naples, and in Auvergne. By this process, 

 fossil shells or corals may often lose their carbonic acid, and the resi- 

 dual lime may enter into the composition of augite, hornblende, garnet, 

 and other hypogene minerals. That the removal of the calcareous- mat- 

 ter of fossil shells is of frequent occurrence, is proved by the fact of such 

 organic remains being often replaced by silex or other minerals, and 

 sometimes by the space once occupied by the fossil being left empty, or 

 only marked by a faint impression. We ought not indeed to marvel at 

 the general absence of organic remains from the crystalline strata, when 

 * See Principles, Index, " Calcareous Springs." 



