302 LYELL'S ELEMENTS OF GEOLOGY. 



Scarcity of Lime in Metamorphic Rocks. 



tonic 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 ther- 

 mal, flowing out from granite and other rocks, and having their 

 waters plentifully charged with carbonate of lime. The quan- 

 tity of calcareous matter which thqse 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 con- 

 siderable.* 



If the quantity of siliceous and aluminous ingredients brought 

 up by such springs were great, instead of being utterly insigni- 

 ficant, 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 constitu- 

 ents, while we know that the same action imparts to newer depo- 

 sits, 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 calcareous rock chemically precipitated, and of many 

 reefs of shelly and coralline stone, must be derived from mineral 

 matter subtracted by plutonic agency, and driven up by gas and 

 steam from fused dnd 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 residual lime may enter into the 

 composition of augite, hornblende, garnet, and other hypogene 

 minerals. That the removal of the calcareous matter 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 we bear in mind how often fos- 

 sils are obliterated, wholly or in part, even in tertiary forma- 

 tions how often vast masses of sandstone and shale, of different 



* See Principles of Geology, Index, " Calcareous Springs." 



