1Q6 SCARCITY OF LIME IN METAMORPHIC ROCKS. [Ch. XXXVII. 



materials winch 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 volcanoes, such as Au- 

 vergne and Tuscany, hundreds of springs, both cold and thermal, 

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

 plentifully charged with carbonate of lime. The quantity of calcare- 

 ous 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 substance 

 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 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 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 volcanoes, as near Naples and in Auvergne. By this pro- 

 cess, 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 cal- 

 careous 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 fossils are ob- 

 literated, wholly or in part, even in tertiary formations — how often 

 vast masses of sandstone and shale, of different ages, and thousands 

 of feet thick, are devoid of fossils — how certain strata may first have 

 been deprived of a portion of their fossils when they became semi- 

 crystalline, or assumed the transition state of Werner — and how the 

 remaining portion may have been effaced when they were rendered 

 metamorphic. Kocks of the last-mentioned class, moreover, have 

 sometimes been exposed again and again to renewed plutonic action. 



* See Principles of Geology, by the Author, Index, " Calcareous Springs." 



