PART 11. CHAPTER XXV. 



301 



Hypogene Rocks, why less calcareous. 



several members of the sedimentary series. In the first place, 

 different assemblages of hypogene rocks occur in different coun- 

 tries ; and secondly, in any one district, the rocks which pass 

 under the same name are often extremely variable in their com- 

 ponent ingredients, or at least in the proportions in which each 

 of these are present. Thus, for example, gneiss and mica- 

 schist, so abundant in the Grampians, are wanting in Cumber- 

 land, Wales, and Cornwall ; in parts of the Swiss and Italian 

 Alps, the gneiss and granite are talcose, and not micaceous, as 

 in Scotland; hornblende prevails in the granite of Scotland- 

 schorl in that of Cornwall — albite in the plutonic rocks of the 

 Andes — common felspar in those of Europe. In one part of 

 Scotland, the mica-schist is full of garnets ; in another, it is 

 wholly devoid of them ; while in South America, according to 

 Mr. Darwin, it is the gneiss, and not the mica-schist, which is 

 most commonly garnetiferous. And not only do the propor- 

 tional quantities of felspar, quartz, mica, hornblende, and other 

 minerals, vary in hypogene rocks bearing the same name ; but, 

 what is still more important, the ingredients, as we have seen, 

 of the same simple mineral are not always constant, (p. 92., 

 and table, p. 102.) 



The Metamorphic strata, why less calcareous than the fos- 

 siliferous. — It has been remarked, that the quantity of calcare- 

 ous matter in metamorphic strata, or, indeed, in the hypogene 

 formations generally, is far less than in fossiliferous deposits. 

 Thus the crystalline schists of the Grampians in Scotland, con- 

 sisting of gneiss, mica-schist, hornblende-schist, and other rocks, 

 many thousands of yards in thickness, contain an exceedingly 

 small proportion of interstratified calcareous beds, although 

 these have been the objects of careful search for economical pur- 

 poses. Yet limestone is not wanting in the Grampians, and it is 

 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 connexion 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 

 metamorphic rocks generally, seems to be the result of some 

 general cause. So long as the hypogene rocks were believed to 

 have originated antecedently 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 plu- 

 A a 



