INTRODUCTION. §• 



same causes, and the relations of the two conditions of these strata are readily 

 understood. The connexion between tlie metamorphic and the non.metamorphic 

 regions, of the mountainous and non-mountainous regions, the gradual dying out 

 of the one and the gradual disappearance of the other, coincide with the lines of 

 the accumulation and the gradual thinning of the strata constituting the series. 

 We see, moreover, why the mineral and chemical constitution of the strata oc- 

 cupying these relations, although of the same geological age, should not correspond 

 with each other. 



In treating of metamorphic rocks, Sir Charles Lyell suggests an explanation 

 of the cause of the general absence of calcareous matter from strata of this 

 character, as follows* : 



'■^ The metamorphic strata, why less calcareous than the fossiliferous. — It has been 

 remarked that the quantity of calcareous matter in metamorphic strata, or indeed in the 

 hypogene formations generally, is far less than in fossiliferous deposits. Thus the crystalline 

 gehists of the Grampians in Scotland, consisting of gneiss, mica-schist, hornblende-schist, 

 and other rocks, many thousands of yards in thickness, contain an exceedingly small pro- 

 portion of interstratified calcareous beds, although these have been the objects of careful 

 search for economical purposes. 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 ^t 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 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 moUusca 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 volcanoes, such as Auvergne and Tuscany, hundreds of springs, both cold and 

 thermal, flowing ont from granite and other rocks, and having their waters plentifully 

 charged with carbonate 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 considerablet X. 



• Manual of Elementary Geology, 5th edition : American edition, page 623. 

 t See Principles of Geology, by the Author : Index, " Calcareous springs." 



t [ It seems to me extremely doubtful if this calcareous matter be derived from any very deep 

 source. H.] 



[ PaljEOntoloot III.] 12 



