616 SCAECITT OF LIME EST METAMORPHIO ROCKS. [Ch. XXXVII. 



Uniformity of mineral character in Hypogene rocks. — Humboldt 

 has emphatically remarked, that when we pass to another hemisphere, 

 we see new forms of animals and plants, and even new constellations in 

 the heavens ; but in the rocks we still recognize our old acquaintances, 

 — the same granite, the same gneiss, the same micaceous schist, quartz- 

 rock, and the rest. It is certainly true that there is a great and striking 

 general resemblance in the principal kinds of hypogene rocks, although 

 of very different ages and countries ; but it has been shown that each 

 of these are, in fact, geological families of rocks, and not definite mineral 

 compounds. They are much more uniform in aspect than sedimentary 

 strata, because these last are often composed of fragments varying greatly 

 in form, size, and colour, and contain fossils of different shapes and min- 

 eral composition, and acquire a variety of tints from the mixture of 

 various kinds of sediment. The materials of such strata, if melted and 

 made to crystallize, would be subject to chemical laws, simple and uni- 

 form in their action, the same in every climate, and wholly undisturbed 

 by mechanical and organic causes. 



Nevertheless, it would be a great error to assume that the hypogene 

 rocks, considered as aggregates of simple minerals, are really more homo- 

 geneous in their composition than the several members of the sediment- 

 ary series. In the first place, different assemblages of hypogene rocks 

 occur in different countries ; and, secondly, in any one district, the rocks 

 which pass under the same name are often extremely variable in their 

 component ingredients, or at least in the proportions in which each of 

 these are present. Thus, for example, gneiss and mica-schist, so abun- 

 dant in the Grampians, are wanting in Cumberland, Wales, and Corn- 

 wall ; 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 garnetif- 

 erous. And not only do the proportional 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. 463 and table, p. 104). 



The mctamorphic strata, why less calcareous than the fossiliferous. — 

 It has been remarked, that the quantity of calcareous matter in meta- 

 morphic strata, or, indeed, in the hypogene formations generally, is far 

 less than in fossiliferous deposits. Thus the crystalline schists of the 

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

 schist, and other rocks, many thousands of yards in thickness, contaiD 

 an exceedingly small proportion of interstratified calcareous beds, al- 

 though these have been the objects of careful search for economical 

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



