764 MINERAL CHARACTER OF HYPOGENE ROCKS. [Ch. XXXVII. 



a particular arrangement may prevail throughout countries of great 

 extent, for the same reason that it is traceable in those sedimentary 

 formations from which crystalline strata are derived. Thus, for 

 example, we have seen that in the Apennines, near Carrara, the de- 

 scending series, where it is metamorphic, consists of, 1st, saccharine 

 marble ; 2 dry, talcose-schist ; and 3dly, of quartz-rock and gneiss : 

 where unaltered, of 1st, fossiliferous limestone; 2dly, shale; and 3dly, 

 sandstone. 



But if we investigate different mountain chains, we find gneiss, 

 mica-schist, hornblende-schist, chlorite-schist, hypogene limestone, 

 and other rocks, succeeding each other, and alternating with each 

 other in every possible order. It is, indeed, more common to meet 

 with some variety of clay-slate forming the uppermost member of a 

 metamorphic series than any other rock ; but this fact by no means 

 implies, as some have imagined, that all clay-slates were formed at 

 the close of an imaginary period, when the deposition of the crys- 

 talline strata gave way to that of ordinary sedimentary deposits. 

 Such clay-slates, in fact, are variable in composition, and sometimes 

 alternate with fossiliferous strata, so that they may be said to belong- 

 almost equally to the sedimentary and metamorphic order of rocks. 

 It is probable that had they been subjected to more intense piutonic 

 action, they would have been transformed into hornblende-schist, 

 foliated chlorite-schist, scaly talcose-schist, mica-schist, or other 

 more perfectly crystalline rocks, such as are usually associated with 

 gneiss. 



Uniformity of Mineral Character in Hypogene Bocks. — It is most 

 true, as Humboldt has happily 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. There is 

 certainly a great and striking general resemblance in the principal 

 kinds of hypogene rocks in all countries, however different their ages ; 

 but each of them, as we have before seen, must be regarded as geo- 

 logical families of rocks, and not as definite mineral compounds. 

 They are more uniform in aspect than sedimentary strata, because 

 these last are often composed of fragments varying greatly in form, 

 size, and color, and contain fossils of different shapes and mineral 

 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 uniform 

 in their action, the same in every climate, and wholly un disturbed by 

 mechanical and organic causes. 



It would, however, be a great error to assume, as some have done, 

 that the hypogene rocks, considered as aggregates of simple minerals, 

 are really more homogeneous in their composition than the several 

 members of the sedimentary series. In the first place, different as- 



