300 LYELL'S ELEMENTS OF GEOLOGY. 



Chronological Relations of Metamorphic Rocks. 



metamorphic, consists of, 1st, saccharine marble; 2dly, talcose- 

 schist ; and 3dly, of quartz rock and gneiss ; where unaltered, 

 of, 1st, fossihferous limestone ; 2dly, shale ; and 3dly, sand- 

 stone. 



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

 mica-schist, hornblende-schist, chlorite-schist, hypogene lime- 

 stone, 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 crystalline strata gave way to that of 

 ordinary sedimentary deposits. Such clay -slates, in fact, are 

 variable in composition, and sometimes alternate with fossihfe- 

 rous strata, so that they may be said to belong almost equally to 

 the sedimentary and metamorphic order of rocks. It is proba- 

 ble that had they been subjected to more intense plutonic 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 rocks. — Hum- 

 boldt 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 re- 

 cognize 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 resem- 

 blance 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 con- 

 tain fossils of different shapes and mineral composition, and ac- 

 quire a variety of tints from the mixture of various kinds of sedi- 

 ment. 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 

 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 homogeneous in their composition than the 



