Oh. XXXYIL] ORDER OF SUCCESSION. 615 



position, giving rise to the superposition of crystalline upon fossiliferous 

 formations. 



It was remarked, in Chap. XXXIV., that as the hypogene rocks, both 

 stratified and unstratified, crystallize originally at a certain depth beneath 

 the surface, they must always, before they are upraised and exposed at 

 the surface, be of considerable antiquity, relatively to a large portion of 

 the fossiliferous and volcanic rocks. They may be forming at all periods ; 

 but before any of them can become visible, they must be raised above the 

 level of the sea, and some of the rocks which previously concealed them 

 must have been removed by denudation. 



In Canada the fossiliferous beds of the Cambrian formation repose un- 

 conforruably on gneiss, which was evidently crystalline before the deposi- 

 tion of the Cambrian (or Potsdam) sandstone. In Anglesea, as was 

 before remarked, the metamorphism of the schists, according to the 

 observations of Professor Ramsay, took place during the Lower Silurian 

 period. Coupling these conclusions with the fact that a hypogene tex- 

 ture has been superinduced in the Alps on Middle Eocene deposits (see 

 p. 600), we cannot doubt that, hereafter, geologists will succeed in de- 

 tecting crystalline schists of almost every age in the chronological series, 

 although the quantity of metamorphic rocks visible at the surface must, 

 for reasons above explained, diminish rapidly in proportion as the monu- 

 ments of newer eras are investigated. 



Order of succession in metamorphic rocks. — There is no universal and 

 invariable order of superposition in metamorphic rocks, although a par- 

 ticular 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 descending series, where 

 it is metamorphic, consists of, 1st, saccharine marble; 2dly, talcos> 

 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 crystalline 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 meta- 

 morphic order of rocks. It is probable 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 

 mth gneiss. 



