Ch. XXXV.] METAMORPHIC ROCKS. 587 



thrust up bodily, and where we may conceive it to have been dis- 

 tended laterally by the repeated injection of fresh supplies of melted 

 materials.* 



CHAPTER XXXV. 



METAMORPHIC ROCKS. 



General character of metamorphic rocks — Gneiss — Hornblende-schist — Mica- 

 schist — Clay-slate — Quartzite — Chlorit e-schist — Metamorphic limestone — Al- 

 phabetical list and explanation of the more abundant rocks of this family — 

 Origin of the metamorphic strata — Their stratification — Fossiliferous strata near 

 intrusive masses of granite converted into rocks identical with different mem- 

 bers of the metamorphic series — Arguments hence derived as to the nature of 

 plutonic action — Time may enable this action to pervade denser masses — From 

 what kinds of sedimentary rock each variety of the metamorphic class may be 

 derived — Certain objections to the metamorphic theory considered — Partial 

 conversion of Eocene slate into gneiss. 



"We have now considered three distinct classes of rocks : first, the 

 aqueous, or fossiliferous ; secondly, the volcauic ; and, thirdly, the plu- 

 tonic, or granitic ; and we have now, lastly, to examine those crystalline 

 (or hypogene) strata to which the name of metamorphic has been assigned. 

 The last-mentioned term expresses, as before explained, a theoretical opin- 

 ion that such strata, after having been deposited from water, acquired, by 

 the influence of heat and other causes, a highly crystalline texture. They 

 who still question this opinion may call the rocks under consideration the 

 stratified hypogene, or schistose hypogene formations. 



These rocks, when in their most characteristic or normal state, are 

 wholly devoid of organic remains, and contain no distinct fragments of 

 other rocks, whether rounded or angular. They sometimes break out in 

 the central parts of narrow mountain chains, but in other cases extend 

 over areas of vast dimensions, occupying, for example, nearly the whole of 

 Norway and Sweden, where, as in Brazil, they appear alike in the lower 

 and higher grounds. In Great Britain, those members of the series 

 which approach most nearly to granite in their composition, as gneiss, 

 mica-schist, and hornblende-schist, are confined to the country north of 

 the rivers Forth and Clyde. 



However crystalline these rocks may become in certain regions, they 

 never, like granite or trap, send veins into contiguous formations, whether 

 into an older schist or granite, or into a set of newer fossiliferous strata. 



Many attempts have been made to trace a general order of succession 



* For the geology of Arran consult the works of Drs. Hutton and MacCullocli, 

 the Memoirs of Messrs. Von Dechen and Oeynhausen, that of Professor Sedgwick 

 and Sir R. Murchison (Geol. Trans. 2d series), Mr. L. A. Necker's Memoir, read to 

 the Royal Soc. of Edin. 20th April, 1840, and Mr. Ramsay's Geol. of Arran, 1841. 

 I examined myself a large part of Arran in 1836. 



