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ELEMENTS OF GEOLOGY. 
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CHAPTER XXXIII. 
METAMORPHIC ROCKS. 
General Character of Metamorphic Eocks.—Gneiss.—Hornblende-schist.— 
Serpentine.—Mica - schist.—Clay - slate.—Quartzite.—Chlorite - schist.— 
Metamorphic Limestone.—^ Origin of the metamorphic Strata.— Their 
Stratification.—Fossiliferous Strata near intrusive Masses of Granite con¬ 
verted into Eocks identical with different Members of the metamorphic 
Series.—Arguments hence derived as to the Nature of Plutonic Action.— 
Hydrothermal Action, or the Influence of Steam and Gases in producing 
Metamorphism.—Objections to the metamorphic Theory considered. 
We have now considered three distinct classes of rocks: 
first, the aqueous, or fossiliferous; secondly, the volcanic; 
and, thirdly, the plutonic; and it remains for us 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 opinion 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 formations 
or crystalline schists. 
These rocks, when in their characteristic or normal state, 
are wholly devoid of organic remains, and contain no dis¬ 
tinct fragments of other rocks, whether rounded or angular. 
They sometimes break out in the central parts of mountain 
chains, but in other cases extend over areas of vast dimen¬ 
sions, occupying, for example, nearly the whole of Xorway 
and Sweden, where, as in Brazil, they appear alike in the 
lower and higher grounds. However crystalline these rocks 
may become in certain regions, they never, like granite, or 
trap, send veins into contiguous formations. In Great Brit¬ 
ain, 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. 
Many attempts have been made to trace a general order 
of succession or superposition in the members of this family; 
clay-slate, for example, having been often supposed to hold 
invariably a higher geological position than mica-schist, and 
