736 METAMOKPHIC ROCKS. [Ch. XXXV. 



volcanic and metamorphic series. Agrees in composition with hornblende- 

 schist, but is not fissile. 



Hornblende-schist, or Slate. Composed of hornblende and felspar. See p. 734. 



Hornblendic or Syenitic Gneiss. Composed of felspar, quartz, and hornblende, 



Hypogene Limestone. See p. 73 5. 



Marble. See pp. 12 and 735. 



Mica-schist, or Micaceous-schist. A slaty rock, composed of mica and quartz, in 



variable proportions. See p. 734. 

 Mica-slate. See Mica-schist, p. 734. 



Phyllade. D'Aubuisson's term for clay-slate, from fyvHag, a heap of leaves. 



Primary Limestone. See Hypogene Limestone, p. 735. 



Protogine. See Talcose-gneiss, p. 734 ; when unstratified, it is Talcose-granite. 



Quartz Rock, or Quartzite. A stratified rock ; an aggregate of grains of quartz. 

 See p. 734. 



Serpentine has already been described (p. 606), because it occurs in both divisions 

 of the hypogene series, as a stratified or unstratified rock. 



Talcose-gneiss. Same composition as talcose-granite or protogine, but stratified 



or foliated. See p. 734. 

 Talcose-schist consists chiefly of talc, or of talc and quartz, or of talc and felspar, 



and has a texture something like that of clay-slate. 



Origin of the Metamorphic Strata. 



Having said thus much of the mineral composition of the meta- 

 morphic rocks, I may combine what remains to be said of their 

 structure and history with an account of the opinions entertained of 

 their probable origin. At the same time, it may be well to forewarn 

 the reader that we are here entering upon ground of controversy, 

 and soon reach the limits where positive induction ends, and beyond 

 which we can only indulge in speculations. It was once a favorite 

 doctrine, and is still maintained by many, that these rocks owe their 

 crystalline texture, their want of all signs of a mechanical origin, or 

 of fossil contents, to a peculiar and nascent condition of the planet at 

 the period of their formation. The arguments in refutation of this 

 hypothesis will be more fully considered when I show, in the thirty- 

 seventh chapter, to how many different ages the metamorphic forma- 

 tions are referable, and how gneiss, mica-schist, clay-slate, and hypo- 

 gene limestone (that of Carrara for example) have been formed, not 

 only since the first introduction of organic beings into this planet, 

 but even long after many distinct races of plants and animals had 

 passed away in succession. 



The doctrine respecting the crystalline strata, implied in the name 

 metamorphic, may properly be treated of in this place ; and we must 

 first inquire whether these rocks are really entitled to be called strati- 

 fied in the strict sense of having been originally deposited as sedi- 

 ment from water. The general adoption by geologists of the term 



