PART I. CHAPTER X. 



135 



Metamorphic Rocks, and their Origin. 



Hornblende Rock, or Amphibolite. The same composition as hornblende- 

 schist, stratified, but not fissile. See p. 100. 



HoRNBLENDE-scHiST, or Slate. Composed chiefly of hornblende, with occa- 

 sionally some felspar. See p. 133. 



HoRNBLENDic or Syenitic Gneiss. Composed of felspar, quartz, and horn- 

 blende. 



Hypogene Limestone. See p. 134. 

 Marble. See p. 1 34. 



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



in variable proportions. See p. 133. 

 Mica-slate. See Mica-schist, p. 133. 



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

 Primary Limestone. See Hypogene Limestone, p. 134. 



Protogine. (See Talcose-gneiss, p. 133.: when unstratified it is Talcose-gra- 

 nite. 



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

 quartz. See p. 133. 



Serpentine occurs in both divisions of the hypogene series, as a stratified or 

 unstratified rock; contains much magnesia; is chiefly composed of the min- 

 eral called serpentine, mixed with diallage, talc, and steatite. The pure 

 varieties of this rock, called noble serpentine, consist of a hydrated silicate 

 of magnesia, generally of a greenish colour ; this base is commonly mixed 

 with oxide of iron. 



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

 stratified or laminated. 



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

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



Whitestone. Same as Eurite. ^ 



Origin of the Metamorphic Strata. 



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

 "amorphic 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 fore- 

 warn the reader that we are here entering upon ground of con- 

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

 and beyond which we can only indulge in speculations. It was 

 once a favourite 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 second part of this volume, to 

 how many different ages the metamorphic formations are refer- 

 able, and how gneiss, mica-schist, clay-slate, and hypogene lime- 

 stone (that of Carrara for example,) have been formed, not only 



