734 HORNBLENDE-SCHIST, MICA-SCHIST, ETC. [Oh. XXXV. 



Instead of consisting of these thin laminae, gneiss is sometimes 

 simply divided into thick "beds, in which the mica has only a slight 

 degree of parallelism to the planes of stratification. 



The term "gneiss," however, in geology is commonly used in a 

 wider sense, to designate the formation in which the above-mentioned 

 rock prevails, but with which any one of the other metamorphic 

 rocks, and more especially hornblende-schist, may alternate. These 

 other members of the metamorphic series are, in this case, considered 

 as subordinate to the true gneiss. 



The different varieties of rock allied to gneiss, into which felspar 

 enters as an essential ingredient, will be understood by referring to 

 what was said of granite. Thus, for example, hornblende may be 

 superadded to mica, quartz, and felspar, forming a syenitic gneiss ; or 

 talc may be substituted for mica, constituting talcose-gneiss, a rock 

 composed of felspar, quartz, and talc, in distinct crystals or grains 

 (stratified protogine of the French). 



Hornblende-schist is usually black, and composed principally of 

 hornblende, with a variable quantity of felspar, and sometimes grains 

 of quartz. When the hornblende and felspar are nearly in equal 

 quantities, and the rock is not slaty, it corresponds in character with 

 the greenstones of the trap family, and has been called " primitive 

 greenstone." It may be termed hornblende rock. Some of these 

 hornblendic masses may really have been volcanic .rocks, which have 

 since assumed a more crystalline or metamorphic texture. 



Mica-schist, or Micaceous Schist, is, next to gneiss, one of the most 

 abundant rocks of the metamorphic series. It is ^ slaty, essentially 

 composed of mica and quartz, the mica sometimes appearing to con- 

 stitute the whole mass. Beds of pure quartz also occur in this forma- 

 tion. In some districts, garnets in regular twelve-sided crystals form 

 an integrant part of mica-schist. This rock passes by insensible gra- 

 dations into clay-slate. 



Clay-slate, or Argillaceous Schist. — This rock sometimes resembles 

 an indurated clay or shale. It is for the most part extremely fissile, 

 often affording good roofing-slate. Occasionally it derives a shining 

 and silky lustre from the minute particles of mica or talc which it con- 

 tains. It varies from greenish or bluish-gray to a lead color ; and it 

 may be said of this, more than of any other schist, that it is common 

 to the metamorphic and fossiliferous series, for some clay-slates taken 

 from each division would not be distinguishable by mineral characters 

 alone. 



Quartzite, or Quartz Rock, is an aggregate of grains of quartz 

 which are either in minute crystals, or in many cases slightly rounded, 

 occurring in regular strata, associated with gneiss or other meta- 

 morphic rocks. Compact quartz, like that so frequently found in 

 veins, is also found together with granular quartzite. Both of these 

 alternate with gneiss or mica-schist, or pass into those rocks by the 

 addition of mica, or of felspar and mica. 



