PART I. CHAPTER X. 133 



Hornblende-Schist Mica-Schist Clay-Slate Quartzite. 



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 a formation in which the above men- 

 tioned rock prevails, but with which any one of the other meta- 

 morphic rocks, and more especially hornblende-schist, may alter- 

 nate. These other members of the metamorphic series are, in 

 this case, considered as subordinate to the true gneiss. In some 

 rare instances fragments of pre-existing rocks may be detected 

 in gneiss. 



The different varieties of rock allied to gneiss, into which fel- 

 spar enters as an essential ingredient, will be understood by re- 

 ferring to what was said of granite. Thus, for example, horn- 

 blende may be superadded to mica, quartz, and felspar, forming 

 a syenitic gneiss ; or talc may be substituted for mica, consti- 

 tuting 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 some- 

 times grains of quartz. When the hornblende and felspar are 

 nearly -in equal quantities, and the rock is not slaty, it corre- 

 sponds in character with the greenstones of the trap family, and 

 has been called " primitive greenstone." Some of these horn- 

 blendic 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 constitute the whole mass. Beds of pure quartz 

 also occur in this formation. In some districts garnets in regu- 

 lar twelve-sided crystals form an integrant part of mica-schist. 

 This rock passes by insensible gradations into clay-slate. 



Clay-slate, or Argillaceous schist. This rock resembles an 

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

 often affording good roofing slate. It may consist of the ingre- 

 dients of gneiss, or of an extremely fine mixture of mica and 

 quartz, or talc and quartz. Occasionally it derives a shining 

 and sill^ lustre from the minute particles of mica or talc which 

 it contains. It varies from greenish or bluish-grey to a lead 

 colour. 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 dis- 

 tinguishable by mineralogical characters. 



Quartzite, or Quartz rock, is an aggregate of grains of 



M 



