Ch. XXXV.] MICA-SCHIST, CLAY-SLATE, ETC. 589 



Hornblende-schist is usually black, and composed principally of horn- 

 blende, 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 com- 

 posed 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 regular 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 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 contains. 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 metamorphic 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. 



Chlorite-schist is a green slaty rock, in which chlorite is abundant in 

 foliated plates, usually blended with -minute grains of quartz, or sometimes 

 with felspar or mica ; often associated with, and graduating into, gneiss 

 and clay-slate. 



Crystalline or Metamorphic limestone. — This hypogene rock, called by 

 the earlier geologists primary limestone, is sometimes a white crystalline 

 granular marble, which when in thick beds can be used in sculpture ; 

 but more frequently it occurs in thin beds, forming a foliated schist much 

 resembling in color and appearance certain varieties of gneiss and mica- 

 schist. When it alternates with these rocks, it often contains some crys- 

 tals of mica, and occasionally quartz, felspar, hornblende, talc, chlorite, 

 garnet, and other minerals. It enters sparingly into the structure of the 

 hypogene districts of Norway, Sweden, and Scotland, but is largely de- 

 veloped in. the Alps. 



Before offering any farther observations on the probable origin of the 

 metamorphic rocks, I subjoin, in the form of a glossary, a brief explanation 

 of some of the principal varieties and their synonyms. 



