Ch. XXXV.] CHLORITE-SCHIST, ETC.— MET AMORPHIC ROCKS. 735 



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 crys- 

 talline granular marble, which when in thick beds. can be used in 

 sculpture ; but more frequently it occurs in thin beds, forming a foli- 

 ated schist much resembling in color and appearance certain varieties 

 of gneiss and mica-schist. When it alternates with these rocks, it 

 often contains some crystals 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 developed 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. 



Explanation of the Names, Synonyms, and Mineral Composition of 

 the more abundant Metamorphic Rocks. 



Actinolite-schist. A slaty foliated rock, composed chiefly of actinolite (an em- 

 erald-green mineral, allied to hornblende), with some admixture of garnet, 

 mica, and quartz. 



Ampelite. Aluminous slate (Brongniart) ; occurs both in the metamorphic and 

 fossiliferous series. 



Amphibolite. Hornblende rock, which see. 



Argillaceous-schist, or Clay-slate. See p. 734. 



Arkose. Xame given hy Brongniart to a compound of the same materials as 

 granite, which it often resembles closely. It is found at the junction of 

 granite with formations of different ages, and consists of crystals of felspar, 

 quart/, and sometimes mica, which, after separation from their original 

 matrix by disintegration, have been reunited by a siliceous or quartzose 

 cement. It is often penetrated by quartz veins. 



Chiastolite-slate scarcely differs from clay-slate, but includes numerous crystals 

 of Chiastolite : in considerable thickness in Cumberland. Chiastolite oc- 

 curs in long slender rhomboidal crystals. For composition, see Table, p. 

 608. 



Chlorite-schist. A green slaty rock, in which chlorite, a green scaly mineral, ia 

 abundant. See p. 735. 



Clay-slate, or Argillaceous-schist. See p. 734. 



Eurite has been already mentioned as a plutonic rock (p. 708), but occurs also 

 with precisely the same composition in beds subordinate to gneiss or mica- 

 slate. 



Gneiss. A stratified or foliated rock ; has the same composition as granite. See 

 p. 733. 



Hornblende Rock, or Amphibolite. See above, p. 605. A member both of the 



