134 



LYELL'S ELEMENTS OF GEOLOGY. 



Chlorite-schist Hypogene Limestone Metamorphic Rocks. 



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. 



Hypogene or metamorphic limestone, — This rock, commonly 

 called primary limestone, is sometimes a thick bedded white 

 crystalline granular marble used in sculpture; but more fre- 

 quently it occurs in thin beds, forming a foliated. schist much 

 resembling in colour and appearance certain varieties of gneiss 

 and mica-schist. It alternates with both these rocks, and in like 

 manner with argillaceous schist. It then usually contains some 

 crystals of mica, and occasionally quartz, felspar, hornblende, 

 and talc. This member of the metamorphic series enters 

 sparingly into the structure of the hypogene districts of Nor- 

 way, Sweden, and Scotland, but is largely developed in the Alps. 



Before offering any further 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 

 synonymes. 



AcTiNOLiTE-scHiST, A slaty foliated rock, composed chiefly of actinolite, (an 

 emerald-green mineral, allied to hornblende,) with some admixture of tel- 

 spar, or quartz, or mica. 



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

 fossiliferous series. 



Amphibolite. Hornblende rock, which see. 



Argillaceous-schist, or Clay-slate. See p. 133. 



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

 tals of Chiastolite ; in considerable thickness in Cumberland. Chiastolite 

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

 102. 



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



is abundant. See p. 134. 

 Clay-slate, or Argillaceous-schist. See p. 133. 



Eurite and Euritic Porphyry. A base of compact felspar, with grains of 

 laminar felspar, and often mica and other minerals disseminated (Brong- 

 niart.) M. D'Aubuisson regards eurite as an extremely fine grained granite, 

 in which felspar predominates, the whole forming an apparently homogene- 

 ous rock. Eurite has been already mentioned as a plutonic rock, but occurs 

 also in beds subordinate to gneiss or mica-slate. 



Gneiss. A stratified or laminated rock, same composition as granite. See p. 132. 



