Ch. xxviil] of volcanic kocks. 473 



DiORiTE. A kind of Greenstone, which see. Components, felspar and horn 

 blende in grains. According to Rose, Ann. des Mines, torn. 8, p. 4, dloritt 

 consists of albiLe and hornblende, but Delesse has shown that the felspar 

 may be Oligoclase or Labradorite. {Ann. des Mines, 184'.', torn. 16, p. 

 323.) Its dark color is due to disseminated plates of hornblende. See 

 above, p. 467. 



DoLERiTE. According to Rose {ibid. p. 32), its composition is black augite and 

 Labrador-felspar; according to Leonhard {Mineralreich, &c., p. 77), augite, 

 Labrador-felspar, and magnetic iron. See above, p. 466. 



DoMiTi2. An earthy trachyte, found in the Puy de Dome, in Auvergne. 



EuPHOTiDE. A mixture of grains of Labrador-felspar and diallage. {Rose, ibid. 

 p. 19.) According to some, this rook is defined to be a mixture of augite 

 or hornblende and Saussurite, a mineral allied to jade. {Allan's Mine- 

 ralogy, p. 158.) Haidinger first observed that in this rock hornblende 

 surrounds the crystals of diallage. 



Felspar-porphyry. Syn. Hornstone-porphyry ; a base of felspar, with crystals 

 of felspar, and crystals and grains of quartz. See also Hornstone. 



Gabbro, see Dialhxge rock. 



Greenstone. Syn. A mixture of felspar and hornblende. See above, p. 467. 



Graystone. (Graustein of Werner.) Lead-gray and greenish rock composed of 



felspar and augite, the felspar being more than seventy-five per cent. 



{Scrope, Jonrn. of Sci. ISTo. 42, p. 221.) Graystone lavas are intermediate 



in composition between basaltic and trachytic lavas. 



Hornblende Rcck, or Amphibolite. This rock, as defined by Leonhard, is com- 

 posed entirely of hornblende; but such a rock appears to be exceptional,, 

 and confined to mineral veins. Any rocks in which hornblende plays a 

 conspicuous part, constituting the "roches amphiboliques" of French 

 writers, may be called hornblende rock. They always contain more or 

 less felspar in their composition, and pass into basalt or greenstone, or 

 aphanite. See p. 466. 



HoRNSTONE-poRPiiYRY. A kind of felspar porphyry {Leonhard, loc. cit.) with a 

 base of hornstone, a mineral approaching near to flint, differing from com- 

 pact felspar in being infusible. 



Hypersthene Rock, a mixture of grains of Labrador-felspar and hypersthene 

 {Rose, Ann. des Mines, torn. 8, p. 13), having the structure of syenite or 

 granite ; abundant among the traps of Skye. It is extremely tough, 

 grayish, and greenish black. Some geologists consider it a greenstone, in 

 which hypersthene replaces hornblende ; and this opinion, says Delesse, 

 is borne out by the fact that hornblende usually occurs in hypersthene 

 rock, often enveloping the crystals of hypersthene. The latter have a 

 pearly or metallic-pearly lustre. 



Laterite. a red, jaspery, brick-like rock, composed of silicate of alumina and 

 oxide of iron, or sometimes consisting of clay colored with red ochre. See 

 above, p. 471. 



Melaphyre. a variety of black porphyry composed of Labrador-felspar and a 

 small quantity of augite. Its black color was formerly attributed to dis- 

 seminated microscopic crystals of augite, but M. Delesse has shown that 

 the paste is discolored by hydrochloric acid, whereas this acid does not 

 attack the crystals of augite, which are seen to be isolated, and few in 

 number. {Ann. des Mines, 4th sei'. torn. xii. p. 228.) From //cAaj, mclas, 

 black. 



