Ce. XXVIIL] OF VOLCANIC ROCKS. 605 



purplish color, like an indurated clay ; passes into hornstone ; generally con- 

 tains scattered crystals of felspar and sometimes of quartz. 



Clinkstone. Syn. Phonolite, fissile Petrosilex, see p. 599 ; a grayish-blue rock, 

 having a tendency to divide into slabs ; hard, -with clean fracture, ringing 

 under the hammer ; principally composed of felspar, and, according to 

 Gmelin, of felspar and mesotype. (Leonhard, Mineralrcich, p. 102.) 



Compact Felspar, -which has also been called Petrosilex ; the rock so called in- 

 cludes the hornstone of some mineralogists, is alhed to clinkstone, but is 

 harder, more compact, and translucent. It is a varying rock, of which the 

 chemical composition is not well defined. (MacCullocJvs Classification of 

 Rocks, p. 481.) 



Cornean or Aphanite. A compact homogeneous rock -without a trace of crystalli- 

 zation, breaking with a smooth surface like some compact basalts ; consists 

 of hornblende, quartz, and felspar in intimate combination. It derives its 

 name from the Latin yrovd-cornu, horn, in allusion to its toughness and com- 

 pact texture. 



Diallage Rocs:. Syn. Euphotide, Gabbro, and some Ophiolites. Compounded of 

 felspar and diallage. 



Diokite. A kind of Greenstone, which see. Components, felspar and hornblende 

 in grains. According to Rose, Ann. des Mines, torn. 8, p. 4, diorite consists 

 of albite and hornblende, but Delesse has shown that the felspar may 

 be Oligoclase or Labradorite. {Ann. des Mines, 1S49, torn. 16, p. 323.) 

 Its dark color is due to disseminated plates of hornblende. See above, 

 p. 599. 



Dolerlte. According to Rose (ibid, p. 32), its composition is black augite and 

 Labrador-felspar ; according to Leonhard (Mineralrcich, &c, p. 77), augite, 

 Labrador-felspar, and magnetic iron. See above, p. 598. 



DoiiiTE. An early 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 rock is defined to be a mixture of augite 

 or hornblende and Saussurite, a mineral allied to jade. (Allans Mineral- 

 ogy, p. 158.) Haidinger first observed that in this rock hornblende sur- 

 rounds the crystals of diallage. 



Felstone. Same as compact felspar (-which see). "When crystals of felspar occur 

 in it, it becomes felstone or felspar-porphyry. See also Hornstone. 



Gabbro, see Diallage rock. 



Greenstone. Syn. A mixture of felspar and hornblende. See above, p. 599. 



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

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

 Journ. of Sci. Xo. 42, p. 221.) Graystone lavas are intermediate in compo- 

 sition between basaltic and trachytic lavas. 



Hornblende Rock, 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 con- 

 spicuous 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. 597. 



Hornstone-poephyrt. A kind of felspar porphyry (Leonhard, he. cit.), -with a base 

 of hornstone, a mineral approaching near to flint, -which differs from com- 

 pact felspar in being infusible. 



