100 LYELL'S ELEMENTS OP GEOLOGY. 



Mineral Composition of Volcanic Rocks. 



olivine, can easily be distinguished from yellowish felspar by its infusibility 

 and having no cleavage. The edges turn brown in the flame ol'the blow- 

 pipe. 



Claystone and Claystone-porphyry. An earthy and compact stone, usually 

 of a purplish colour, like an indurated clay; passes into hornstone; gene- 

 rally contains scattered crystals of felspar and sometimes of quartz. 



Clinkstone. Syn. Phonolite, fissile Petrosilex; a greenish or greyish rock, 

 having a tendency to divide into slabs and columns ; hard, with clean frac- 

 ture, ringing under the hammer ; principally composed of compact felspar, 

 and, according to Gmelin, of felspar and mesotype. (Leonhard, Mineral- 

 reichs, p. 102.) A rock much resembling clinkstone, and called by some 

 Petrosilex, contains a considerable percentage of quartz and felspar. As 

 both trachyte and basalt pass into clinkstone, the rock so called must be very 

 various in composition. 



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

 includes the hornstone of some mineralogists, is allied to clinkstone, but is 

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

 chemical composition is not well defined, and is perhaps the same as that of 

 clay. {MacCulloch's Classification of Rocks, p. 481.) Mr. MacCulloch says, 

 that it contains both potash and soda. 



CoRNEAN. A variety of claystone allied to hornstone. A fine homogeneous 

 paste, supposed to consist of an aggregate of felspar, quartz, and hornblende, 

 with occasionally epidote, and perhaps chlorite ; it passes into compact fels- 

 par and hornstone. {De la Beche, Geol. Trans, second series, vol. 2. p. 3.) 



DiALLAGE ROCK. Syn. Euphatide, Gabbro, and some Ophiolites. Compounded 

 of felspar, and diallage, sometimes with the addition of serpentine, or mica, 

 or quartz. (MacCulloch, ibid. p. 648.) 



DiORFTE. A kind of greenstone, which see. Components, felspar and horn- 

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

 consists of albite and hornblende. 



DiORiTic-poRPHYRY. A porphyritic greenstone, composed of crystals of albite 

 and hornblende, in a greenish or blackish base. (Rose, ibid. p. 10.) 



DoLERiTE. Formerly defined as a synonym of greenstoiie, which see. But 

 according to Rose (ibid. p. 32.), its composition is black augite and Labrador- 

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

 felspar, and magnetic iron. 



DoMiTE. An earthy condition of trachyte, found in the Puy de Dome, in Au 

 vergne. 



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 m.ineral allied to jade. (Allan's Mineralogy, 

 p. 158.) See Diallage rock. 



Felspar-porphyry. Syn. Homstone-porphyry ; a base of felspar, with crys- 

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



Gabbro, see Diallage rock. 



Greenstone ; Syn. Dolerite and diorite ; components, hornblende and felspar, 



or augite and felspar in grains. See above, p. 96. 

 Greystone. (Graustein of Werner.) Lead grey and greenish rock, composed 



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



(Scrope, Joarn. of Sci. No. 42. p. 221.) Greystone lavas are intermediate in 



composition between basaltic and trachytic lavas. 



Hornblende rock. A greenstone, composed wholly or principally of granular 

 hornblende, or augite. (Leonhard, Mineralreichs, &c., p. 85.) 



Hornstone, Hornstone-porphyry. A kind of felspar- porphyr}% (Leonhard, 

 ibid.) with a base of hornstone, a mineral approaching near to flint, differing 

 from compact felspar in being infusible. 



