472 MINERAL COMPOSITION [Ch. XXVHL 



had a similar origin. In India, however, especially in the Deccan, the 

 term " laterite" seems to have been used too vaguely. 



It would be tedious to enumerate all the varieties of trap and lava 

 which have been regarded by different observers as sufficiently abundant 

 to deserve distinct names, especially as each investigator is too apt to 

 exaggerate the importance of local varieties which happen to prevail in 

 districts best known to him. It will be useful, however, to subjoin here, 

 in the form of a glossary, an alphabetical list of the names and synonyms 

 most commonly in use, with brief explanations, to which I have added a 

 table of the analysis of the simple minerals most abundant in the volcanic 

 and hypogene rocks. 



Explanation of the Names, Synonyms, and Mineral Composition of the 

 more abundant Volcanic Rocks. 



Agglomerate. A coarse breccia, composed of fragments of rock, cast out f 

 volcanic vents, for the most part angular and without any admixture af 

 water-worn stones. " Volcanic conglomerates " may be applied to mix- 

 tures in which water-worn stones occur. 



Apiianite. See Cornean. 



AMrniBOLiTE, or Hornblende Rock, which see. 



Amygdaloid. A particular form of volcanic rock ; see p. 468. 



Augite Rock. A rock of the basaltic family, composed of felspar and augite. 

 See p. 466. 



Augitic-forphyry. Crystals of Labrador-felspar and of Augite, in a green or 

 dark gray base. (Hose, Ann. des J/me.s, torn. 8, p. 22, 1835.) 



Basalt. An intimate mixture of felspar and augite with magnetic iron, olivine, 



«fec. See p. 466. 

 Basanite. Name given by Alex. Brongniart to a rock, having a base of basalt, 



with more or less distinct crystals of augite disseminated through it. 



Claystone and Claystone-forfiiyry. An earthy and compact stone, usually of 

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

 contains scattered crystals of felspar and sometimes of quartz. 



Clinkstone. Si/n. Phonolite, fissile Petrosilex, see p. 467 ; 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, Mineralreich, p. 102.) 



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. (AfacCulloch's Classification of 

 Hocks, p. 481.) 



Corneas or Aphanite. A compact homogeneous rock without a trace of crystal- 

 lization, breaking with a smooth surface like some compact basalts ; con- 

 sists of hornblende, quartz, and felspar, in intimate combination. It 

 derives its name from the Latin word cornu, horn, in allusion to its 

 toughness and compact texture. 



Diallage Rock. Syn. Euphotide, Gabbro, and some Ophiolites. Compounded 

 of felspar and diallage. 



