PART I. CHAPTER VII. 99 



Volcanic Rocks. 



sembling the trap which may be found in a dike. These may 

 sometimes be tuffs, notwithstanding their density or compactness. 

 The chocolate-coloured mud, which was poured for weeks out of 

 the crater of Graham's Island, in the Mediterranean, in 1831, 

 must, when unmixed with other materials, have constituted a 

 stone heavier than granite. Each cubic inch of. the impalpable 

 powder which has fallen for days through the atmosphere during 

 some modern eruptions, has been found to weigh, without being 

 compressed, as much as ordinary trap rocks, which are often 

 identical in mineral composition. 



The fusibility of the igneous rocks generally exceeds that of 

 other rocks, for there is much alkaline matter and lime in their 

 composition, which serves as a flux to the large quantity of silica, 

 which would be otherwise so refractory an ingredient. 



It is remarkable, that notwithstanding the abundance of this 

 silica, quartz is wanting in the volcanic rocks, or is present only 

 as an occasional mineral, like mica. The elements of mica, as 

 of quartz, occur in lava and trap, but the circumstances under 

 which these rocks are formed, are evidently unfavourable to the 

 development of mica and quartz, minerals so characteristic of the 

 hypogene formations. 



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

 lava which have been regarded by different observers as suffi- 

 ciently abundant to deserve distinct names, especially as each 

 investigator is too apt to exaggerate the importance of local va- 

 rieties which happen to prevail in districts best known to him. It 

 will be useful, however, to subjoin here, in the form of a glos- 

 sary, an alphabetical list of the names and synonyms most com- 

 monly 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. f 



Explanation of the names, synonyms, and mineral composition 

 of the more abundant volcanic rocks. 



AMPHIBOLITE. See Hornblende rock,amphibole being Haiiy's name for horn- 

 blende. 



AMYGDALOID. A particular form of volcanic rock; see p. 96. 



AUGITE ROCK. A kind of basalt or greenstone, composed wholly or principally 

 of granular augite. (Leonhord's Mineralreichs, 2d edition, p. 85.) 



AUGITIOPORPHYRY. Crystals of Labrador-felspar and of augite, in a green or 

 dark grey base. (Rose, Ann. des Mines, torn. 8. p. 22. 1835.) 



BASALT. Chiefly augite an intimate mixture of augite and felspar with mag- 

 netic iron, olivine, &c. See p. 95. The yellowish green mineral called 



