According to Mr. Scrope, the Italian geologists confine the term tuff, or tufa, to felspathose mixtures, and those composed principally of pumice, using the term peperino for the basaltic tuffs.[374-B] The peperinos thus distinguished are usually brown, and the tuffs grey or white.
We meet occasionally with extremely compact beds of volcanic materials, interstratified with fossiliferous rocks. These may sometimes be tuffs, although their density or compactness is such as to cause them to resemble many of those kinds of trap which are found in ordinary dikes. 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, and to be often identical with these 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, that is, crystalline silica, is usually 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 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.
Amphibolite. See [Hornblende rock], amphibole being Haüy's name for hornblende.
Amygdaloid. A particular form of volcanic rock; see [p. 372.]
Augite rock. A kind of basalt or greenstone, composed wholly or principally of granular augite. (Leonhard's Mineralreich, 2d edition, p. 85.)
Augitic-porphyry. Crystals of Labrador-felspar and of augite, in a green or dark grey base. (Rose, Ann. des Mines, tom. 8. p. 22. 1835.)