On the site of the ancient city of the Aurunci, and on the volcanic phenomena which it exhibits / [Charles Daubeny].

  • Daubeny, Charles, 1795-1867
Date:
1846
Catalogue details

Licence: Public Domain Mark

Credit: On the site of the ancient city of the Aurunci, and on the volcanic phenomena which it exhibits / [Charles Daubeny]. Source: Wellcome Collection.

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    to have been such an accession of alkali and of earths, that the whole of the silica entered into combination, and consequently no quartz exists in the rock. But when we proceed to the lava-currents which have been emitted from actual volcanos, or to the analogous trap formations which are regarded as the effects of submarine eruptions, we find a still further diminution in the proportion of silica, indicated by the substitution of labradorite for orthoklase, or in other words, of one atom of felspar instead of three, coupled with the presence of hornblende or augite b in both which minerals the silica bears a still smaller proportion to the base with which it is combined. In these last minerals two new elements also make their appearance, which are seldom or never present, except in small quantities, in granite or in trachyte—I mean lime and magnesia; thus evincing already a change in the nature of the igneous operations, or in the materials upon which they were exerted. Thus the modern lavas of mount Etna have been determined by Lowe to consist of an intimate mixture of labradorite and of augite, and a lava which had recently flowed from Stromboli was as- t Hornblende is R Si + Si‘-2, where R is generally lime, but sometimes protoxide of iron, or soda; and R-j is generally magnesia, but sometimes protoxide of iron. In some horn¬ blendes the silica seems to be partially replaced by alumina. • • Bonsdorff. Augite is R3 Si‘^, where R is either lime, magnesia, pro¬ toxide of iron, or protoxide of magnesia. The silica is sometimes replaced by alumina, as is the case also in hornblende. See Rammelsberg’s Dictionary of Mineralogy, Berlin 1841. u Jameson’s Journal, 1837.