72 A nalyses of Books . [Jan. 



mary. The porphyries, euphotides, or compounds of jade and dial- 

 lage, serpentines, black porphyry, or ophites and dolomites, are more 

 variable in their position. 



Among volcanic products the trachites are considered most an- 

 cient, and are sometimes stratified. The traps, or basalts afford 

 many minerals ; the lava group contain also many species. Both 

 iEtna and Vesuvius have been known to eject granite, in addition 

 to the pulverulent and solid matter which they continue to emit at 

 intervals. 



Decomposition of Rocks — Veins. — According to Becquerel, 

 veins are not to be considered as products of one general cause, but 

 of a concurrence of several causes. The veins in the most ancient 

 rocks are smaller than in the newer rocks, the largest existing in the 

 schists and transition limestones. Werner considered that rocks were 

 decomposed by two acids: 1. By carbonic acid as when granite 

 and gneiss or felspar alone are decomposed and form kaolin. 2. Sul- 

 phuric acid derived from pyrites, as in veins of felspar, mica, and 

 amphibole. Arsenic acid he considered produced a similar effect. 



M. Fournet, who has paid much attention to veins, distinguishes 

 two kinds : those of igneous origin, such as porphyries, trachites, &c. 

 in which the silica has formed combinations by means of heat ; and 

 those of aqueous origin, as we see illustrated in mineral waters. To 

 exemplify the former, he cites those instances where sulphuret of 

 iron, silica, and iron pyrites have been deposited upon the fragments 

 of primitive rocks, and with regard to the latter, he mentions cases 

 where talc and mica are changed into a grey substance, and granites 

 where felspar is altered into kaolin, likewise talcose schists where 

 steatite is isolated in veins. In the veins of Pont Gibaud, he ob- 

 served four other epochs. At the second period new branches were 

 formed, which were filled with secondary and tertiary products, 

 especially quartz, but likewise sulphurets, which have formed alter- 

 nating zones of pyrites, galena, and hyaline quartz in small crystals. 

 A third period distinguishes a dilatation which disturbed the sources 

 of the galena and introduced solutions of sulphate of barytes. At 

 the fourth epoch, the incrusting power of these sources appears to 

 have been enfeebled, when pyrites and minute veins of carbonates 

 were deposited. The fifth epoch was contemporaneous with the 

 basaltic eruptions. It is obvious, that for an explanation of the mode 

 in which these veins are tilled, we must have recourse to chemistry. 

 Thus, hydrate of iron proceeds from the decomposition of pyrites; 

 the powder of hydrous oxide is derived from the decomposition of the 

 carbonate, galena is gradually converted into a black pulverulent 

 substance, which gives birth to black and white carbonate. With 

 regard to the formation of rock-salt, Dumas has observed that in one 

 variety of it which decrepitated when placed in water, the cause 

 was attributable to hydrogen which condensed in its cavities. 



Granite. — Saussure attributed the decomposition of this rock to a 

 corrosive juice which dissolved the gluten uniting all its parts. 

 Vanquelin and Alluan traced the cause to disintegration of the rock, 

 and the removal of the alkali in the felspar by water. But Berthier 

 has shewn that silica as well as potash is removed, a silicate of 



