M. Cordier Found At Teneriffe Xeolite In An Amygdaloid Which Covers The Basalts Of La Punta Di Naga.) In Like Manner The Lava Of Scala, With Which The City Of Naples Is Paved, Contains A Close Mixture Of Basalt, Nepheline, And Leucite.

With respect to this last substance, which has hitherto been observed only at Vesuvius and in the environs of Rome, it exists perhaps at the peak of Teneriffe, in the old currents of lava now covered by more recent ejections.

Vesuvius, during a long series of years, has also thrown out lavas without leucites: and if it be true, as M. von Buch has rendered very probable, that these crystals are formed only in the currents which flow either from the crater itself, or very near its brink, we must not be surprised at not finding them in the lavas of the peak. The latter almost all proceed from lateral eruptions, and consequently have been exposed to an enormous pressure in the interior of the volcano.

In the plain of Retama, the basaltic lavas disappear under heaps of ashes, and pumice-stone reduced to powder. Thence to the summit, from 1500 to 1900 toises in height, the volcano exhibits only vitreous lava with bases of pitch-stone* (* Petrosilex resinite. Hauy.) and obsidian. These lavas, destitute of amphibole and mica, are of a blackish brown, often varying to the deepest olive green. They contain large crystals of feldspar, which are not fissured, and seldom vitreous. The analogy of those decidedly volcanic masses with the resinite porphyries* (* Pechstein-porphyr.

Enter page number Go  Previous Next
Page 176 of 779
Words from 47660 to 47918 of 211363