Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 1 - By Alexander Von Humboldt And Aime Bonpland.
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The Currents Of Lava, Which We Discover On
The Slope Of Monte Verde, Between The Beautiful Spring Of Dornajito
And Caravela, Are Black Masses, Altered By Decomposition, Sometimes
Porous, And With Very Oblong Pores.
The basis of these lower lavas
is rather wacke than basalt; when it is spongy, it resembles the
amygdaloids* of Frankfort-on-the-Main.
(* Wakkenartiger
mandelstein. Steinkaute.) Its fracture is generally irregular;
wherever it is conchoidal, we may presume that the cooling has been
more rapid, and the mass has been exposed to a less powerful
pressure. These currents of lava are not divided into regular
prisms, but into very thin layers, not very regular in their
inclination; they contain much olivine, small grains of magnetic
iron, and augite, the colour of which often varies from deep
leek-green to olive green, and which might be mistaken for
crystallized olivine, though no transition from one to the other of
these substances exists.* (* Steffens, Handbuch der Oryktognosie
tome 1 s. 364. The crystals which Mr. Friesleben and myself have
made known under the denomination of foliated olivine (blattriger
olivin) belong, according to Mr. Karsten, to the pyroxene augite.
Journal des Mines de Freiberg 1791 page 215.) Amphibole is in
general very rare at Teneriffe, not only in the modern lithoid
lavas, but also in the ancient basalts, as has been observed by M.
Cordier, who resided longer at the Canaries than any other
mineralogist. Nepheline, leucite, idocrase, and meionite have not
yet been seen at the peak of Teneriffe; for a reddish-grey lava,
which we found on the slope of Monte Verde, and which contains
small microscopic crystals, appears to me to be a close mixture of
basalt and analcime.* (* This substance, which M. Dolomieu
discovered in the amygdaloids of Catania in Sicily, and which
accompanies the stilbites of Fassa in Tyrol, forms, with the
chabasie of Hauy, the genus Cubicit of Werner.
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