Thus The Crater At The Extremity Of The Piton, Which Is Called The Caldera, Is Extremely Small.

Its diminutive size struck M. de Borda, and other travellers, who took little interest in geological investigations.

As to the nature of the rocks which compose the soil of Teneriffe, we must first distinguish between productions of the present volcano, and the range of basaltic mountains which surround the Peak, and which do not rise more than five or six hundred toises above the level of the ocean. Here, as well as in Italy, Mexico, and the Cordilleras of Quito, the rocks of trap-formation* are at a distance from the recent currents of lava (* The trap-formation includes the basalts, green-stone (grunstein), the trappean porphyries, the phonolites or porphyrschiefer, etc.); everything shows that these two classes of substances, though they owe their origin to similar phenomena, date from very different periods. It is important to geology not to confound the modern currents of lava, the heaps of basalt, green-stone, and phonolite, dispersed over the primitive and secondary formations, with those porphyroid masses having bases of compact feldspar,* which perhaps have never been perfectly liquified, but which do not less belong to the domain of volcanoes. (* These petrosiliceous masses contain vitreous and often calcined crystals of feldspar, of amphibole, of pyroxene, a little of olivine, but scarcely any quartz. To this very ambiguous formation belong the trappean porphyries of Chimborazo and of Riobamba in America, of the Euganean mountains in Italy, and of the Siebengebirge in Germany; as well as the domites of the Great-Sarcouy, of Puy-de-Dome, of the Little Cleirsou, and of one part of the Puy-Chopine in Auvergne.)

In the island of Teneriffe, strata of tufa, puzzolana, and clay, separate the range of basaltic hills from the currents of recent lithoid lava, and from the eruptions of the present volcano. In the same manner as the eruptions of Epomeo in the island of Ischia, and those of Jorullo in Mexico, have taken place in countries covered with trappean porphyry, ancient basalt, and volcanic ashes, so the peak of Teyde has raised itself amidst the wrecks of submarine volcanoes. Notwithstanding the difference of composition in the recent lavas of the Peak, there is a certain regularity of position, which must strike the naturalist least skilled in geognosy. The great elevated plain of Retama separates the black, basaltic, and earthlike lava, from the vitreous and feldsparry lava, the basis of which is obsidian, pitch-stone, and phonolite. This phenomenon is the more remarkable, inasmuch as in Bohemia and in other parts of Europe, the porphyrschiefer with base of phonolite* (* Klingstein. Werner.) covers also the convex summits of basaltic mountains.

It has already been observed, that from the level of the sea to Portillo, and as far as the entrance on the elevated plain of the Retama, that is, two-thirds of the total height of the volcano, the ground is so covered with plants, that it is difficult to make geological observations. 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. 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.

Enter page number Go  Previous Next
Page 47 of 208
Words from 46838 to 47846 of 211363