Volume I Part 6 (1/2)
Isolated volcanoes, in the most distant regions, are very a.n.a.logous in their structure. At great elevations all have considerable plains, in the middle of which arises a cone perfectly circular.
Thus at Cotopaxi the plains of Suniguaicu extend beyond the farm of Pansache. The stony summit of Antisana, covered with eternal snow, forms an islet in the midst of an immense plain, the surface of which is twelve leagues square, while its height exceeds that of the peak of Teneriffe by two hundred toises. At Vesuvius, at three hundred and seventy toises high, the cone detaches itself from the plain of Atrio dei Cavalli. The peak of Teneriffe presents two of these elevated plains, the uppermost of which, at the foot of the Piton, is as high as Etna, and of very little extent; while the lowermost, covered with tufts of retama, reaches as far as the Estancia de los Ingleses. This rises above the level of the sea almost as high as the city of Quito, and the summit of Mount Lebanon.
The greater the quant.i.ty of matter that has issued from the crater of a mountain, the more elevated is its cone of ashes in proportion to the perpendicular height of the volcano itself. Nothing is more striking, under this point of view, than the difference of structure between Vesuvius, the peak of Teneriffe, and Pichincha. I have chosen this last volcano in preference, because its summit*
enters scarcely within the limit of the perpetual snows. (* I have measured the summit of Pichincha, that is the small mountain covered with ashes above the Llano del Vulcan, to the north of Alto de Chuquira. This mountain has not, however, the regular form of a cone. As to Vesuvius, I have indicated the mean height of the Sugar-loaf, on account of the great difference between the two edges of the crater.) The cone of Cotopaxi, the form of which is the most elegant and most regular known, is 540 toises in height; but it is impossible to decide whether the whole of this ma.s.s is covered with ashes.
TABLE 3: VOLCANOES:
Column 1: Name of the volcano.
Column 2: Total height in toises.
Column 3: Height of the cone covered with ashes.
Column 4: Proportion of the cone to the total height.
Vesuvius : 606 : 200 : 1/3.
Peak of Teneriffe : 1904 : 84 : 1/22.
Pichincha : 2490 : 240 : 1/10.
This table seems to indicate, what we shall have an opportunity of proving more amply hereafter, that the peak of Teneriffe belongs to that group of great volcanoes, which, like Etna and Antisana, have had more copious eruptions from their sides than from their summits. 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 cla.s.ses 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 ma.s.ses 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 ma.s.ses 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 ma.s.ses, 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 ma.s.s 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 a.n.a.lcime.* (* This substance, which M. Dolomieu discovered in the amygdaloids of Catania in Sicily, and which accompanies the stilbites of Fa.s.sa 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. .h.i.therto 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, dest.i.tute 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 a.n.a.logy of those decidedly volcanic ma.s.ses with the resinite porphyries* (* Pechstein-porphyr. Werner.) of the valley of Tribisch in Saxony is very remarkable; but the latter, which belong to an extended and metalliferous formation of porphyry, often contain quartz, which is wanting in the modern lavas. When the basis of the lavas of the Malpays changes from pitchstone to obsidian, its colour is paler, and is mixed with grey; in this case, the feldspar pa.s.ses by imperceptible gradations from the common to the vitreous. Sometimes both varieties meet in the same fragment, as we observed also in the trappean porphyries of the valley of Mexico. The feldsparry lavas of the Peak, of a much less black tinge than those of Arso in the island of Ischia, whiten at the edge of the crater from the effect of the acid vapours; but internally they are not found to be colourless like that of the feldsparry lavas of the Solfatara at Naples, which perfectly resemble the trappean porphyries at the foot of Chimborazo. In the middle of the Malpays, at the height of the cavern of ice, we found among the vitreous lavas with pitch-stone and obsidian bases, blocks of real greenish-grey, or mountain-green phonolite, with a smooth fracture, and divided into thin laminae, sonorous and keen edged. These ma.s.ses were the same as the porphyrschiefer of the mountain of Bilin in Bohemia; we recognised in them small long crystals of vitreous feldspar.
This regular disposition of lithoid basaltic lava and feldsparry vitreous lava is a.n.a.logous to the phenomena of all trappean mountains; it reminds us of those phonolites lying in very ancient basalts, those close mixtures of augite and feldspar which cover the hills of wacke or porous amygdaloids: but why are the porphyritic or feldsparry lavas of the Peak found only on the summit of the volcano? Should we conclude from this position that they are of more recent formation than the lithoid basaltic lava, which contains olivine and augite? I cannot admit this last hypothesis; for lateral eruptions may have covered the feldsparry nucleus, at a period when the crater had ceased its activity. At Vesuvius also, we perceive small crystals of vitreous feldspar only in the very ancient lavas of the Somma. These lavas, setting aside the leucite, very nearly resemble the phonolitic ejections of the Peak of Teneriffe. In general, the farther we go back from the period of modern eruptions, the more the currents increase both in size and extent, acquiring the character of rocks, by the regularity of their position, by their division into parallel strata, or by their independence of the present form of the ground.
The Peak of Teneriffe is, next to Lipari, the volcano that has produced most obsidian. This abundance is the more striking, as in other regions of the earth, in Iceland, in Hungary, in Mexico, and in the kingdom of Quito, we meet with obsidians only at great distances from burning volcanoes. Sometimes they are scattered over the fields in angular pieces; for instance, near Popayan, in South America; at other times they form isolated rocks, as at Quinche, near Quito. In other places (and this circ.u.mstance is very remarkable), they are disseminated in pearl-stone, as at Cinapecuaro, in the province of Mechoacan,* (* To the west of the city of Mexico.) and at the Cabo de Gates, in Spain. At the peak of Teneriffe the obsidian is not found towards the base of the volcano, which is covered with modern lava: it is frequent only towards the summit, especially from the plain of Retama, where very fine specimens may be collected. This peculiar position, and the circ.u.mstance that the obsidian of the Peak has been ejected by a crater which for ages past has thrown out no flames, favour the opinion, that volcanic vitrifications, wherever they are found, are to be considered as of very ancient formation.
Obsidian, jade, and Lydian-stone,* (* Lydischerstein.) are three minerals, which nations ignorant of the use of copper or iron, have in all ages employed for making keen-edged weapons. We see that wandering hordes have dragged with them, in their distant journeys, stones, the natural position of which the mineralogist has not yet been able to determine. Hatchets of jade, covered with Aztec hieroglyphics, which I brought from Mexico, resemble both in their form and nature those made use of by the Gauls, and those we find among the South Sea islanders. The Mexicans dug obsidian from mines, which were of vast extent; and they employed it for making knives, sword-blades, and razors. In like manner the Guanches, (in whose language obsidian was called tabona,) fixed splinters of that mineral to the ends of their lances. They carried on a considerable trade in it with the neighbouring islands; and from the consumption thus occasioned, and the quant.i.ty of obsidian which must have been broken in the course of manufacture, we may presume that this mineral has become scarce from the lapse of ages. We are surprised to see an Atlantic nation subst.i.tuting, like the natives of America, vitrified lava for iron. In both countries this variety of lava was employed as an object of ornament: and the inhabitants of Quito made beautiful looking-gla.s.ses with an obsidian divided into parallel laminae.
There are three varieties of obsidian at the Peak. Some form enormous blocks, several toises long, and often of a spheroidal shape. We might suppose that they had been thrown out in a softened state, and had afterwards been subject to a rotary motion. They contain a quant.i.ty of vitreous feldspar, of a snow-white colour, and the most brilliant pearly l.u.s.tre. These obsidians are, nevertheless, but little transparent on the edges; they are almost opaque, of a brownish black, and of an imperfect conchoidal fracture. They pa.s.s into pitch-stone; and we may consider them as porphyries with a basis of obsidian. The second variety is found in fragments much less considerable. It is in general of a greenish black, sometimes of murky grey, very seldom of a perfect black, like the obsidian of Hecla and Mexico. Its fracture is perfectly conchoidal, and it is extremely transparent on the edges. I have found in it neither amphibole nor pyroxene, but some small white points, which seem to be feldspar. None of the obsidians of the Peak appear in those grey ma.s.ses of pearl or lavender-blue, striped, and in separate wedge-formed pieces, like the obsidian of Quito, Mexico, and Lipari, and which resemble the fibrous plates of the crystalites of our gla.s.s-houses, on which Sir James Hall, Dr.
Thompson, and M. de Bellevue, have published some curious observations.* (* The name crystalites has been given to the crystalized thin plates observed in gla.s.s cooling slowly. The term glastenized gla.s.s is employed by Dr. Thompson and others to indicate gla.s.s which by slow cooling is wholly unvitrified, and has a.s.sumed the appearance of a fossil substance, or real gla.s.s-stone.)
The third variety of obsidian of the Peak is the most remarkable of the whole, from its connexion with pumice-stone. It is, like that above described, of a greenish black, sometimes of a murky grey, but its very thin plates alternate with layers of pumice-stone. Dr.
Thomson's fine collection at Naples contained similar examples of lithoid lava of Vesuvius, divided into very distinct plates, only a line thick. The fibres of the pumice-stone of the Peak are very seldom parallel to each other, and perpendicular to the strata of obsidian; they are most commonly irregular, asbestoidal, like fibrous gla.s.s-gall; and instead of being disseminated in the obsidian, like crystalites, they are found simply adhering to one of the external surfaces of this substance. During my stay at Madrid, M. Hergen showed me several specimens in the mineralogical collection of Don Jose Clavijo; and for a long time the Spanish mineralogists considered them as furnis.h.i.+ng undoubted proofs, that pumice-stone owes its origin to obsidian, in some degree deprived of colour, and swelled by volcanic fire. I was formerly of this opinion, which, however, must be understood to refer to one variety only of pumice. I even thought, with many other geologists, that obsidian, so far from being vitrified lava, belonged to rocks that were not volcanic; and that the fire, forcing its way through the basalts, the green-stone rocks, the phonolites, and the porphyries with bases of pitchstone and obsidian, the lavas and pumice-stone were no other than these same rocks altered by the action of the volcanoes. The deprivation of colour and extraordinary swelling which the greater part of the obsidians undergo in a forge-fire, their transition into pitch-stone, and their position in regions very distant from burning volcanoes, appear to be phenomena very difficult to reconcile, when we consider the obsidians as volcanic gla.s.s. A more profound study of nature, new journeys, and observations made on the productions of burning volcanoes, have led me to renounce those ideas.
It appears to me at present extremely probable, that obsidians, and porphyries with bases of obsidian, are vitrified ma.s.ses, the cooling of which has been too rapid to change them into lithoid lava. I consider even the pearlstone as an unvitrified obsidian: for among the minerals in the King's cabinet at Berlin there are volcanic gla.s.ses from Lipari, in which we see striated crystalites, of a pearl-grey colour, and of an earthy appearance, forming gradual approaches to a granular lithoid lava, like the pearlstone of Cinapecuaro, in Mexico. The oblong bubbles observed in the obsidians of every continent are incontestible proofs of their ancient state of igneous fluidity; and Dr. Thompson possesses specimens from Lipari, which are very instructive in this point of view, because fragments of red porphyry, or porphyry lavas, which do not entirely fill up the cavities of the obsidian, are found enveloped in them. We might say, that these fragments had not time to enter into complete solution in the liquified ma.s.s. They contain vitreous feldspar, and augite, and are the same as the celebrated columnar porphyries of the island of Panaria, which, without having been part of a current of lava, seem raised up in the form of hillocks, like many of the porphyries in Auvergne, in the Euganean mountains, and in the Cordilleras of the Andes.