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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 masses, the cooling of which has been too rapid to change them into lithoid lava.
The objections against the volcanic origin of obsidians, founded on their speedy loss of colour, and their swelling by a slow fire, have been shaken by the ingenious experiments of Sir James Hall. I applied the blowpipe to some black pumice-stone from the volcano of the isle of Bourbon, which, on the slightest contact with the flame, whitened and melted into an enamel.
To the glowing coals of gems succeeded the dead cinders of obsidians, black stones scarcely swelling, without a gleam above the tarnished gold of their mountings; one entered no Holy Week, everywhere the "Pange Lingua" and the "Stabat Mater" wailed under the arches, and then came the "Tenebræ," the lamentations, and the psalms, whose knell shook the flame of the brown waxen tapers, and after each halt, at the end of each of the psalms, one of the tapers expired, and its column of blue smoke evaporated still under the lighted circumference of the arches, while the choir recommenced the interrupted series of complaints.
At one place, we were obliged to dismount and walk, on account of the sharp descent, and found ourselves upon an ugly piece of limestone or sandstone rock, which soon, to our surprise, we found replaced by a solid mass of obsidian. The cochero, says that the place is known as itzlis the obsidians, the knives.
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.
Stokes' fine collection of obsidians from Mexico, I observe that the sphaerulites are generally much larger than those of Ascension; they are generally white, opaque, and are united into distinct layers: there are many singular varieties, different from any at Ascension.
To judge of the augmentation of their bulk, we measured pieces exposed to a forge-fire of moderate heat, by the water they displaced from a cylindric glass, enveloping the spongy mass with a thin coating of wax. According to our experiments, the obsidians swelled very unequally: those of the Peak and the black varieties of Cotopaxi and of Quinche increased nearly five times their bulk.
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 glass.
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 masses 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 glass-houses, on which Sir James Hall, Dr.
Darwin attributes the lamination and fissile structure of volcanic rocks of the trachytic series, including some obsidians in Ascension, Mexico, and elsewhere, to their having moved when liquid in the direction of the laminae. The zones consist sometimes of layers of air-cells drawn out and lengthened in the supposed direction of the moving mass.
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