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Rocks weighing many tons are often thrown from a volcano at the beginning of an outburst by the breaking up of the solidofied floor of the crater; and during the progress of an eruption large blocks may be torn from the throat of the volcano by the outrush of steam. But the most important fragmental materials are those derived from the lava itself.
In the class of eruption illustrated by these examples, there is an absence of fragmental material, such as explosive volcanoes hurl into the air, and a person may stand within a few yards of a rushing stream of molten rock, or examine closely the opening from which it is being poured out, without danger or serious inconvenience.
In the case of a volcanic mountain, however, the same form is due either to accumulation of fragmental material piled around the cup-shaped hollow, or crater, which is usually placed at the apex of the cone, and owing to which it is bluntly terminated, or else to the welling up from beneath of viscous matter in the manner presently to be described. Views of Sir Humphrey Davy and L. von Buch.
It was this excentrical arrangement which gave rise to the supposition that such volcanic ash-beds had been tilted up by a force acting in the direction of the volcanic throat, or orifice of eruption. The interior wall of Monte di Somma, the original crater of Vesuvius, presents a good illustration of such fragmental beds.
To this class of craters those of Etna, Vesuvius, and Auvergne belong. It has also been shown that in many instances these crater-walls have been breached on one side, and that streams of molten lava which once occupied the cup to a greater or less height, have poured down the mountain side. Hence the form or outline of many of these fragmental craters is crescent-shaped.
Mine too whose else? thy costly fruit-garden, with its sun-baked southern wall; the ampler pleasure-garden, rising backwards from the house in triple terraces, with flower-pots now of palest lead, save that a speck here and there, saved from the elements, bespeak their pristine state to have been gilt and glittering; the verdant quarters backwarder still; and, stretching still beyond, in old formality, thy firry wilderness, the haunt of the squirrel, and the day-long murmuring woodpigeon, with that antique image in the centre, God or Goddess I wist not; but child of Athens or old Rome paid never a sincerer worship to Pan or to Sylvanus in their native groves, than I to that fragmental mystery.
The boy was sympathetic, without being obtrusively so, and his comments on the paintings were confident and unconventional. "So different from ce cher Pelouse," said Foster, with a grimace. He enjoyed immensely the fragmental half-hours given him through those two days.
These loose, or partially consolidated, fragmental materials are rudely stratified, and slope downwards and outwards from the edge of the crater, so as to present the appearance of what is known as "the dip" of stratified deposits which have been upraised from the horizontal position by terrestrial forces.
And ever and anon, as she plied the industrious needle over the heel of the too fragmental stocking, the low melody would burst unconsciously forth of, "Is there nobody coming to marry me? Nobody coming to woo-oo-oo?" Lady, not in vain was the burden of that votive song. There was somebody coming.
Only Ceres and Pallas had been discovered when he put forth his theory, but when Juno and Vesta were found they fell in with his predictions so well that the theory was generally regarded as being virtually established; while the fluctuations in the light of Vesta, as we have before remarked, led Olbers to assert that that body was of a fragmental shape, thus strongly supporting his explosion hypothesis.
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