OCR Text |
Show 392 TRUNCATION OF VOLCANIC CONES. water exceeding many times that of the atmosphere, must impede the escape of the elastic fluids and of lava, until the resistance is augmented in the same proportion; hence the explosions will be more violent, and when a cone is formed it will be liable to be blown up and truncated at a lower level than in shallower water or in the open air. Add to this, that when a submarine volcano has repaired its cone, it is liable to be destroyed again by the waves, as in several cases before adverted to. The vent will then become choked up with strata of sand and fragments of rock, swept in by the tides. and currents. These materials are far more readily conso]i. dated under water than in the air, especially as mineral matter is so copiously introduced by the springs which issue from the ground in all volcanic regions hitherto carefully investigated. Beds of solid travertin, also, and in hot countl·ies coral reefs, must often, during long intervals of quiescence, obstruct the vent, and thus increase the repressive force and augment the violence of eruptions. 'l'he probabilities, there· fore, in a submarine volcano, of the destruction of a larger part of the cone, and the formation of a more extensive crater, are obvious ; nor can the dimensions of "craters of elevation," if referred to such operations} surprise us. During an eruption in 1444, accompanied by a tremendous earthquake, the summit of Etna was destroyed, and an enormous crater was left, from which lava flowed. The segment of that crater may still be seen near the Casa Inglese, and, when complete, it must have measured several miles in diameter. The cone was afterwards repaired ; but this would. not have happened so easily had Etna been placed like Stromboli in a deep sea, with its peak exposed to the fury of the waves. Let us suppose the Etnean crater of 1444 to have been filled up with beds of coral and conglomerate, and that during succeeding eruptions these were thrown out by violent explosions, so that the cone became truncated down to the upper margin of the woody 1·ecrion, a circular basin would then be formed thirty Italian mDes in circumference, exceeding by five or six miles the cir· cuit of the Gulf of Santorin. Yet we know by numerous sections that the strata of trachyte, basalt, and trachytic breccia, would, in that part of the great cone of Etna, dip on all sides off from the centre at a gentle angle to every point of the com· St~ppoud Serlion of Darrm 11/allll, in tho Day of Dt ngal. ~aves, and .part~y by explosions which preceded the form . o the ne~v mt:nor c~ne, f e g. Whether the outer and·J::lOn cone has m tlus particular case, tocrcther with tl b ger the ocean. on which it rests, been upheaved, or wh;~het~~~om. ~f ~ally proJected in great part like Stromboli above the 1 or;gtf :i~e s~a~ ~a!, probably, be determined by geological inv::t~cr~- ns .' OI' m th~ former case, some beds rc Jete with m t; re~ams may be mtcrstratified with volcanic e)ections. anne l orne of the accounts transmitted to us by eye-witn f t le gradual manner in which New K . esses, o with r · 1 amem first rose covered tvmg s Jells in the Gulf of Santorin, appear, certainly, * For the measurement f <l'ff, boschi dell' Etna, Scuderi s;cti J d <:~~nAt pa~tsGo~ the cone of Etna, sec Tmtlato dci ' · c cau. 1011, de Catan., vol. i, |