OCR Text |
Show 348 FLOWING OF LAVA UNDER WATER. · f 18GlGl But some violent hypothesis must always eruptton ° ~<~t(J. 1 · 1 · 1 h be resor t e d t o, in Order to explain away fac.t s w 11e 1 Imp· y t e um.m pa1. re d ene1. gy of repro. ductive causes, m our own tim. esb. We have hitherto descnbed the structure .of the cone , ut a sma1 1 part on l y of the eJi ected matter remams so n• ea•r tbo the volcam·c or1'f1i ce. A larror e portion of sand an. d scor1·r e Is ofarlnle b the winds and scattered over the surroundmg plams, or . s • Y h . d much more is swept down by torrents mto 111 to t e sea , an . l d dln·ing the intervals, often protracted for many cen-t Je eep, . l d . f t f: t . between eruptions. There, hon2:onta eposits o . u a-unes, . d' f th k d ceous rna tte r become intermixed with se 1ment o o · erd £m s, an d wi' tl1 sl 1 e lls and corals ' and ' when af.t erwards· rmse 'dJ orm1 rocks of a mixed character, such as tuffs, pepermos, an :o. canic conglomerates. Some of the lavas, also, ~f Vesuvms, reac h t h e sea, as do those of almost all volcanos ; smce they£ a rde general l y m. I.s 1a n ds , or· lJorderinror the coast. Here theby Dn a bottom already rendered nearly level, for reasons e ore exp1 a .m e d b y us , 'vhen spealdng of deltas. Instead, .t herefore, of being highly inclined, as around the cone, ?r m narrow bands as in a val1ey, they spread out in bro~d honzontal she~ts o lonrr as they retain their fluidity; and this process may pro~ ably ~ontinue for a considerable time, since, as upon. th~ land, . the upper coating of hardened lava protects the hqmd and movinrr mass below from contact with the air, so beneath the sea th~ same superficial crust may prevent the gre~t bod~ of lava from cooling, and, being pressed upon by the wet?h~ o an m. creas.m g coltJmn of water as the current descends·1,1 I· t Is ptr ot-bably squeezed down: thus the subjacent matter, sti . m a;~ e of fusion, may be made to flow rapidly towards a.ll P.omts o tIe This would take place the more readily If the Hut-compass. . 1 1 ls down more slowly tonian assumptiOn be true, t Jat ava coo . h' 1 under the pressure of a deep sea t 1l an m. tl Ie open mr ' w Ic .1 was supposed to be a corollary from ~ir Ja~e~ Hadll'shextpe:~~ ments respcctm· rr compressi·O n, wI l en ce 1t was .1 n1erre t a Bv t 0 pressure prevented water. fw· m expan ding mto steam. u {i ll that the heat even if such be the case: It. by no means o ows . the air, and of the lava could be carried off more slow Jy than ~n h lted in seas of ordinary depth there can be no doubt t at t e me matter would cool far more rapi'd ly un d er wha ter . nd that Besides the ejections which fall on t e cone, a VOLCANIC ALLUVIONS. 349 much greater mass which finds its way gradually to the neighbouring sea, there is a third portion often of no inconsiderable thickness, composed of alluvions, spread over the valleys and plains at small distances from the volcano. Immense volumes of aqueous vapour are evolved from a crater during eruptions, and often for a long time subsequently to the discharge of scm·ire and lava. 'l'hese vapours are condensed in the cold atmosphere surrounding the high volcanic peak, and heavy rains are caused sometimes even in countries where, under other circumstances, such a phenomenon is entirely unknown. The floods thus occasioned sweep along the impalpable .dust and light scorire, till a current of mud is produced, which is called, in Campania, "lava d'acqua," and is often more dreaded than an igneous stream (lava di fuoco), from the greater velocity with which it moves. So late as the ~'7th of October, 18~~, one of these alluvions descended the cone of Vesuvius. After overspreading much cultivated soil, it flowed suddenly into the villages of St. Sebastian and Massa, and, filling the streets and interior of some of the houses, suffocated seven persons. It will therefore happen very frequently, that, towards the base of a volcanic cone, alternations will be found of lava, alluvions, and showers of ashes. 'l~o which of these two latter divisions the mass enveloping Herculaneum and Pompeii should be referred, has been a question of the keenest controversy ; but the discussion might have been shortened, if the combatants had reflected that, whether volcanic sand and ashes were conveyed to the towns by running water, or through the air, during an eruption, the interior of buildings, so long as the roofs remained entire, and all undergi'Ound vaults and cellars, could only be filled by an alluvion. . We learn from history, that a heavy shower of sand, pumice, and lapilli, sufficiently great to render Pompeii and Herculaneum uninhabitable, fell for eight successive daJ'S and nio-hts • b ' ln the year 79, accompanied by violent rains. We ought, therefore, to find a very close resemblance between the strata covering these towns, and those composing the minor cones of the Phlegrooan Fields, accumulated rapidly like Monte N uovo du n.n g a continued shower of ejected matte' r; with this differ-' ence, that the strata incumbent on the cities would be horizontal, whereas those in the cones are highly inclined, and that |