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Show 390 THEORY OF ELEVATION CRATERS. presents, on almost all sides, a surface of bare rocks, which rise up with a moderate declivity towards the interior ; but at one point there is a narrow cleft, by which we can penetrate into the centre, and there discover that it is occupied by a great Cone n n<l Crttfe~· qf Da o·r en Ttlu 11<1,1 rll he Day nf D<11~nl. circular basin, fi1leu by the waters of the sea, bordered all around by steep rocks, in the midst of which rises a volcanic cone, very frequently in eruption. The summit of this cone is 1690 French feet in height, corresponding to that of the circular border which incloses the basin ; so that it can only be seen from the sea through the ravine, which precisely resembles the deep gorge by which we penetrate into the Caldera of the Isle of Palma, and of which an equivalent, more or less decided in its characters, is said to occur in all elevation craters. '!'he cone of the high peak of 'l'eyda, in Teneriffe, is also 1 ·epresented -as rising out of the middle of a crater of elevation, standing like a tower surrounded b{ its foss and bastion ; ~he foss beino- the remains of the ancient gulf, and the bastion the escar~ment of the circular. inclosure. So that Teneriffe ~s an exact counterpart of Barren Island, except that one 1s raised to an immense height, while the other is still on a level with the sea, and in part concealed beneath its waters. . Now, without enumerating more examples, let us cons1der wl1at form the products of submarine volcanos may naturally be expected to assume. There is every reason to conclude, from t11e few accounts which we possess of eruptions at the bottom of the sea, that they take place in the same manner t~ere as on · the open surface of a continent*. That the volcamc phenomena, if they are ever developed at unfathomable depths, may ,.. Scropo on Volcanos, p. 171. SUBMARINE VOLCANIC ROCKS. 391 be extremely differ t · . been witnessed b ~~ 'Is very possible; but when they have explosions o. f ... ~ 1e flcr~ws of vessels casually passing the acr1wrm mds b tl l ' resembled those f l enea 1 t le waters have closely o vo canos on the land R k fi ignited scorire, and comminuted 1 . oc y ragments, in several cases conical islands h as ~es, ~e thrown. up, and wards disappeared; as when, i:v;69~e:n~rmcd, wluch a~terwere thrown up off' St M' h 1. 17~0, small Isles • • lc ae 111 the Azores, or as Sabrina rose m 1811 near the same spot, and, in 1783, N oe off the coast of Iceland. Where the cones h a· Y ' b bl . ave Isappeared they pro a y consisted of loose matters easil d d b' d ' y re uce y the wave·s and fi current.s to a submarine shoal · Wh en 1. s1 a n d s h ave remam~ . rm' as m the case of Hi era, and the New and Little Kamem I~ the. Gulf of Santorin (see wood-cut No. 15) the hha ve bco nsisted m .p art of solid lava · Wh a t ever d ou b ts m' i. ghYt av: een en!ertamed as to the action of volcanos entire! sub mat·me, yet It must always have been clear that . yth - · 1 . ' m ose numewus c~ses w 1ere they JUSt raise their peaks above the waves, the eJected sand, scm·ire, and frao-ments of rock must accumulate round the vent into a cone ~ith t 1 ' wh'l tl 1' h 'll a cen ra crater, I e 1e Ig ter WI be borne to a distance by tide d rent b . d d . s an cur- . s, a.s y wm s urmg eruptions in open air. The lava bw hUiC h Issueks' fromh the crater spreads over the su b aqueous o o~, see mgt e lowest levels, or accumulating upon itself ;c~or~mg ~o its liquidity, volume, and rapidity of congelation ~ o owmg, m short, the same laws as when flowing in the at: mosphere *· Bu:. we ma~ ~ext. enquire, what characters may enable a geolooist to d1stmgmsh between cones formed entirely or in great part beneath the waters of the sea, and those for~ed on land. In the first place, large beds of shells and corals often ~row o~ the sloping sides of submarine cones, particularly in le Pacific, and these often become interstratified with la Inh~tead of alluvions containing land-shells like some of thvoass. w tch c H 1 ' e 1 over ercu aneum, great beds of tufaceous sand and cong omerate, mixed with marine remains, might be expected o\ such parts of the flanks of a volcano like Stromboli as are su merged beneath the waters. The pressure of a column of * Scrope on Volcanos, p. 173, |
