1849.] LYELL ON THE STRUCTURE OF VOLCANOS. 217 



continuous and dove-tailed into each other, except one grand deposit 

 of white tufaceous conglomerate, which forms the capping of all the 

 islands. M. Virlet found that the vesicles or pores of the beds of tra- 

 chyte were lengthened in the several directions in which they would 

 naturally be drawn out, had they flowed as melted matter towards 

 different points of the compass from the summit of a cone, the axis 

 of which once occupied the centre of the gulf. From the structure, 

 irregularity, and interrupted nature of the beds, and their moderate 

 dip, not exceeding in Thera three or four degrees, there seems no 

 reason to assume that they have undergone any change from their 

 original position, except such as may have arisen from general up- 

 ward and downward movements of the whole island. 



The length of the outer coast-line of the three islands taken together 

 is about thirty miles. Aspronisi is not more than a mile in circuit, 

 and only 300 feet high. It is surrounded by dangerous shoals for a 

 distance of about a third of a mile, as if it had recently wasted away 

 by the action of the sea ; and in the geological chapter before cited 

 of the 'Expedition of the Morea,^ it is stated that the waves are con- 

 stantly preying on the marginal cliffs of the three islands, so as to en- 

 large the intervals between them *. That these islands were once 

 united has been the conclusion of every geographer and geologist who 

 has seen them ; but the late survey by Capt. Graves may be said to 

 have set the question for ever at rest. The subaqueous rim of the 

 crater has been traced, first from Cape Acrotiri in Thera to Aspro- 

 nisi, the depth of water varying from five to ten fathoms, and then 

 from Aspronisi to Therasia, where there is the same depth, one 

 spot, called Mansell's shoals (fig. 4, C), being no more than nine 

 feet under water. 



Throughout the circuit of these two lines of shoal, constituting the 

 unbroken subaqueous rim of the crater, the water was found to deepen 

 suddenly on the inside, or towards the gulf; but in the third channel, 

 called the northern entrance (fig. 4, B), about a mile wide, between 

 Therasia and the nearest part of Thera, a remarkable breach is disco- 

 vered in the continuity of the submarine walls of the great bowl. Near 

 the land on both sides the water is shallow for a certain distance, as if 

 the cliffs had wasted away. Then there is a plunge to 100 fathoms, 

 and in the middle of the passage no less than 195 fathoms, or 1170 

 feet. This deep ravine in the bed of the sea is significantly spoken 

 of by Lieut. Leycester, as the "door into the crater." It is evi- 

 dently the "portillo" of Santorin, or its " Baranco de las angustias." 

 It is the single chasm through which, when the Santorin archipelago 

 stood more than 1000 feet higher, the contents of the vast crater of 

 denudation were swept by the sea ; and it is a remarkable fact, that 

 its depth precisely agrees, to within a few inches, with the greatest 

 depth discovered, after minute soundings, in any part of the gulf. It 

 is also important to remark, that outside the islands, where the sound- 

 ings deepen much more gradually than in the inside of the gulf, they 

 reach in some places, as for example about two miles south-west of 

 Aspronisi, to depths of 250 and 260 fathoms, showing that the Gulf 

 * Expedition de Moree, vol. ii. p. 259. 



