J. W. Judd — On Volcanos. 



299 



The Ponza Islands, which lie off the entrance to the Gulf of 

 Gaeta, form two small groups of islets and rocks, which are evi- 

 dently the highest points of submerged tracts of considerable size — 

 for round the islands the depth of water increases very gradually, 

 and the 200-fathom line is only reached at distances of about three 

 miles from the shores ; yet the part of the Mediterranean immediately 

 around them affords soundings up to 700 fathoms or more, as in the 

 case of the Lipari Islands. 



About thirty miles west of Ischia rise the islands of Yentotiene 

 and San Stefano. These are evidently two fragments, which have 

 escaped denudation, of a great volcano composed of materials precisely 

 similar in character to those forming the island of Ischia — namely, 

 ordinary trachytes with the agglomerates and tuffs derived from them. 

 The foundations of both the islands consist of masses of rock of great 

 hardness and solidity, evidently, as shown by their highly scoriaceous 

 upper surfaces, portions of vast lava-streams ; and these are covered 

 by thick masses of more or less stratified tuffs and agglomerates. 

 Yentotiene is one mile and a half long, by half a mile broad, and it 

 rises to a height of 470 feet above the sea-level. The form assumed 

 by this island, on account of the inclined position of its masses of 

 lava and tuffs, is familiar to all geologists from the sketch given in 

 Mr. Scrope's "Yolcanos," page 209. San Stefano is similar in 

 character, but of smaller size, being less than half a mile in diameter, 

 and rising to a height of only 272 feet above the sea ; its form is 

 illustrated in the accompanying sketch, Fig. 17. By an elevation of 

 200 fathoms the sea-bottom around these two islands would be con- 

 verted into an island of conical form, having a diameter of sis miles, 

 and a height of nearly 1700 feet. 



Fig. 17. — The Island of San Stephano as seen from the South. 



a. Tracnytic lava-stream, with scoriaceous surface, b, Stratified„tuffs. c, Prison and 



Barracks. 



The same remark applies to the Botte Bock, between Yentotiene 

 and Ponza, a projecting point of another, but much smaller, sub- 

 merged mountain mass. It is composed of ordinary trachyte; and 

 if elevation to the extent of 200 fathoms were to take place, a conical 

 mountain of about two miles in diameter, and having the Botte Bock 

 as its apex, would be exposed to view. 



