Part 61 (2/2)

[Ill.u.s.tration: Fig. 458. Map of the Island of St. Paul, in the Indian Ocean, lat. 38 44' S., long. 77 37' E., surveyed by Capt. Blackwood, R.N., 1842.]

[Ill.u.s.tration: Fig. 459. View of the Crater of the Island of St. Paul.]

The island of St. Paul may perhaps be motionless; but if, like many other parts of the earth's crust, it should begin to undergo a gradual upheaval, or if, as has happened to the sh.o.r.es of the Bay of Baiae, its level should oscillate, with a tendency upon the whole to increased elevation, the same power which has cut away part of the cone, and caused the cliffs now seen on the north-east side of the island, would have power to undermine the walls of the crater, and enlarge its diameter, keeping open the channel, by which it enters into it. This ravine might be excavated to the depth of 180 feet (the present depth of the crater), and its length might be extended to many miles according to the size of the submerged part of the cone. The crater is only a mile in diameter, and the surrounding cliffs, where loftiest, only 800 feet high, so that the size of this cone and crater is insignificant when compared to those in the Sandwich Islands, and I have merely selected it because it affords an example of a cla.s.s of insular volcanos, into the craters of which the sea now enters by a single pa.s.sage. The crater of Vesuvius in 1822 was 2000 feet deep; and if it were a half submerged cone, like St. Paul, the excavating power of the ocean might in conjunction with gaseous explosions and co-operating with a gradual upheaving force, give rise to a caldera on as grand a scale as that exhibited by Palma.

[Ill.u.s.tration: Fig. 460. Side view of the Island of St. Paul (N.E. side).

Nine-pin rocks two miles distant. (Captain Blackwood.)]

If, after the geographical changes above supposed, the volcanic fires long dormant should recover their energy, they might, as in the case of Teneriffe, Vesuvius, Santorin, and Barren Island, discharge from the old central vent, long sealed up at the bottom of the caldera, new floods of lava and clouds of elastic vapours. Should this happen, a new cone will be built up in the middle of the cavity or circular bay, formed, partly by explosion, partly perhaps by engulphment, and partly by aqueous denudation. In the island of Palma this last phase of volcanic activity has never occurred; but the subterranean heat is still in full operation beneath the Canary Islands, so that we know not what future changes it may be destined to undergo.

FOOTNOTES:

[378-A] I have been favoured with this drawing by Captain B. Hall.

[381-A] Cambridge Transactions, vol. i. p. 402.

[382-A] Cambridge Trans., vol. i. p. 410.

[382-B] Ibid. vol. ii. p. 175.

[382-C] Dr. Berger, Geol. Trans., 1st series, vol. iii. p. 172.

[382-D] Geol. Trans., 1st series, vol. iii. p. 210. and plate 10.

[382-E] Ibid. p. 201.

[383-A] Geol. Trans., 1st series, vol. iii. p. 205.

[383-B] Ibid. p. 213.; and Playfair, Ill.u.s.t. of Hutt. Theory, p. 253.

[383-C] Geol. Trans., 1st series, vol. iii. p. 206.

[383-D] Sedgwick, Camb. Trans. vol. ii. p. 37.

[383-E] Ill.u.s.t. of Hutt. Theory, -- 253. and 261. Dr. MacCulloch, Geol.

Trans., 1st series, vol. ii. p. 305.

[383-F] Syst. of Geol. vol. i. p. 206.

[384-A] Camb. Trans. vol. ii. p. 180.

[385-A] MacCul. Syst. of Geol. vol. ii. p. 137.

[385-B] Seale's Geognosy of St. Helena, plate 9.

[386-A] Fortis. Mem. sur l'Hist. Nat. de l'Italie, tom. i. p. 233. plate 7.

[387-A] Scrope, Geol. Trans. vol. ii. p. 205. 2d series.

<script>