SUBMARINE VOLCANOS. 



71 



enabled to carry backwards his researches in the same regions, through connecting 

 links, from the existing phenomena, into volcanic products of high antiquity. 



Again, many of the ancient volcanic rocks in Auvergne afford distinct proofs of 

 having been poured into lakes, for they are interstratified and commingled with the 

 freshwater and terrestrial spoils which constitute the lacustrine limestones and sand- 

 stones of that region j thus explaining the geological period during which they were 

 erupted, whilst the lofty positions they occupy, on the opposite sides of deep valleys 

 over which they once spread in continuous masses, by demonstrating the enormous 

 amount of denudation to which they have been subsequently exposed, convey to the 

 mind some measure of their venerable age. 



In these tracts the phenomena are probably all referrible, either to subaerial volcanic 

 action, or to vents of eruption from which currents of lava have been poured out into 

 contiguous lakes. The latter phenomena are analogous in kind, to that greater class of 

 subaqueous lavas which have been formed under the sea, the condition of the waters 

 alone constituting the difference. Now, as the greater number of existing active vol- 

 canos are near the coast, so currents of lava have been repeatedly propelled into the sea. 

 The coasts of Naples and Sicily, the latter particularly, present many examples of such 

 currents, which have flowed into the sea in broad sheets, and have been consolidated 

 under pressure into hard columnar lava, very similar to the basalts of older date 1 . 



We are thus naturally led to enter on the consideration of such volcanos of sub- 

 aqueous origin, as have been attested by the sudden rise of volcanic cones and islands 

 from the bottom of the sea. The isle of Nyoe on the coast of Iceland, the island Sabrina 

 off St. Michael in the Azores, an island in the Grecian Archipelago, and Graham 

 Island near the coast of Sicily, are well-known historical examples. The Jast-men- 

 tioned island, having been thrown up so recently as the year 1831, when naturalists 

 were quite alive to the importance of the deductions to which its apparition might give 

 rise, has received much attention, and the series of phenomena which accompanied its 

 appearance and disappearance have been faithfully recorded. They may be concisely 

 stated as having occurred in this order : 



1st. The soundings of Capt. Smyth, R.N. 2 , had established that the sea was upwards 

 of 600 feet deep at this spot, anterior to the eruption. 



2ndly. Ships passing near the place a short time before the eruption, were affected 

 by earthquakes. 



3rdly. Commencement of the eruption : the sea thrown up in waterspouts, accom- 

 panied by columns of steam and vapour. 



4thly. The formation of a small island, at first only twelve feet high, with a central 



1 Dr. Daubeny has clearly explained how, under the pressure of an ocean sufficiently deep to prevent the 

 formation of steam, the heat being carried off more slowly, the lava would longer retain its fluidity, and would 

 ultimately arrange itself in crystalline forms, more or less prismatic and regular. 



2 Phil. Trans., 1832, p. 255. 



i 2 



