
j eh 
: 3 
_ Parr I. Sucr. i. §3.] SUBMARINE VOLCANOES. 253 
The numerous volcanoes which dot the Pacific Ocean,’ probably 
‘in most cases began their career as submarine vents, their eventual 
appearance as subaerial cones being due to the accumulation of 
erupted material, and perhaps, also, as in the case of Santorin, to actual 
upheaval of the sea-bottom. The lonely island of St. Paul? (Fig. 60), 
lying in the Indian Ocean more than 2000 miles from the nearest 
land, is a notable example of the summit of a volcanic mountain 
rising to the sea-level in mid-ocean. Its circular crater, broken 
down on the north-east side, is filled with water, having a depth of 30 
fathoms. . 
Recent observations by Von Drasche have shown that at Réunion, 
during the early submarine eruptions of that volcano coarsely 
erystalline rocks (gabbro) were emitted, that these were succeeded by 
andesitic and trachytic lavas: but that when the vent rose above the 
sea basalts were poured out.* It is interesting to find that the order 
of appearance of the lavas in a submarine volcano so closely resembles 

Fic. 60.—VoLcanic CRATER OF St. Paun Is~taAnp, INDIAN QOcBAN. 
that generally noticed in terrestrial volcanic districts. Fouqué 
observes that at Santorin some of the early submarine lavas are 
identical with those of later subaerial origin, but that the greater 
part of them belong to an entirely different series, being acid rocks, 
belonging to the group of hornblende-andesites, while the subaerial 
rocks are augite-andesites. The acidity of these lavas has been 
largely increased by the infusion into them of much silica, chiefly in 
the form of opal. They differ much in aspect, being sometimes 
compact, scoriaceous, hard like millstone, with perlitic and spheru- 
litic structures, while they frequently present the characters of trass 
impregnated with opal and zeolites. Among the fragmental ejections 
there occur blocks of schist and granitoid rocks, probably repre- 
senting the materials below the sea-floor through which the first 
* See Darwin’s “ Volcanic Islands,” 2nd edit., 1876. 
2 See Assoc. Francaise, iv. p. 581. 
5 Tschermak’s Mineralogische Mittheil. 1876, pp. 42, 157, give an interesting 
account of the Philippine volcanoes. A similar structure occurs at Palma. Cohen. 
Neues Jahrb. 1879, p. 482. 
