1^2 Washington — Submarine Eruptions of 1831 and 1891. 



attributing it to the accumulation of bombs or the viscosity of 

 pahoehoe lava, ejected subaerially, while the greater number 

 connect the structure with eruption under submarine condi- 

 tions or with intrusions into mud or silt. The authors of the 

 last three papers cited, as well as Geikie and Teall, attribute it 

 to truly submarine eruptions, and in the discussion of the 

 latest paper Dr. Flett* cites "bombs " of the 1891 eruption in 

 analogy with the ellipsoids of Cornwall. It is noteworthy 

 that, according to Butler's descriptions, the " bombs " of this 

 agree in many respects with the Cornwall masses, especially 

 in their highly vesicular texture, arranged in bands of some- 

 what varying characters. As according to the account of 

 Ricco, some of the "bombs" of the Foerstner Yolcano were 

 red-hot, even after floating for some time, it is evident that 

 they must have been distinctly viscous, so that those which 

 did not reach the surface and were piled up on one another 

 must have been more or less flattened and distorted by the 

 superincumbent mass, and thus have given rise to the pecu- 

 liarities of shape and mutual fitting together of the curved 

 surfaces so well shown in the illustrations to. the papers cited 

 above. 



Some experiments by Johnston-Lavis, who found that 

 globules were produced by injecting one highly viscous liquid 

 into another, are cited by G. Plataniaf as illustrating the 

 formation of the spheroidal basalts of Aci-Castello. Such a 

 division into spheroids would be favored were the injected 

 material molten rock, when the influence of steam and the 

 spheroidal state of water, or rather here of the injected lava 

 surrounded by a layer of steam, would come into play, 

 especially if the depth, and hence the supply, of water was so 

 great as to prevent its exhaustion by evaporation. 



It is notable in this connection that the examples of pillow- 

 lavas seem to be wholly confined to the so-called " basic " rocks, 

 those low in silica and high in femic constituents : whereas w r e 

 find at eruptions in the open sea of lavas rich in silica (as at 

 Santorini) that the mass has solidified into angular blocks. 

 The difference is to be ascribed to the difference in the relative 

 fusibility and viscosity of the two kinds, the highly femic 

 magmas lending themselves readily to the assumption of a 

 more or less spherical form as the mass separates on issuing 

 into the water, while the very viscous siliceous magmas would, 

 on comparatively small cooling, become so nearly solid as to 

 crack, and their rapidly complete solidification would prevent 

 the rounding of the sharp angles of the blocks. 



* Quart. Jour, Geol. Soc, vol. lxiv, p. 270, 1908. 



fin H. J. Johnston-Lavis, the South Italian Volcanoes, Naples, p. 42, 1891. 



