456 PKOF. J. W. JUDD AND G. A. J. COLE ON THE 



lane of Iceland,, which has 49 per cent, of silica. Its specific 

 gravity, however, is given by Sartorius von Waltershausen * as 

 2-531. This low density is the more remarkable when we find 

 that the proportion of oxide of iron is above 20 per cent., and that 

 there is little or no water in the rock. If this determination of the 

 specific gravity be correct, it would appear that this sideromelane 

 differs in a very marked manner from the ordinary basalt-glasses. 



While we have proof in these very basic glasses of Scotland that 

 even basalts with a low percentage of silica sometimes assume the 

 vitreous form, yet it is at the same time made evident that the 

 tendency to do so diminishes as the silicia-percentage declines. The 

 basic glasses of Some, Screpidale, and Gribun form much narrower 

 selvages to the dykes than the more acid ones of the Beal and 

 Lamlash. 



The vitreous rock of Beinn Shiant, which contains 58*67 per 

 cent, of silica, is, judging from its composition, probably like that 

 of Eskdale, not a true basalt-glass, but an augite-andesite-glass. 

 Similar vitreous conditions of augite-andesite have been described 

 by Fouque and Levy f from Santorin, and by Cohen J from between 

 New Britain and New Ireland, this latter being a pumiceous variety. 

 Cohen also refers a Sandwich-Island rock, showing remarkable 

 effects under polarized light due to internal strain, to the pumiceous 

 condition of augite-andesite-glass §. 



In the Sandwich Islands, as we have already pointed out, we 

 find a striking exception to the rule that basalt-lavas only assume 

 the glassy condition locally and as the result of rapid cooling. In 

 those islands the whole mass of lava-streams appears to frequently 

 consist of basalt-glass. 



If the older analyses of J. C. Jackson, B. Silliman, jun., and other 

 American chemists were to be relied upon, the exceptional cha- 

 racters of these Sandwich-Island rocks would seem to be sufficiently 

 explained by their peculiar composition. According to these 

 analyses, the Hawaiian lavas contain a proportion of silica varying 

 from 39 to 52 per cent. ; the proportion of alumina is small, and 

 that of iron very great, sometimes from 17 to 33 per cent. ; while 

 the percentage of the alkalies is often very high indeed, occasionally 

 over 20. 



But the more recent investigations of Cohen || have suggested 

 grave doubts as to the reliability of these older analyses. The 

 analyses of six glassy lavas from the Sandwich Islands have given 

 Cohen, "Wagner, and van Werveke closely concordant results, the 

 average composition of these rocks being, according to their analyses, 

 as follows : — 



* Vulkan. Gesteine in Sicilien und Island, p. 203. 

 t Mineralogie Micrographique, pi. xxxv. fig. 3. 

 \ Neues Jahrbuch fur Min. &c. 1880, toI. ii. p. 37. 

 § Ibid. p. 38. 

 || Neues Jahrb. fur Min. &c. 1880, vol. ii. p. 41. 



