BASALT-GLASS OF THE WESTERX ISLES OF SCOTLAND. 449 



of crystalline structure is always higher than that of a rock of the 

 same chemical composition, hut having a more or less vitreous or 

 colloid structure. In the same way the glassy products formed by 

 the fusion of crystalline rocks are always of less density than 

 the rocks themselves. Delesse fused a basalt having the specific 

 gravity of 2-85, and found the glass thus produced to have a 

 density of 2*77. The well-known melaphyre (altered basalt) called 

 Rowley Eag has a specific gravity of 2-84 according to our deter- 

 minations ; the black glass formed by Messrs. Chance, of Birming- 

 ham, from the fusion and rapid cooling of the same, we found to 

 have a density of 2*75 ; the same material when slowly cooled, 

 yielding a product full of crystals of felspar and skeleton-crystals 

 of magnetite, has a density of 2-77. 



But although the rule that the crystalline forms of rocks are 

 more dense than the glassy is very generally true, there is, as 

 Delesse himself pointed out *, a remarkable exception to it in the 

 case of a basalt-glass of the Western Isles of Scotland. Delesse 

 found that a portion of the basalt-vein of Lamlash has a density of 

 2-649, while the glassy basalt forming the selvage of the dyke is 

 denser, having a specific gravity of 2*714. This anomalous circum- 

 stance has been confirmed by a series of careful determinations 

 which Mr. Thomas Davies, acting on our suggestion, has made on a 

 specimen of this dyke in the British-Museum collection. He found 

 that the basalt of the dyke had a specific gravity of 2- 6 7, the part 

 of the glassy selvage adjoining and graduating into basalt had a 

 specific gravity of 2 # 72, while nearer to the side it was 2*74, and 

 on the extreme edge 2*78. 



Delesse endeavoured to explain this anomalous rise of density in 

 the more vitreous portions of the rock by a slight difference of 

 composition in the two parts of the mass. It is true that his 

 analysis shows the basalt to contain a little less silica and rather 

 . more water than the glass ; but the difference is so slight as to be 

 almost within the limits of errors of analysis, and perhaps does not 

 sufficiently explain this curious difference of density. The micro- 

 scopic study of this rock shows that the basalt of the dyke is a 

 magma-basalt, in which only incipient crystallization has taken place. 

 It is possible that the basalt has suffered greater alteration than 

 the more compact glass of the selvages of the dyke. 



The average density of basalt-glass may probably be taken as 

 about 2-7. Von Lasaulx gives the range as from 2*51 to 2*56 f ; 

 but this is certainly below the truth. The basalt-glass (sidero- 

 melane) of Iceland has, according to Sartorius von AValtershausen + 

 a specific gravity of 2-53 ; the " tachylyte " of the Sasebiihl has a 

 specific gravity of 2-58 § ; and the " slaggy augite " of Sicily, 



* Ann ales des Mines, vol. xiii. pp. 369, 370. 



t Elemente der Petrographie, p. 225 (1875). 



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



§ Hausmann, 'Neues Jabrb. fur Min.' &c. 1844, p. 70. Breithaupt (Kasteer'a 

 Archiv fur die gesammte Naturl. vol. viii. p. 112) gives specific gravity 250 to 

 2-54. 



