BASALT-GLASS OF THE WESTERN ISLES OP SCOTLAND. 461 



crystallized, but a large amount of glass of a brownish or greenish 

 colour is seen between the crystals (PI. XIII. fig. 7 ; compare fig. 8 

 and PL XIY. fig. 1). 



In the Lamlash basalt-glass the crystallites of magnetite, and 

 probably of some other minerals, are numerous, and the basalt is 

 only slightly more de vitrified than the glass. This basalt is a true 

 magma-basalt or limburgite, with porphyritic crystals of augite, 

 olivine, and plagioclase felspar. 



8. Summary of Results. 



Prom the foregoing descriptions it will appear that in the 

 Western Isles of Scotland we have examples of a somewhat rare 

 class of materials of considerable interest to the petrographer. 



These materials, though often classed as mineral species, are 

 really rocks, and, indeed, constitute merely a local condition of 

 certain types of basaltic lava. The names tachylyte and hyalo- 

 melane, as applied to mineral species, ought therefore to be aban- 

 doned altogether. The supposed distinction between tachylyte and 

 hyalomelane, founded on their behaviour with acids, altogether 

 fails in practice as a means of discrimination between the different 

 varieties. 



Of the several names proposed for the rocks of this class, that of 

 " basalt-glass " appears to us to be the most convenient and least 

 open to objection. This name indicates its mode of origin and its 

 relation to basalt, and the only possible source of error which we 

 can anticipate from its use is its confusion with the glassy magma, 

 or uncrystallized residue, found in many basalts — a substance which 

 may be of totally different composition. We would advocate in the 

 same way calling the glassy varieties of other rocks by similar 

 names, as rhyolite- glass, trachyte-glass, andesite-glass, phonolite- 

 glass, &c, the names of obsidian and pitchstone or retinite being 

 still used for the types with vitreous and resinous lustre respec- 

 tively, and the terms spherulite-rock, perlite, pumice, &c. being- 

 applied to varieties exhibiting special modifications of structure. 

 Tachylyte may, in the same manner, be a useful alternative name 

 for the basic glasses, to be employed in contradistinction to obsi- 

 dian or acid glass ; but there seems to be no reason for the reten- 

 tion of the term hyalomelane. 



From the glasses of more acid composition basalt-glass is at once 

 distinguished by its higher specific gravity. While ordinary obsi- 

 dians (rhyolite- and trachyte-glass) have a density varying from 

 23 to 2*5, the average being 2-4 or under, the density of basalt- 

 glass varies from 2*5 to 2*9, the average being 2*7. The basalt- 

 glass of several Scotch localities is of exceptionally high density, 

 between 2*8 and 2*9. The glass, when unaltered, is probably in 

 all cases of less density than the same material in a more crystalline 

 condition. 



The striking magnetic properties of basalt-glass enable us to dis- 

 tinguish it from other vitreous rocks, as does also its remarkable 

 opacity even in the thinnest splinters. Perhaps the most noticeable 



