On the Basalt-glass of the Western Isles of Scotland. 69 
May 23.— J. W. Hulke, Esq., F.R.S., President, 
in the Chair. 
The following communications were read : — 
1. "On the Basalt-glass (Tachylyte) of the Western Isles of 
Scotland." By Prof. J. W. Judd, F.R.S., See. G.S., and G. A. J. 
Cole, Esq., F.G.S. 
Basalt-glass or tachylyte is a rare rock, although very widely 
distrihuted. 
In the Western Isles of Scotland it has, by the authors of the 
paper, been detected in five localities only, namely Lamlash (Holy 
Isle) near Arran, the Beal near Portree in Skye, Gribun and Some 
in Mull, and Screpidale in Baasay. 
Basalt-glass is always found in the Hebrides as a selvage to dykes, 
though elsewhere it has been described as occurring under other 
conditions, where rapid cooling of basaltic lava has taken place. 
Some of the varieties of basalt-glass in the Hebrides differ from any 
hitherto described by their high specific gravity (2-8 to 2-9) and by 
their low percentage of silica (45 to 50). 
This basalt-glass is frequently traversed by numerous joints ; it 
is occasionally finely columnar, and sometimes perlitic in structure. 
From the acid glasses (obsidian) it is distinguished by its density, 
its opacity, its magnetic properties, and especially by its easy 
fusibility, from which the name of tachylyte is derived. By its 
greater hardness it is readily distinguished form its hydrated forms 
(palagonite, &c). 
In its microscopic characters basalt-glass is found to resemble 
other vitreous rocks ; thus it exhibits the porphyritic, the banded 
and fiuidal, the spherulitic, and the perlitic structures. In the 
gradual transition of this rock into basalt, all the stages of devitri- 
fication can be well studied. 
The difference between these locally developed basalt-glasses and 
the similar materials forming whole lava-streams in the Sandwich 
Islands was pointed out in the paper, and the causes of this difference 
were discussed. 
It was argued that the distinction between tachylyte and hyalome- 
lane, founded on their respective behaviour when treated with acids, 
must be abandoned, and that these substances must be classed as 
rocks and not as mineral species ; the name basalt-glass was adopted 
as best expressing their relations to ordinary basalt, the term 
tachylyte being applied to all glasses of basic composition and being 
used in contradistinction to obsidian. 
2. " On a Section recently exposed in Baron Hill Park, near 
Beaumaris." By Prof. T. G. Bonney, M.A., F.R.S., Sec. G.S. 
The author, about three years since, observed some imperfect ex- 
posures of a felsitic grit in the immediate vicinity of the normal 
schists of the district in a road which leads from Beaumaris cemetery 
