PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 11 
Crystalline system the orthorhombic, form like muscovite,—optic axis in 
plane of longer diagonal of the crystal,—optic angle 67° 5’. 
A twin crystal, with two intersecting systems of rings, is in the author’s 
possession. 
A small quantity was treated with sulphuric acid, and a similar quantity 
with hydrochloric. 
The sulphuric acid seemed rapidly to decompose the mineral, the silica 
rising in light clouds when stirred. 
The hydrochloric acid was rapidly turned yellow; but, even after repeated 
evaporation with fresh quantities of acid, perfect decomposition could not be 
effected ; the mineral remaining as a heavy white powder. 
Specific gravity 2°825; H. 2:25. 
22°87 grains yieldea— 
Silica, ; ; . 45°426 
Alumina, . - 29652 
Ferric Oxide, . : TESio2S 
Manganous Oxide, . ‘ 022 
Lime, ; : : *788 
Magnesia, . 1° 702 
Potash, : : . 6°94 
Soda, : ; : -. « 28267 
Water, 5293 
100°418 
Insoluble silica, 5:68 per cent. ; possible impurity, kyanite. 
Was examined under the suppositicn that it was Damourite. 
3. It is very probable that most of what has been regarded as talc-slate in 
the Highlands of Scotland will in almost all cases prove to be margarodite- 
slate, or other hydrated mica-slates. 
HipBERT mentions many such slates as occurring in the Shetlands. One, 
which he specially draws attention to, he says is to be found a little to the 
north of Vanleep in Hillswickness. He describes the pellicular form, brilliant 
pearly lustre, and remarkable unctuosity of the mineral to which the schistose 
character of the rock is due. He also notes the want of elasticity in the 
“pellicles.” These pellicles were analysed specially to ascertain if so-called 
tale-slates were of the nature assigned to them in the name. 
The pellicles were picked from the more quartzy bands of the rock, as there 
they seemed always to be of increased size. 
They had little or no elasticity, but this I find to be not unusual with 
margarodite. 
