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PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 
The iron of muscovite has hitherto been almost invariably set down as being 
in the ferric state :—it is very probable, however, that its condition had not 
been often determined. 
The amount of water is here high ; most analyses of micas would seem to 
have been executed on material dried at 212°. 
Two other delicately-tinted micas may be suggested to future investigators 
for examination; namely, a brilliant-lustred yellow mica, from Struay Bridge in 
Ross-shire, and a somewhat rose-tinted variety, from Glen Skiag in une same 
county. 
Muscovite is not so markedly typical of granites in Scotland as elsewhere, 
being largely replaced by the dark-coloured micas ;—doubtless some dark grey 
muscovite may exist, but I have never myself found such, or any of a brown or 
black colour. 
The finest specimens of muscovite in Scotland are found at the following 
localities :— 
Large rosette crystallisations occur in a very quartzose vein to the west of 
Bigsetter Voe, in Mainland, Shetland ; still larger at Loch Glass, in Ross-shire, 
at Glen Skiag in the same county, and at Struay Bridge in Inverness-shire. 
A crystal 15 inches in length was found in the very singular vein, if vein 
it should be called, in Glen Skiag. The windows of the smaller houses in 
Duffus are said to have been at one time “ glazed” with sheets of muscovite. 
I have determined the optic angles of the following micas :— 
Axis im plane of longer diagonal. 
Rich brown, great vein of Rubislaw, ; : é; ; 67° 45’ 
Yellow green, great vein of Ben Capval, . ; : : We le: 
Light brown, third vein east of Portsoy, . ; ‘ : 64° 30’ 
Pale rose, Glen Skiag, . ; ! ; ; : : Tl 45" 
Silvery, exfiltrative vein, Rubislaw, 2 : : ; 69° 5’ 
Light brown, Loch Glass, ‘ : 71° 40° 
In taking the specific gravity of the Glen Skiag mica, it was found that after 
having been boiled in water—to expel air bubbles—and being suddenly cooled, 
its specific gravity was 2° 832; but after lying for twenty-four hours in water, its 
specific gravity was only 2°782. Upon being suddenly cooled after boiling, the mica 
seems to contract beyond its normal condition ; there being a difference of ‘05. 
A. specimen of the Rubislaw mica, treated in the same way, gave after 
boiling and sudden cooling a specific gravity of 2°813 ; after lying in water for 
four hours, of 2°783. Here there would appear to be the same undue contrac- 
tion, resulting from the sudden cooling, though not to the same extent. 
The larger plates of mica contain imbedded substances which will be after- 
wards noticed. 
Muscovite chiefly occurs in veins, either intrusive or exfiltrative ; in these 
