42 PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 
I have noted, as regards the optical properties of Haughtonite (where its 
extreme opacity permits of observation), that it was found to be biaxial, though 
only to a small extent—2* to 3°. It is not this fact so much as the habit of its 
plates, which induces the belief that it is orthorhombic in form. 
Distinct, or at least free, crystals I have not met with. In many instancesit 
is found in plates devoid of regular form and definite appearance: this is the 
case at Roneval, Nishibost, Rispond, and Clach-an-Eoin ; but at most of the 
localities in which it occurs in exfiltration veins in granite, the crystals are 
disposed in lengthened, radiating, somewhat fan-shaped arrangements, with 
oblique terminations ; these crystals are frequently three or four inches in 
length, by a fourth of an inch in width: they so occur at Lairg, Rubislaw, 
Cove, the graphic granite and the adjacent granitic vein at Portsoy, at Bliry- 
drine, and at Craig Burn, near Rhynie. 
The accompanying sketch of a Craig specimen, expresses a not unusual 
appearance. 
The peculiarity of the association with the muscovite of Cove has already 
been noted. ; 
Before the blowpipe, the three species function differently, though to but a — 
slight extent. All give with fluxes the iron reaction,—Biotite to the smallest, 
Haughtonite to markedly the largest extent. All fuse to a black magnetic 
bead,—Biotite with ease, Haughtonite-with considerable difficulty,—lepidome- 
lane, again, standing intermediate. The Biotite bead is but feebly magnetic ; 
that of lepidomelane distinctly so; that of Haughtonite powerfully so. 
Under the breath of the blowpipe flame the plate of Biotite, even if brown, 
whitens ; that of lepidomelane pales; that of Haughtonite, if of a pale tint, or 
however black, becomes still blacker from increased opacity. 
Under the action of acids, thin scales of the three substances are affected 
in the same order. When treated in the cold with hydrochloric and sulphuric 
