31 6 Mr. Knox’s experiments and observations 
a loose contexture ; and likewise a considerable quantity of 
compact stilbite, approaching to fibrous, and of a schistose 
structure. 
I obtained specimens of the pitch-stone of various kinds, 
compact, thin slaty, and disintegrated ; and one of the thin 
slaty was of a different colour from the rest, darker, more 
porphyritic, and of a more shining lustre. 
The external characters of all agreed in the following par- 
ticulars. 
Specific gravity 2,31. Fracture, small conchoidal ; frag- 
ments indeterminately angular, rather sharp edged and rhom- 
boidal. — Surface, smooth and glistening. — Feel, unctuous. — 
Lustre, resinous. — Hardness, semi-hard, the sharp edges 
scratching glass. — Streak and powder whitish grey, passing 
into greenish grey. — Smell, oily . — Before the blow-pipe, fuses 
without addition into a pale leek-green glass. Colour of the 
most porphyritic specimen, perfect leek-green ; of the rest, 
olive-green, passing into oil- green. 
Although the peculiar character of this variety of pitch- 
stone is its smell, yet, I believe, it differs from all others, in- 
cluding those from Arran, in the degree in which it is disposed 
to divide into thin laminae ; its proneness to disintegrate, and 
the regularity of its rhomboidal fragments. So much is it 
inclined to disintegrate, that a piece, nearly compact, after 
lying for some days on the table of a warm laboratory, fell 
into large rhomboidal fragments. 
It will appear, I think, that these qualities proceed from the 
same cause. 
A piece of the compact specimen was exposed for half an 
hour to a red heat in an open crucible. It lost 7,75 per cent. 
