£54 Dir Traill on the Mineralogy qf HaTkin Mountain^ 
pure siliceous slate. Near the Old-rake a weathered variety of 
this slaty rock occurs on the surface, beautifully veined and 
clouded with reddish-brown and ochre-yellow, on a light yel- 
lowish-grey ground ; but the more usual colour-delineations are 
blackish-grey, streaked with fine parallel lines of a smoke-grey 
colour. 
The rock called by the miners cliert^ is a massive siliceous 
slate, not disposed to divide into thin layers, but occurring in 
thick beds. Its colour is bluish-grey, mixed with yellowish-grey. 
Its fracture is splintery, sometimes passing into imperfectly flat 
conchoidal, with a very feeble lustre on the latter surfaces ; but 
is dull where splintery. Its edges, in the purest specimens (as 
in that of Mr Bishop’s quarry), are highly translucent. It is 
often traversed by many small veins of quartz ; but large blocks, 
of great purity and uniform structure, may be procured in some 
places, especially in the above-mentioned quarry, which is in a 
rocky crest on the eastern ridge of the mountain. The specific 
gravity of this sort is £.6363. The blocks here raised are 
brought to Flint, and there shipped at the rate of L. 1 : £ : 6 
per ton, for the potteries in our midland counties, where they 
are employed to grind flints for the better sort of porcelain. 
The purity of this rock augments the product of fine siliceous 
earth, by its own attrition during the process. The rock of 
this quarry, it may be remarked, bears a striking similarity to 
the siliceous substance found, in small quantity, resting on the 
massive limestone of Windmill-hill at Gibraltar, which appears 
to me also a siliceous slate The rock of the above quarry is 
known to our potters by the name of Blue Cherty to distinguish 
it from another kind, of a whiter colour, which Mr Bishop pro- 
cures from a quarry on the southern part of Halkin, and sells 
to the manufacturers of the finer sort of china-ware, at a some- 
what higher price. This white chert seems to be the transition 
of siliceous slate into quartz; or rather a species of quartz-farcilite, 
with translucent nodules. The white chert of Mr Bishop’s 
quarry is now used not only for grinding flints, but as common 
millstone. It is in great request, and is shipped at L. I, 5s. 
per ton. 
* It is the “ flint of a sap-green colour” of Colonel Imrie — See his valuable 
paper in Edin^ Phil, Trans, vol, vi. p. 3, 
