with m Account of the recently discovered Buhrstone^ S55 
Near Lord Grosvenor’s Main, there is another variety of the 
siliceous rock, with a multitude of minute pores, which is ex- 
ceedingly hard and tough, and is by some intelligent persons 
thought to be well adapted for grinding barley. It might be 
pierced and framed for this purpose, like buhrs ; but none of 
such millstones have yet been manufactured. 
About half a mile from Lord Grosvenor’s main, lies the quar- 
ry of a still more valuable variety of the chert, the substitute for 
the buhrstone of France. This substance seems the transition 
of the siliceous slate into a hornstone, rendered porous chiefly 
by containing siliceous petrifactions of different species of corals. 
A thin covering of soil here rests on debris of siliceous slate ; 
under this occurs a bed of siliceous rock, four feet in thickness. 
This bed is generally dense and compact ; but its lowest part 
becomes porous, both from its structure and abundance of coral- 
lites, A second bed contains the finest buhrs. Large masses of 
this stone may be p’ocured, exceedingly hard, extremely porous, 
and very uniform in their quality. This sort of rock passes, in 
some parts of the quarry, into the compact chert ; but there is 
at this spot a large body of the vesicular rock, admirably adapt- 
ed for millstones of the best quality. Masses of it, from half a 
ton to upwards of two tons, may be procured of very uniform 
quality. Its general colours are greyish-white, and various 
shades of light grey. It is full of pores, which are partly owing 
to the abundance of petrifactions it contains, and in part to the 
vesicular texture of the rock itself. From the number of its ve- 
sicles, the fracture is not always easily detected ; but it is decid- 
edly splintery in the more solid parts. It is hard, and is broken 
M'ith great difficulty. The edges are translucent. The specific 
gravity of fragments of a medium quality is 2.50^2. 
The equal distribution of the pores in large masses of this 
rock, and its extreme hardness, induced the discoverers, who 
had experienced its efficacy in grinding flints for the potteries, 
to try it as a millstone ; and experience has justified their most 
sanguine expectations. When wrought up like French buhr, 
it has been found to equal the best foreign millstones. I saw a 
pair, five feet in diameter, in the water flour-mill at Flint, 
They had been nearly three years in use, and, according to the 
miller’s testimony, proved as sharp and totigh, as the best 
