MILL-STONE, or BUHR-STONE. This interesting form of silica, which occurs in great masses, has a texture essentially cellular, the cells being irregular in num ber, shape, and size, and are often cross ed by thin plates, or coarse fibres of silex. The buhr-stone has a straight fracture, but it is not so brittle as flint, though its hardness is nearly the same. It is feebly translucent ; its colors are pale and dead, of a whitish, grayish, or yellowish cast, sometimes with a tinge of blue.
The buhr- stones usually occur in beds, which are sometimes continuous, and at others interrupted. These beds are placed amid deposites of sand, or argilla mous and ferruginous marls, which pene trate between them, filling their fissures and honeycomb cavities. Bohr-stones constitute a very rare geological forma tion, being found in abundance only in the mineral basin of Paris, and a few ad joining districts. Its place of superposi tion is well ascertained : it forms a part of the lacustrine, or fresh-water forma tion, which, in the locality alluded to, lies above the fossil-bone gypsum, and the stratum of sand and marine sandstone which covers it. Buhr-stone constitutes, therefore, the uppermost solid stratum of the crust of the globe ; for above it there is nothing but alluvial soil, or diluvial gravel, sand, and loam.
Buhr-stones sometimes contain no or ganic forms, at others they seem as if stuffed full of fresh-water shells, or land shells, and vegetables of inland growth.
There is no exception known to this ar rangement; but the shells have a silielous nature, and their cavities are often bedecked with crystals and quarts. The best buhr-stones for grinding corn, have about an equal proportion of solid matter, and of vacant space. The finest quarry of them is upon the high ground, near la Ferte-saus-Jouarre. The stones are quarried in the open air, and are cut out in cylinders, from one to two yards in diameter, by a series of iron and wood en wages, gradually but equally inserted. The pieces of buhr-stones are afterwards cut in parallelopipeds, called panes, which are bound with iron hoops into large millstones. These pieces are exported chiefly to England and America. Good millstones of a bluish white color, with a regular proportion of cells, when six feet and a half in diameter, fetch 1200 francs a-piece, or $220. A coarse conglomerate sandstone or breccia is, in some cases, used as a substitute for buhr-stones; but it is a poor one.
Very good buhr millstones are now sent from Georgia and Arkansas, which are said to equal the French stone in quality. A set of 51 feet Georgia stones have been put up in the mills of Hacks hall, Bro. & Co., at Richmond, Virginia.