Buhrstone, the best material known for constructing millstones. It is a silicious rock found interstratified with the sands, marls, and sandstones of the tertiary formations of the Paris basin, peculiar for its regular cellular structure and hardness like flint, with which it is identical in composition, both being mere varieties of quartz. It is these qualities of extreme roughness, derived from its honeycombed structure, and great hardness and strength, that render it the best stone for grinding. The fossil shells of land and fresh-water origin, with which the rock is sometimes filled, are converted into the same hard silicious substance as the rest of the stone, and their cavities are often lined with crystals of quartz. The color of the rock is whitish, with- a shade sometimes of gray, and sometimes of yellbw and blue. The best quality is that about equally made up of solid silex and of vacant spaces. The stones are quarried at numerous localities near Paris, whence they are transported in large quantities into the interior, and to Bordeaux and Havre for exportation. La Ferte-sous-Jouarre, in Seine-et-Marne, is one of the most important points where they are procured.
The quarries are worked open to the day, and the stones, when extracted from" their beds, are split with wedges into cylindrical forms. The pieces are cut into parallelopipeds, which are called panes. These are to be hooped together into the shape of millstones, answering the purpose perfectly well, while they are of much more convenient size for transportation than single stones. Good millstones of a bluish white color, and 6 1/2- feet diameter, are worth 1,200 francs, or about $250, each. In this country numerous substitutes for the French buhrstone have been found, the most important of which is furnished by the buhrstone rock of the bituminous coal measures of northwestern Pennsylvania and eastern Ohio, immediately underlying the principal iron ore deposits of that region. This rock has been wrought into millstones ever since the revolution, but the French rock has maintained a decided preference in all the. great markets.