Hornstone

occurs under three modifications; splintery hornstone, conchoidal hornstone, and woodstone. The colours of the first two are gray, white, and red; they are all ma ive; dull, or of a glimmering lustre. Translucent only on the thin edges. Difficult to break. Hornstone is le brittle than flint; and by its infusibility before the blowpipe it may be distinguished from petrosilex, which it resembles in external appearance. The geological locality of hornstone is remarkable; for it occurs in both ancient and recent formations. It is found frequently in the veins that traverse primitive crystalline rocks, filling up the interstices, and enveloping their metallic ores. In the lead mine of Huelgoët in Britt any it is whitish; but its prevailing colour is gray. It occurs likewise in the middle beds of the coarse limestone ( calcaire gro ier ) in the Paris basin, which is a comparatively modern formation, as well as in the sand beds of the upper parts of this district, near Saint Cloud, Neuilly, . The hornstone which occurs in secondary limestone is called chert by the English miners. It is valuable for forming the grinding blocks of flint mills in the pottery manufacture.