The sandstones of the so-called Roubidoux formation, which occurs in south central Missouri, commonly show well-preserved ripple marks on the rock slabs. These marks were formed exactly as their name suggests—in sand which was thrown into ripples by the shallow water in which it accumulated and was covered and cemented so as to retain the ripple forms. [Illustration: Ripple marks in limy sandstone.] Sandstone is used for building stone, walks, grindstones, furnace linings, and rock gardens. Large quantities are mined each year near Pacific, Festus and Crystal City, Klondike, and Hermann, for the manufacture of glass and other uses. Common glass is a cooled melt of relatively pure silica sand, soda ash, and lime. Asphaltic sandstone is used in road building. Sand-lime brick are made of sand. Sand is used as a molding material for metal castings, a parting substance between brick in kilns, and in large quantities in concrete and mortar mixtures. Chert, Flint The names chert and flint have in some regions been used for the same hard, fine-grained rock found so abundantly in Missouri, but correct usage employs chert for the white and gray varieties, and flint for the black variety. Flint may be thought of as slightly impure chert, a chert which is colored black by a small amount of pigment, usually fine carbon, or perhaps iron sulphide, scattered through it like fine dust. [Illustration: Chert, fossiliferous and slightly speckled. Note typical sharp edges, smooth surfaces, and conchoidal fracture. From near Columbia.] Chert is characterized by being harder than glass, brittle, very fine-grained, and by breaking with a smooth, rounded or hollowed clam shell-like (conchoidal) fracture and sharp edges. It was used by Indians to make arrow heads. It accumulates in abundance both in stream beds as gravel which has been more or less rounded by wear, and on the hillsides within the soil and sub-soil. Yellow and red iron oxides may stain and penetrate weathered chert gravel so that it becomes reddish, rusty, tan, yellow or brown. Chert remains abundant because of its extreme resistance to weathering. It is so hard that stream action wears it only very slowly. Its chemical composition is silica, SiO₂, a substance which is but little affected chemically by ground water. Where chert has contained fine grains of calcite scattered through it, the calcite may be removed in solution, leaving pores, and a zone of porous, light weight, tripolitic chert, harsh to the feel and enveloping an unaltered interior (See WEATHERED CHERT). Not uncommonly, fossil remains of calcite which were embedded in chert have been dissolved, leaving their hollow impressions.