CHERT, in geology, a rock consisting mainly of silica in a finely granular or chalcedonic form, closely allied to flint (q.v.). Cherts are probably formed in several different ways; for example, by solution and re-deposition of the silica contained in the spicules of sponges and other siliceous organisms in sedimentary rocks, such as the Carboniferous limestone and Upper Greensand of England; by accumulation on the floor of the sea in ancient times of radiolaria and diatoms, like some of the modern deep-sea de posits; by precipitation of silica from magmatic solutions belong ing to the later stages of submarine volcanic eruptions. In some instances it is uncertain which of these causes was concerned in their formation. Cherts rich in compounds of iron, of volcanic origin, were the primary source of the iron ore of the Lake Su perior region of the United States. The Rhynie cherts of Devonian age, in Scotland, are noteworthy for the remarkable preservation of the minute details of the structure of fossils embedded in them.
(R. H. RA.)