Millstone Grit, a geological formation, principally a conglomerate, composed of sili-cious sand and small pebbles; it is also called grit rock and grindstone grit. It is named from the frequent use to which it is put, particularly in England. The formation lies at the commencement of the coal period, being located between the subcarboniferous period and the lower coal measures, and marks the transition from the marine to the terrestrial period. The area that had been covered with fields of crinoids was swept during the millstone grit epoch by currents and waves which left the surface under a great depth of pebbles and -and. The coarseness of the beds along the Appalachian region in Pennsylvania indicates that this was the border reef of the continent, and its great thickness, exceeding 1,200 ft. shows that it was also a region of great subsidence. The formation here is mostly a whitish silicious conglomerate, with some sandstone layers and a few thin beds of carbonaceous shells. At Tamaqua the thickness is 1,400 ft.; at Pottsville, 1,000 ft.; in the Wilkesbarre region. from 200 to 300 ft.; and where it caps the mountain at Blossburg it is from 50 to 100 ft. In Virginia the thickness sometimes reaches 1,000 ft., and the rock is mainly a sandstone, but contains heavy beds of conglomerate.
It may be remarked that the conglomerate of the Bubcarboniferous period becomes also in Virginia a sand rock. In Alabama it is a quartzose grit ot great thickness, and is used for millstones. In Tennessee there are two heavy beds of conglomerate, with several thick coal beds between them, and also below both, which are generally referred to the false coal mea-sures of the millstone grit epoch. The mill-stone grit formation extends over parts of some of the southern counties of New York having a thickness of from 20 to 60 ft. In Cattaraugus and Alleghany counties, on account of the regularity of the joints, it stands out in huge blocks, forming walls and square structures which have received the names of " Rock City " and " Ruined City." Among the plants of this formation, according to Lesquereux, are lepidodendrons, sigillaria, and calamites, with several species of ferns.