DIABASE, a word used by petrographers with varied meanings; but the present usage in this country restricts it to crystalline igneous rocks, of the basalt family, generally occurring as dikes, having as essential constituents plagio clase feldspar, augite and magnetite. Olivene may or may not be present. Diabases differ from the granitoid rocks, to which they have a somewhat similar texture, in that the crystals of feldspar are long and narrow, or lath-shaped, and the dark silicates are arranged in the inter stices of the feldspar crystals, giving an ophitic texture. Diabases are of common occurrence in the United States. They form dikes and lac eoliths at various places along the Atlantic sea board from Nova Scotia to Georgia. The so called traps of the Connecticut Valley and the Palisades of the Hudson are familiar examples. In the Lake Superior country diabases are of frequent occurrence; notably in the Marquette Iron Range and on Keweenaw Point; the lat ter sometimes carry native copper. The con
venient field name greenstone is frequently given to old, more or less altered, diabases. By pressure and shearing stresses, and the intru sion of other igneous rocks, diabases change to hornblende and chlorite schists, showing no trace of original structure. Such rocks are common in the Lake Superior region and else where along the Algonkian and Archaean rocks that characterize the formation of so much of northeastern North America. Diabases are of common occurrence in the Rocky Mountains also, and frequently, over deposits, are found along their contacts with sedimentary rocks, particularly limestones. The typical diabase of the Palisades contains: SiO, 53.13; Ali°. 13.74; Fes°, 1.08; FeO 9.10; CaO 9.47; MgO 8.58; Nai0 2.30; Ki0 1.03. The specific grav ity is 2.96. See BASALT; TRAP.