Anthracite deposits near Scranton formed 345 to 280 million years ago from ancient swamps, where decaying plants were transformed by time and the pressure of overlying rock into hard carbon known as coal. The coal formed in layers or veins, and the Clark Vein in the Lackawanna Mine is especially rich because an unusually heavy deposit of organic matter settled and decayed there. In the Lackawanna Mine, holes were drilled in the anthracite for explosive charges, then coal and rock were blasted loose, loaded into cars, and brought from underground to the tipple at the surface. There the coal and rock were dropped through a chute into train cars and carried to an off-site colliery for cleaning and separation. Anthracite, or hard coal, lies beneath 484 square miles of northeastern Pennsylvania and is composed of almost pure carbon, mainly used as domestic fuel, while bituminous coal is used in industry. Pennsylvania ranks first in the nation in anthracite production, and in the 1940s the state's underground deposits were estimated at 15 billion tons.