The most remarkable instance of the addition of volcanic rocks to the surface of North America is in the case of the Columbia River lava, which covers some 200,000 or more square miles of country in Washington, Oregon, and neighbouring States. In that region outwellings of highly liquid rock came from fissures and spread widely over the surface as veritable inundations, which on cooling became black, basaltic rock, but without forming mountains or craters. Where the Snake River has excavated its magnificent canyon in these still horizontal layers of basalt, a thickness of 4,000 feet is revealed, although the stream has not as yet cut through the formation, and in Stein Mountain, Oregon, a similar series of lava-sheets over 5,000 feet thick has been measured. The Columbia River lava was spread over the surface of a deeply eroded land in a series of vast overflows of molten material. The liquid rock covered the broad plains and extended into the valleys in the adjacent mountains, giving them level floors of basalt. Mountain spurs became capes and headlands and outstanding buttes were transformed into islands in the molten sea. The lava since cooled and crystallized has in places been folded and tilted; streams like the Columbia, Snake, Spokane Rivers, etc., have carved great canyons in it, and the surface, especially where it is still nearly horizontal, has decayed and yielded a wonderfully rich soil. It is the fine, rich residual material of these lava plains, redistributed in part by the wind, which furnishes the basis for the immense wheat industry of the northwestern portion of the United States. The extrusion of molten rock from deep within the earth so as to form volcanoes or fissure eruptions at the surface is only a part of a widely extended and highly varied process. As geologists have discovered, particularly in deeply eroded regions, by no means all of the fissures which permit of the forcing upward of molten material in them reach the surface. Many of them died out before coming to the light and favoured the production of various forms of intrusion.