VOLCANIC ACTION. Molten lava, rising from within the earth toward the surface, sometimes reaches the surface, but often rises into the crust and remains there. Small masses filling cracks in the rocks are called dikes; masses thrust in be tween the layers of the strata form sills or in truded sheets, like the Palisades of the Hudson; still larger masses, which lift the rock and form great wells of lava, are called laccoliths; and huge masses, with irregular boundaries, common in the cores of mountains, are known as bosses. Instances of each of these classes of igneous rock have been revealed by the denudation which has stripped off the overlying strata.
Where the molten rock reaches the surface it usually rises through a fissure; and when the volcanic energy is vigorous, as it was during the formation of the mountains of the Western United States, the lava may well out through these fissures, and form vast floods which inun date great areas on either side of the fissure.
Hundreds of thousands of square miles in the West are covered by these ancient lava floods. In no part of the world is this form of fissure eruption well developed at the present day, though the volcanoes of Iceland approach this type.
The geological effects of volcanic eruptions are of very great importance. The heat of intruded masses causes change in the rocks with which they come in contact. By the outflowing of the lava extensive changes are made in the topog raphy, and highly important, though usually destructive effects, are produced on life. Much rock material is added to the crust, mostly near the volcanoes, in the form of ash and lava flows, but partly as intrusions into the crust and partly as deposits, on the land and in the sea, derived from ash drifted about by the air and water •currents. See VOLCANO; LACCOLITE.