Some idea of the quantitative geological importance of volcanism may be conveyed to the reader when we assert that, according to a conservative estimate, fully one-half of a million cubic miles of molten rocks have been poured out upon the surface of the earth through volcanic action in relatively recent geological time! The Cascade Range with its lofty peaks, including Mount Shasta and Mount Rainier, each rising more than 14,000 feet above the sea, has been built up very largely by volcanic action during the last era of geologic time. Many other mountain peaks and various ranges have been similarly developed either wholly or in part. The great chain of Aleutian Islands extending hundreds of miles into the sea, is the scene of much volcanic activity where a great mountain range is now literally being born out of the sea by the processes of vulcanism. Before this the reader has more than likely wondered about the source of the heat, vapors (mainly water), and power involved in volcanic action. Answers to these questions are closely tied up with the precise cause (or causes) of volcanic action which remains one of the most uncertain of the larger problems of geologic science. Before briefly discussing the causes, a few additional facts should be stated. First, in regard to the heat, a careful determination of the temperature of the molten lava of Kilauea in 1911 showed it to be 1,260 degrees Centigrade, or 2,300 degrees F. This is, however, a relatively low temperature, because many lavas from other regions show melting points all the way up to at least 2,000 degrees Centigrade (3,600 degrees F.). Water in the form of steam is quantitatively one of the greatest products of volcanoes. A fair idea of the tremendous volumes of water involved may be gained from the statement that a careful estimate shows that fully 460,000,000 gallons of water in the form of steam erupted from a single secondary cone of Mount Etna during a period of 100 days. Among other gases which are given off in greater or less amounts during volcanic activity are carbonic acid gas, sulphureted hydrogen, sulphur dioxide, and hydrochloric acid. Some idea of the power back of volcanoes may be gained not only from the tremendous explosions such as those above described, but also from the fact that the pressure necessary to raise the column of lava from sea level to the top of Mauna Loa (nearly 14,000 feet) is about 1,150 atmospheres, or about 17,000 pounds per square inch. The actual pressure must there be much greater because the lava is forced up from far below sea level.