The proximate cause of earthquakes, though by no means accurately defined, seems referable to the action of internal heat or fire. That the earth was once subject to the action of a vast internal power springing probably from the development of subterranean or central heat, the elevations and depressions, and the generally scarred and torn character of its exterior, make sufficiently evident. A power similar in kind, but more restricted in degree, is still at work in the bowels of the earth, and occasionally breaks down all barriers and devastates certain parts of the world. There is good reason for holding that earthquakes are closely connected with volcanic agency. Both probably spring from the same cause; and may be regarded as one mighty influence operating to somewhat dissimilar results. Volcanic agency, therefore, is an indication of earthquakes, and traces of the first may be taken as indications of the existence (either present or past, actual or possible) of the latter.

The manifestation of these awful phenomena is restricted in its range. Accordingly, geologists have laid down certain volcanic regions or bands within which this manifestation takes place. Over these regions various traces of volcanic agency are found, such as either gaseous vapours or hot springs, or bituminous substances, and in some instances (occasionally) active volcanoes. See Physical Geography, sec. v.; Ætna; Antioch; Lesbos; &c.