That fire is the origin of earthquakes had been held by various philosophers, who, however, differed as to the manner in which the fire acts. Anaxagoras thought it was by explosions caused from the collision of underground clouds (236); others held that the immense mass of vapour produced by the subterranean conflagrations as it accumulates may exert such a pressure as to disrupt all obstructions; or when the pressure is less may cause no more than a heaving of the surface. The idea that the shock of an earthquake results from the removal of material underneath, whereby the stability of the overlying portion is undermined, and a collapse of the ground ensues, was held in various forms. Some thought that this destruction arose from extensive combustion within the earth. Anaximenes supposed that just as at the surface, rocks and old buildings yield to the ravages of time and fall down, so in the interior of the earth similar landslips may occur and cause shocks to the districts above them (237).[119] [119] The collapse of the roofs or sides of underground caverns may undoubtedly be in some instances the cause of local earthquakes. This origin is enforced by Lucretius: terra superne tremit magnis concussa ruinis, subter ubi ingentes speluncas subruit aetas. --_De Rer. Nat._ vi. 544. But the favourite opinion of antiquity regarded earthquakes as primarily due to the violent commotion of air. Seneca comments on the views of various philosophers, and more especially Aristotle’s, as to the way in which the air acts, and he then proceeds to deliver his own judgment. He has no doubt that, though some of the other agencies may co-operate, the chief motive force in earthquakes is air. By no part of nature, he affirms, is such violent energy displayed as by air; it kindles fire, tosses the surface of the waters into waves, destroys large tracts of the earth, uplifts new mountains, and raises in the midst of the sea islands never seen before. Not only does air exist above ground, but it also fills the hollows and interstices of the interior of the earth, into which it freely enters from the surface. Nothing in nature is so restless as air, and the earth cannot but be affected by the movements of the air included in its inside. The author agrees with the general opinion that when the air begins to be agitated in a subterranean cavern which it has filled, pressed by that which is still entering, it struggles to escape, and, when it does so, emerges with a violence proportionate to the narrowness of the passage for its exit. But if unable to make its way out, it becomes furious, acts like a swollen impetuous river, and that overthrows everything in its path.[120] [120] Lucretius gives a picturesque recital of these views (_De Rer. Nat._ vi. 535–607).