Although the violent and paroxysmal outbursts of volcanic mountains arrest the attention, and powerfully impress us with a sense of the volcanic activity going on beneath the earth's surface, yet it may well be doubted whether the quantity of heat, which the earth gets rid of by their means, at all approaches in amount that which is quietly dissipated by means of the numerous 'stufas,' gaseous exhalations, and thermal springs which occur in such abundance all over its surface. For while the former are intermittent in their action, and powerful outbursts are interrupted by long periods of rest, the action of the latter, though feeble, is usually continuous. [Sidenote: EFFECTS OF HOT-SPRINGS.] Most people may regard the hot spring of Bath as a very slight manifestation of volcanic activity. This spring issues at a constant temperature of 49° C, or 120° Fahr. As, however, no less than 180,000 gallons of water issue daily from this source, we may well understand how great is the amount of heat of which the earth's crust is relieved by its agency. It may indeed be doubted whether its action in this way is not at least equal to that of a considerable volcano which, though so much more violent, is intermittent in its action. Nor are thermal springs by any means ineffective agents in bringing materials from the interior of the earth's crust and depositing it at the surface. The Bath spring contains various saline substances, principally sulphates and chlorides, in solution in its waters. These are quietly carried by rivers to the sea, and are lost to our view. The spring has certainly maintained its present condition since the time of the Romans, and I find that if the solid materials brought from the interior of the earth during the last 2,000 years had been collected, they would form a solid cone equal in height to Monte Nuovo. Yet we usually regard the Campi Phlegræi as a powerfully-active volcanic district, and the subterranean action in our own country as quite unworthy of notice. When we remember the fact that on the continent of Europe the hot and saline springs may be numbered by thousands, and that they especially abound in districts like Hungary, the Auvergne, the Rhine provinces, and Central Italy, where volcanic action has recently become extinct, we shall be able to form some slight idea of the work performed by these agents, not only in relieving the earth's crust of its superfluous heat, but in transporting materials in a state of solution from the interior of that crust and depositing them at the surface. The vast deposits of siliceous sinter and of travertine also bear witness to the effects produced by hot and mineral springs.