The volcanoes of the chain of _Puys_, represented on opposite page (PL. III.) are so perfectly preserved, their lava is so frequently superposed on sheets of basalt, and presents a composition and texture so distinct, that there is no difficulty in establishing the fact that they are posterior to the basaltic formation, and of very recent age. Nevertheless, they do not appear to belong to the historic ages, for no tradition attests their eruption. Lyell places these eruptions in the Lower Miocene period, and their greatest activity in the Upper Miocene and Pliocene eras. “Extinct quadrupeds of those eras,” he says, “belonging to the genera mastodon, rhinoceros, and others, were buried in ashes and beds of alluvial sand and gravel, which owe their preservation to overspreading sheets of lava.”[22] [22] Ibid, p. 677. [Illustration: Fig. 9.--Section of a volcano in action.] All volcanic phenomena can be explained by the theory we have already indicated, of fractures in the solid crust of the globe resulting from its cooling. The various phenomena which existing volcanoes present to us are, as Humboldt has said, “the result of every action exercised by the interior of a planet on its external crust.”[23] We designate as volcanoes all conduits which establish a permanent communication between the interior of the earth and its surface--a conduit which gives passage at intervals to eruptions of _lava_, and in Fig. 9 we have represented, in an ideal section, the geological mode of action of volcanic eruptions. The volcanoes on the surface of the globe, known to be in an occasional state of activity, number about three hundred, and these may be divided into two classes: the _isolated_ or _central_, and the _linear_ or those volcanoes which belong to a _series_.[24] [23] “Cosmos,” vol. i., p. 25. Bohn. [24] “Cosmos,” vol. i., p. 237.