Some volcanic cones exhibit evidence that during the series of eruptions by which they have gradually been built up, the centre of volcanic action has shifted to another point within the mountain. Thus Lyell has shown, in the case of Etna, that during the earlier periods in the history of the mountain the piling up of materials went on around a centre which is now situated at a distance of nearly four miles from the present focus of eruptive activity. Some of our old British volcanoes, of which the denuded wrecks exist in the Western Isles of Scotland, show similar evidence of a shifting of the axis of eruption. One of the most conspicuous features of a volcanic cone is the great depression or crater found at its summit. In describing the internal structure of volcanic cones, we have seen how these craters are produced and acquire their inverted conical form, by the slipping and rolling back of materials towards the centre of eruptive action. Almost all volcanic cones exhibit craters, but in those which are formed entirely by the outwelling of viscid lavas the central depression is often slight and inconspicuous, and occasionally altogether wanting. It frequently happens, however, that eruptive action has ceased at the centre of a volcano, and its summit-crater may by denudation be entirely destroyed, while new and active craters are formed upon its flanks. Stromboli furnishes us with an admirable example of this kind (see fig. 1, facing p. 10). Other volcanoes may exhibit several craters, one at the summit of the mountain and others upon its flanks. Of this we find a good example in Vulcano (fig. 6, p. 43). [Illustration: Fig. 68.--Cotopaxi (19,600 feet), as seen from a distance of ninety miles.] When a volcano has been built up by regular and continuous eruptions from the same volcanic vent, the size of the crater remains the same, while the volcano continually grows in height and in the diameter of its base. The size of the crater will be determined by the eruptive force at the volcanic centre, the size of the mountain by the duration of the volcanic activity and the quantity of material ejected. In the earliest stage of its history, such a volcano will resemble Monte Nuovo, which has a crater reaching down almost to the base of the mountain; in the later stages of its history, such a volcano will resemble Cotopaxi (fig. 68) and Citlaltepetl (fig. 69), in which the crater, though of far greater absolute dimensions than that of Monte Nuovo, bears but a small proportion to the vast cone at the summit of which it is situated. [Illustration: Fig. 69.--Citlaltepetl, or the Pic d'Orizaba, in Mexico (17,370 feet), as seem from the forest of Xalapa.] [Sidenote: ORIGIN OF VOLCANIC CRATERS.] In the great majority of volcanoes, however, eruptive action does not go on by any means regularly and continuously, but terrible paroxysmal outbursts occur, which suddenly enlarge the dimensions of the crater to an enormous extent.