The series of changes in the shape of Vesuvius, which are proved by documentary evidence to have been going on during the last 2,000 years, probably find their parallel in all active volcanoes. In all of these, as we shall hereafter show, the activity of the vents undergoes great vicissitudes. Periods of continuous moderate activity alternate with short and violent paroxysmal outbursts and intervals of complete rest, which may in some cases last for hundreds or even thousands of years. During the periods of continuous moderate activity, the crater of the volcano is slowly filled up by the growth of smaller cones within it; and the height of the mountain is raised. By the terrible paroxysmal outbursts the mountain is often completely gutted and its summit blown away; but the materials thus removed from the top and centre of the mass are for the most part spread over its aides, so that its bulk and the area of its base are thereby increased. During the intervals of rest, the sides of the mountain which are so largely composed of loose and pulverulent materials are washed downwards by rains and driven about by winds. Thus all volcanoes in a state of activity are continually growing in size every ejection, except in the case of those where the materials are in the finest state of subdivision, adding to their bulk; the area of their bases being increased during paroxysmal outbursts, and their height during long-continued moderate eruptions. [Sidenote: DEVIATIONS FROM CONICAL FORM.] We have pointed out that the conical form of volcanic mountains is due to the slipping of the falling materials over one another till they attain the angle at which they can rest. There are, however, some deviations from this regular conical form of volcanoes which it may be well to refer to. The quantity of rain which falls during volcanic eruptions is often enormous, owing to the condensation of the great volumes of steam emitted from the vent. Consequently the falling lapilli and dust often descend upon the mountain, not in a dry state but in the condition of a muddy paste. Many volcanic mountains have evidently been built up by the flow of successive masses of such muddy paste over their surfaces. Some volcanic materials when mixed with water have the property of rapidly 'setting' like concrete. The ancient Romans and modern Italians, well acquainted with this property of certain kinds of volcanic dust and lapilli, have in all ages employed this 'puzzolana,' as it is called, as mortar for building. The volcanic muds have often set in their natural positions, so as to form a rock, which, though light and porous, is of tolerably firm consistency. To this kind of rock, of which Naples and many other cities are built, the name of 'tuff' or 'tufa' is applied. A similar material is known in Northern Germany as 'trass.'