When masses of scoriæ in a semi-fluid condition are thrown to only a little distance above the volcanic vent, so that they have not time to assume a perfectly solid condition before they fall round the vent, the rugged masses of lava unite to form heaps of most irregular shape. In such cases, the falling fragments being in a semi-plastic state, stick to the masses below, and do not tend to roll down the sides of the heap. Irregular heaps of such volcanic scoriæ abound on the surfaces of lava-streams, being piled up around each 'bocca' or vent which the steam-jets escaping from the lava-currents form at their surfaces. Such irregular accumulations of scoriæ were observed on the lavas of Vesuvius during the eruptions of 1822, 1855, and 1872, and have also been described in the case of many other volcanoes. In fig. 26 (p. 101) we have given representations of a group of such irregular scoria-cones which was observed by Schmidt on the Vesuvian lava of 1855. It will be seen from this drawing that there is scarcely any limit to the steepness of the sides of such scoria-heaps, in which the materials are in an imperfectly solidified condition when they reach the ground. But in the majority of cases, the scoriæ ejected from volcanic vents are thrown to a great height, and are in a more or less perfectly solidified condition when they fell to the ground again. In such cases the fragments obey the ordinary mechanical laws of falling bodies, rolling and sliding over one another, till they acquire a slope which varies according to the size and form of the fragments. In this way the great conical mounds are formed which are known as 'cinder-cones,' or more properly as 'scoria-cones.' Scoria-cones usually vary in the slope of their sides from 35° to 40° and may differ in size from mere monticules to hills a thousand feet or more in height. Scoria-cones of this character abound in many volcanic districts, as the Auvergne, where they may be numbered by thousands. The materials forming such scoria-cones vary in size from that of a nut to masses as large as a man's head, and fragments of even larger dimensions are by no means uncommon. When the lava in a volcanic vent is perfectly glassy, instead of being partially crystalline in structure, we find not scoriæ but pumice ejected. In such cases, as in the Lipari Islands for example, we see cones entirely built up of pumice. Such pumice-cones resemble in the angle of their slope (see fig. 41, facing p. 124), the ordinary scoria-cones, but are of a brilliant white colour, appearing as if covered with snow. [Sidenote: PRESERVATION OF SCORIA-CONES.]