Sometimes, however, scoria-cones are found reduced by atmospheric waste to mere heaps of cinders, in which the position of the crater is indicated only by a slight depression, as in [fig. 42].
CONES COMPOSED OF LAVA.
When but little explosive action takes place at the volcanic vent, and only fluid lava is ejected, mountains are formed differing very greatly in character from the cones composed of fragmentary materials.
If the lavas be of very perfect liquidity, like those erupted in the Sandwich Islands, they flow outwards around the vent to enormous distances. By the accumulation of materials during successive outbursts, a conical mass is built up which has but a slight elevation in proportion to the area of its base. Thus in Hawaii we find great volcanic cones, composed of very fluid lavas, which have a height of nearly 14,000 feet with a diameter of base of seventy miles. In these Hawaiian mountains the slope of the sides rarely exceeds 6° to 8°.
But if, on the other hand, the lavas be of much more viscid consistency, the character of the volcanic cones which are produced by their extrusion will be very different. The outwelling material will tend to accumulate and heap itself up around the vent. By successive ejections the first-formed shell is forced upwards and outwards, and a steep-sided protuberant mass is formed, exhibiting in its interior a marked concentric arrangement. Dr. Ed. Reyer, of Grätz, has devised a very ingenious method for reproducing on a miniature scale the characteristic features of these eruptions of viscid lavas. He takes a quantity of plaster of Paris reduced to a pasty consistence, which he forces through a hole in a board. The plaster accumulates in a great rounded boss about the orifice through which it has been forced. If the plaster have some colouring matter introduced into it, the mass, on being cut across, will exhibit in the disposition of its colour-bands the kind of action which has gone on during its extrusion, [fig. 43].
Fig. 43.—Experimental illustration of the Mode of Formation of volcanic cones composed of viscid lavas.
Fig. 44.—The Grand Puy of Sarcoui, composed of trachyte, rising between two breached scoria-cones (Auvergne).
There are many volcanic cones which exhibit clear evidence of having thus been formed by the extrusion of a viscid mass of lava through a volcanic fissure. Among such we may mention the domitic Puys of Auvergne, [fig. 44], many andesitic volcanoes in Hungary, the phonolite hills of Bohemia, and the so-called 'mamelons' of the Island of Bourbon. See figs. [45] and [46]. When the interior of these masses is exposed by natural or artificial sections, they are all found to exhibit the onion-like structure which occurs in the plaster models.