Fig. 17.—Section to illustrate the structure of the Plateau type.
This geological history is illustrated by the diagram in [Fig. 17]. The stippled ground underneath (x, x) represents the original undulating surface of the country on which the plateau eruptions were poured out. The lavas of these eruptions are shown by the horizontal lines to have entirely buried the heights and hollows of the old land, and to have risen up to the upper dotted line, which may be taken to mark the limit reached by the accumulation of volcanic material. The dark lines (d, d) which come up through the bedded lavas indicate the dykes with their connected vents. Denudation has since stripped off the upper part of the volcanic series down to the uppermost continuous black line which represents the existing surface of the ground. The level sheets of lava have been deeply trenched, and in one instance the valley has not only been cut through the volcanic pile, but has been partly eroded out of the older rocks below. To the right and left, the lavas end off abruptly in great escarpments.
The succession of events here depicted has occurred more than once in Britain. The Plateau type is chiefly developed in this country among the great Tertiary basalt districts of Antrim and the Inner Hebrides, which reappear in the Faroe Islands, and again still farther north in Iceland. But it also occurs among the volcanic rocks of the Old Red Sandstone and Carboniferous periods.
As compared with the other volcanic types, that of the Plateaux is distinguished by the wide extent of surface which its rocks cover, by the great preponderance of lavas over tuffs, and by the regularity and persistence of the individual sheets of rock. In Britain, the plateau-lavas are even still often approximately horizontal, and lie piled on each other in tolerably regular beds to a thickness of 1000, and in one place to more than 3000 feet. They form wide level or gently undulating tablelands, which rise in bold escarpments from the surrounding country and have been deeply carved into valleys. The sides of their cliffs and slopes are marked by parallel lines of terrace, arising from the outcrop of successive sheets of lava (Figs. [11], [265]).
With the Tertiary basalt-plateaux are connected thousands of dykes, that follow each other along nearly parallel lines in a general north-westerly direction, and mark the position of fissures up which the molten lava ascended. Occasional necks of agglomerate or basalt indicate the sites of some of the eruptive vents.
The Carboniferous volcanic plateaux have been more extensively denuded than those of Tertiary age, so that a large number of their vents have been laid bare. In general these vents are of comparatively small size, though larger than those of the Carboniferous Puys. In some districts, abundant dykes traverse the rocks on which the plateaux rest, though the fissures seem to have been less numerous than in Tertiary time.
3. The Puy type, as before remarked, takes its name from the well-known puys, or volcanic cones, of Central France. Volcanoes of this type form conical hills, generally of small size, consisting usually of fragmental materials, sometimes of lava. Where a cone is partially effaced by a second, and even by a third, successive slight shiftings of the vent are to be inferred (see Figs. [29] and [214]). In many cases, no lava has issued from such cones, nor were the ashes and cinders dispersed far from the vent. Hence, in the progress of denudation, cones of this kind are easily effaced.
From the uniformity of composition of their materials, the simplicity and regularity of their forms, and their small size, it may be inferred that many of these cones were the products of single eruptions. They may conceivably have been thrown up in a few days, or even in a single day. The history of Monte Nuovo, in the Bay of Naples, which was formed within twenty-four hours in the year 1538, is a memorable example of the rapidity with which a cone more than 400 feet high may be thrown up at some distance from a central vent.
The smallest independent volcanoes are included in the Puy type. In many instances the diameter of the funnel has not exceeded a few yards; the largest examples of the type seldom exceed 1000 feet in width.
Where lavas have been discharged, as well as ashes and stones, a more vigorous activity is indicated than where merely cones of tuff were formed. The lavas may come from more than one side of a cone, and may flow in opposite directions for a distance of several miles. It is observable that considerable streams of lava have issued from the base of a cinder-cone without disturbing it. The molten rock has found a passage between the loose materials and the surface on which they rest,[20] though, in some cases, the cone may have been thrown up after the emission of the lava.