CHAP. IV 
PLATEAU OR FISSURE TYPE 
43 
silts and drift-wood. But the rain will continue to fall, and the drainage 
to seek its way seaward. When the last eruption ceases, and the rivers are 
at length left undisturbed at their task of erosion, they will carve that lava- 
floor into deep gorges or open valleys. Where they flow between the 
lavas and the slopes against which these ended, they will cvit back the 
volcanic pile, until in course of time the lavas will present a bold mural 
escarpment to the land that once formed their limit. Ihe volcanic plain 
will become a plateau, ending off in this vertical wall and deeply trenched by 
the streams that wind across it. And if the denudation is continued long 
enough, the plateau will be reduced to detached hills, separated by deep and 
wide valleys. 
This geological history is illustrated by the diagram in Fig. 17. The 
stippled ground underneath (a:, .*) 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 ac- 
cumulation of volcanic material. Tire 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 wdiich 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 Bed 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 ai’e even still 
often approximately horizontal, and lie piled on each other in tolerably regulai 
beds to a thickness of 1000, and in one place to more than 3000 feet. 
