Book 8 — CHAPTER XXXV THE SYSTEM OF DYKES—continued (continued 2)

The association of fissures and dykes with the accumulation of thick and extensive volcanic plateaux, over so wide a region of North-western Europe as from Antrim to the North of Iceland, finds its parallel in different parts of the world. One of the closest analogies presents itself among the Ghauts of the Bombay Presidency, where vast basaltic sheets, probably of Cretaceous age, display topographical and structural features closely similar to those of the Tertiary volcanic plateaux of the British Isles. The dykes connected with these Indian basaltic outflows correspond almost exactly in their general character and stratigraphical relations to those of this country. They occur in great numbers, rising through every rock in the district up to the crests of the Ghauts, 4000 feet above the sea. They vary from 1 or 2 to 10, 20, 40, and even occasionally 100 or 150 feet in width, and are often many miles in length. They observe a general parallelism in one average direction, and show no perceptible difference in character even when traced up to elevations of 3000 and 4000 feet.[219]

Thousands of square miles in the Western States and Territories of the American Union have been similarly flooded with basic lavas. Denudation has not yet advanced far enough to lay bare much of the platform on which these lavas rest. But the dykes that traverse the rocks outside of the lava-deserts afford an example of the structure which will ultimately be revealed when the wide and continuous basalt-plains shall have been trenched by innumerable valleys and reduced to fragmentary plateaux with lofty escarpments (p. 267).

It is to the modern eruptions of Iceland, however, that we turn for the completest illustration of the phenomena connected with dykes and fissures. An account of these eruptions will therefore be given in Chapter xl. as an explanation of the history of the Tertiary basalt-plateaux of Britain.