TOPOGRAPHY AND GEOLOGY. 45 including the interior, lowlying at that time, would he flooded; the volcanic vent converted for a period into a submarine volcano, and the molten rock that issued at such times be deposited in the form of tuffs and conglomerates. In the course of ages the whole of the spaces between the fragments of the older plateau were gradually upbuilded above the encroachment of the sea ; the currents and tides carrying away the greater part of the dis- integrated igneous rock that found its way beyond the outlying tertiary bulwarks that now form a considerable part of the coastal region; the climate became more temperate, no glaciers being found at a lesser altitude than 2,500 feet, and the later discharges of molten rock were deposited unchanged, save by congelation, in the form of the basaltic and other lava-strata that lie uppermost in the post-tertiary plateau forming the greater part of the interior. The regularity and evenness with which these vast sheets have been deposited seem to point to a remarkably peaceful welling forth of the floods of molten rock, accompanied by the discharge of little or no fragmentary material, for no layers of ash interpose between the strata, thin layers of clinker-like crust alone being found. Their deposit, however, appears to have first greatly narrowed in, and ultimately to have sealed the vent whence they issued; and then came troublous times. As to the site of this volcanic vent, I believe that immediately following the disturbances of the glacial epoch a rift in the earlier rock formations of the earth’s crust extended from south to north under those portions of the island now known as the Vatna Johull and the O'daftcihraun as far north as Krnfla. Fragments of the older plateau being left standing on both sides of the southern end of the rift, it was narrower than in the centre, and the superincumbent masses of igneous rock now existing there wore sooner piled up than where the rift was wider; as also was the case towards the northern end where it also was less in width, the volcanic vent being thus at a comparatively early period of its post-tertiary history narrowed down to the limits of the O'dar&ahraun. The mountain masses lying upon these rifts on the north and south stand to this day, as a glance at the map will show. The greater number of the foundation strata of the island were probably de- posited during the time the outlet extended north and south through the O'da/Sahraun ; and by the welling forth of the later sheets of molten rock, the outlet was still further narrowed down to the limits of the Askja crater. The OdtP&ahraun lies higher

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