46 GUIDE TO ICELAND. than the deserts that surround it on the north, west and east, strong presumptive evidence that it lies nearer the vent whence the sheets of rock issued, and also that the later ones were less in hulk than those preceding, and consequently did not extend farther round the vent than the borders of the 0'clcP6ahraun ; the very last that issued prior to the upbuilding of Asiya’s mountainous periphery being the comparatively small ones, that form its widely extending base. One thing is certain, Askja is surrounded with a tract covered with lava which, subsequently to the deposit of the most superficial sheet of rock on the plateau, has issued from that crater, and welled forth from rifts and smaller volcanic moun- tains around it, having as large an area as the whole of the other tracts similarly overspread in the island ; and this fact in itself is strongly corroborative of the view taken that the Askja crater marks the focus of volcanic activity in Iceland. The period following the close sealing of the great central vent was, beyond doubt, the most troublous one in the history of the island since the glacial epoch ; most of the mountains, not portions of the older plateau, now standing in the island being formed by the volcanic disturbances that succeeded. It is an easy matter to trace the rift, of which Askja is the present great central outlet, for it forms to this very hour a covered channel running almost due north for a distance of forty miles, and another south to the coast, both clearly traceable by signs of active volcani- city in several places, where gases from the central vent force their way through the porous lava filling in the rift, and form solfatarar in spots where cracks and fissures exist. The Fremri- ndmar (Farther-solfatarar) lie upon the northern channel on the very verge of the O'dd’&ahraun, and mark where an immense flood of lava in comparatively recent times burst forth through the over- lying strata, roofing in the rift, at a weak spot between the site of former disturbance farther north, where volcanic mountains and vast lava-floods piled above offered more resistance, and another volcanic range, the Northern Dl/ngjufjoll, which mark the course of the rift through the O'dcPSahraun. Farther north, upon the line of this rift are also the Jllriarndmar, the solfatarar in which are situated the largest boiling mud wells in Iceland, Leirhnulcr and Krajla ; and it is possible at the latter place, which has been the scene of terribly violent eruptions, ramifications extend to the ]}eistareykir solfatarar and the Uxahver group of hot springs. From Leirhnulcr, and a number of fissures between that volcanic mountain and the Fremrindmar (all lying in. a line running from south to

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