The younger rocks of Iceland (Pliocene to Present Day) occupy the broad median zone shown in Text-fig. I; the boundaries are only approximate in places, and information is lacking with regard to the eastern one particularly. Outside this zone early Tertiary basalts predominate but (I) the uppermost basalts are regarded by some authors as Quaternary, (2) Quaternary and Recent volcanic rocks occur in the Snaefells Peninsula and in Skagi. The thickness of the early Tertiary series has been estimated at 3,000 m.; it may well be more, and it is reasonable to suppose that originally these basalts extended over the site of the median zone. Hitherto all workers in Icelandic geology have considered the zone to be a graben where the older rocks have been faulted or tilted downwards, but recently R. A. Sonder (1, 2) has put forward the startling idea that it is a horst, a view which is discussed, and not unfavourably, by H. G. Backlund (3). Before considering this new conception attention will be drawn to evidence in the neighbourhood of the north-western boundary of the zone, evidence which is not mentioned by Sonder and which, although it has been known for over 70 years, is often ignored or forgotten by writers on Icelandic tectonics.