This paper was in continuation of the series of memoirs on the volcanic rocks of Scotland previously read by the author before the Society, and contained the first portion of the results of a survey of the western region, extending from the south of Antrim to the north of Syke. The districts more especially dwelt upon were the islands of Mull, Eigg, and Staffa. After alluding to the writings of previous geologists upon these tracts, more particularly to the discovery by the Duke of Argyll of tertiary leaves under basalt at Ardtun Head, in Mull, the author remarked that up to this time the great mass of volcanic rocks in the Western Islands has been usually regarded as of Oolitic age—an opinion in which he himself had shared. His object in the present communication was to show that as regards Mull and the adjoining islets this opinion was erroneous, that the enormous volcanic accumulations of these islands belonged in reality to the Miocene period, and that, in all likelihood, the long chain of basaltic masses, extending from the north of Ireland along the west coast of Scotland to the Faroe Islands, and beyond these to Iceland, was all erupted during the same wide interval in the Tertiary periods.