XVI. The Sequence in Islay and Jura

Abstract

Islay has the special geological interest that the Dalradian rocks there are associated with those that have been identified as Lewisian, Moine and Torridonian, and contain fossiloid impressions that have been regarded as Cambrian. The Dalradian sequence in Islay includes representatives of all the Series to the uppermost division, the Ardnoe Beds, and the Schichallion Boulder Bed. There is still marked disagreement1 as to the structure of the island and the succession of its rocks. Macculloch in 1819 was the pioneer in Islay Geology (Western Islands of Scotland, Vol. 11, p. 223); he compared the western rocks to the Lewisian gneiss, identified those of the northern Rhinns and their continuation in Oronsay and Colonsay as mica-schists, and with characteristic insight correlated the Portaskaig Conglomerate with the Schichallion Boulder Bed and recognised its unconformity on the Islay Limestone. Murchison and Geikie in 1861 (Quart. Journ. Geol. Soc., Vol. XVII, pp. 206-10), believing in the Palaeozoic age of the Highland metamorphic rocks, correlated the Islay Limestone with the Durness Limestone (Ordovician) and the upper quartzite with that of Sutherland which is Cambrian. Thomson2 in 1877 (Trans. Geol. Soc., Glasgow , v, pp. 200-222) correlated the gneiss of the Rhinns with the Lewisian, the rocks of north-eastern Islay with the Torridonian, the main quartzite with the Upper Cambrian and the quartzite and schists of the Mull of Oa with the Silurian. He recognised that some of the rocks represent those now referred to as Dalradian, and in his valuable study of the boulder bed, This 250-word extract was created in the absence of an abstract