Abstract

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This was not established until Allison (1933) confirmed the sequence on Islay proposed by Bailey (1916). The formation everywhere lies at the same horizon, between a limestone formation beneath and a dolomitic formation, overlain by a quartzite, above. The formation is presumed to be late Pre-Cambrian in age, for it lies low down in the thick Dalradian sequence (Table 1), probably some 10000m (Rast 1963, p. 126) beneath the Leny Limestone, which has yielded a late Lower Cambrian fauna (Pringle 1940; Rayner 1965; Stubblefield 1956, p. 29).

The Port Askaig Formation consists of mixtites (unsorted and usually unstratified till-like beds) separated by stratified siltstone, sandstone, conglomerate and dolomite interbeds. The mixtites contain abundant rock fragments scattered randomly through them (Pis. 2, 3). Granite stones of up to l-5m in diameter are present and must presumably be far-travelled, having been brought from outside the basin of deposition (the Caledonian geosyncline). The largest intra-basinal fragment measures 320 x 64 x 45m. Adjacent mixtite beds often have subtly different lithologies and are more easily distinguished in some cases by those differences than using the thin, and sometimes discontinuous, stratified interbeds. Shackleton and Pitcher (Kilburn et al. 1965, fig. 3) distinguished 38 successive mixtites in the sequence on the east and south coasts of Garbh Eileach in the Garvellachs, numbering them from the base upwards (PI. 10). Despite difficulties in applying these numbers to the type sequence at Port Askaig, the introduction of a new numbering system was not thought to be justified and