IV.— Pillow-Structure in Spilitic Lavas at Downan Point, Ballantrae

Abstract

1. Introduction Pillow-lavas of Middle Arenig age are associated with serpentinite and basic intrusions in the Girvan-Ballantrae complex, South Ayrshire (Geol. Surv. Sheet 7, Scotland). The area has been described by Peach and Horne (1899), and in more recent publications by Bailey and McCallien (1952, 1957), Bloxam (1955, 1958), and Bloxam and Allen (1959). The reader is referred to the papers by Bailey and McCallien who have provided a detailed historical account of previous research in the district, and have also advanced some new interpretations. The present account is confined to a description of the Downan Point group of pillow-lavas which form the most southerly outcrop of the Girvan-Ballantrae volcanic rocks, 1½ miles south of Ballantrae (Fig. 1). No direct fossil evidence has so far been recorded indicating the age of the Downan Point pillow lavas, but graptolitic black shales associated with pillow-lavas at Bennane Head, 4 miles to the north, are Middle Arenig in age (Lapworth, 1889), and a similar age is inferred for the Downan Point lavas. The literature on pillow-lavas is extensive, and various descriptions of their structures and possible mode of origin have been summarised by Shrock (1948). 2. The Downan Point Pillow-Lavas (i) General Features and “way-up” criteria. —The Downan Point pillow-lavas form almost uninterrupted exposures along the shore, most of which are readily accessible. Unlike many pillow-lavas in the Girvan-Ballantrae complex, they have suffered little deformation and, in the writer’s experience, are among the finest examples of pillow-structure in the British Ordovician (Peach and Horne, This 250-word extract was created in the absence of an abstract