THE GARS-BHEINN ULTRABASIC SILL, ISLE OF SKYE

Abstract

Gars-bheinn, the most southerly peak of the Cuillin Hills in the Isle of Skye, has on its flank two minor intrusions, the lower of which is of notable petrological interest. They were mentioned by Harker (1904, p. 67), who observed that the lower intrusion “is in visible continuity with a dyke below . . . cutting the basaltic lavas and the underlying Torridon Sandstone”, and that, as in the main Sgùrr Dubh mass, banding occurs in the Gars-bheinn intrusions. The type of banding developed at Gars-bheinn differs from that of the main mass, which is more readily comparable with that in the Rhum ultrabasic rocks ( Brown 1956 ); in Sgùrr Dubh and Rhum the banding is due to varying proportions of medium-grained primary precipitate crystals. In the Gars-bheinn intrusions, the banding is of medium-grained olivine-rich bands alternating with pegmatitic feldspar-rich bands, individual feldspar crystals in the latter often exceeding three inches in length. The intrusions occur on the southern flank of Gars-bheinn at 1000 and 1500 feet respectively. The lower is associated at its eastern side with a vertical feeder dyke, the continuation of which may be traced above the sill, trending towards the higher intrusion. The lower sill (Fig. 1), which at outcrop is approximately 900 feet in length and 250 feet thick, is moderately well exposed, whereas much of the upper is obscured by a scree of large detached blocks. The term “Gars-bheinn sill” refers only to the lower sill, which has the greater petrological interest. The bulk