Abstract

Dr. R. G. Park expressed great interest in these new dates from the southern region of the Lewisian. He was pleased to learn that Dr Bowes had apparently abandoned his view that the gneisses of the southern region were Laxfordian supracrustals. Since Rona lay less than 10 km across the strike from the Torridon-Shieldaig area, at the southern end of the region recently described by Dr Moorbath and himself, it was reasonable to expect some correlation between the chronologies of the two areas. Several points emerge from such a comparison.

(1) Of the 50 K–Ar dates in the Moorbath & Park study, only two, from the Gruinard Bay area, yield Badcallian (early Scourian) dates comparable with the c. 2700 m.y. event of the authors, although it was suggested from the lead isotope data that the whole region was in existence at 2900 m.y. In view of the Inverian and Laxfordian re-metamorphism which seems to have affected especially the acid gneisses of the region, it is perhaps puzzling that such an old date has been preserved in the Rona gneisses. The Inverian event which appears to be pervasive throughout the southern region may die out southwards towards Rona.

(2) The later discordant basic intrusives of Rona seem to be members of the Scourie dyke suite—the similarity of the sequence described to the mainland sequence leaves very little room for doubt on this point.

(3) The two Laxfordian dates recognized by the authors are associated with a pegmatite (at 1740

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