Extract

Dr Peach recently drew the writer's attention to the occurrence of a dyke of unusual characters in Lauderdale. This dyke has proved to be a monchiquite, and is apparently distinct petrographically from all other intrusions described from south-east Scotland. It occurs on the left bank of the Earnscleugh Burn (a tributary of the Leader Water) three-quarters of a mile south of the farm of Earnscleugh, its outcrop making a prominent scar on the grassy hillside. Though contacts with the surrounding sediments are not exposed, the dyke is clearly seen to have a north-east trend. The length and breadth of the outcrop are about 50 yards and 10 yards respectively. The grits of Upper Old Red Sandstone age, into which the dyke has been intruded, do not appear to have undergone much alteration.

Petrography.—In the hand specimen the rock is seen to be strikingly porphyritic, large phenocrysts of unaltered biotite and hornblende being imbedded in a decomposed purple groundmass of very fine grain. The prophyritic constituents are not abundant, but their large size makes them exceedingly conspicuous. The biotite forms well-shaped hexagonal plates up to 1 inch in diameter, while the hornblende prisms are usually corroded and are seldom more than ½ inch across. The marginal portions of the dyke are crowded with xenoliths.

Owing to the decomposed nature of the groundmass it was found exceedingly difficult to retain the large phenocrysts in thin sections during grinding, and a separate microscopic examination was therefore made of the biotite and hornblende. The