The phenomenon of which this paper treats occurs in a basalt dyke exposed in a small quarry by the roadside north-east of Dippin, near the twelfth milestone from Brodick. This dyke runs from north-west to south-east, and is intrusive into the great teschenite (or crinanite) sill of Dippin. It is probably the one referred to in the Geological Survey Memoir on North Arran, South Bute, and the Cumbraes (1903), p. 119. It averages 12 feet in thickness, although it is extremely irregular, owing perhaps to the difficulty experienced in penetrating the tough, coarse, massive rock of the sill. The contacts are not plane, but about against the crinanite in a very intricate manner, sometimes along vertical or horizontal joints, sometimes along irregular surfaces unconnected with jointing. Both contacts show thin films of tachylyte, which, as the rock is quarried, are left adhering to the irregular surfaces of the sill rock. Another dyke-like mass, 1 foot thick, is seen in the same quarry, and is doubtless an offshoot from the larger dyke.