Salisbury Crags and Stirling Castle became quite well known to geologists as places to observe the intrusion of whinstone into pre-existing limestone. A third site was discovered by John MacCulloch on the Isle of Skye. There is a location on the northern shore, at Trotternish, where there is a long three-fingered sill of whinstone that intrudes between the layers of limestone. MacCulloch first illustrated the Trotternish sill in a paper, and it was picked up by many other writers as further evidence of the volcanic origin of whinstone.

Leonhard’s book on basalt formations illustrates a great number of intrusions, often putting five or six on a single plate. We reproduce a detail of the plate that depicts the Trotternish sill.