The dyke in question occurs at the extreme north end of Pentreath Beach, which is about a quarter of a mile to the south-east of Kynance Cove.

The dyke forms a portion of the “granulitic group” of Prof. Bonney, which is now known to be of igneous origin, and consists of banded gneissic rocks, some of which are like a granite, other portions like a diorite, the latter always more or less porphyritic.