With the J urassic beds is associated an extensive series of eruptive rocks (gabbro, peridotite, serpentine, diorite, granite, &c.); they are chiefly of Jurassic age, but the eruptions may have continued into the Lower Cretaceous.
Gabbro occurs in the peninsula of Fethland; diorite in Northmavine between Rinas Voe and Mavis Grind; and epidote-syenite in Dunrossness.
To N.E.; this is succeeded by a belt of gabbro, and finally the eastern border is again occupied by micaceous and chloritic schists.
A soil of very close texture, the gabbro, is found, most largely in the north-east.
It appears to consist chiefly of gabbro, peridotite, serpentine and other very basic eruptive rocks, which are believed to be of Cretaceous age.