At some points the limestone has been dolomitized; near Bonsall it has been converted into a granular silicified rock.
In Arran there are pitchstone dikes, some of which are very completely vitreous, while others are changed to spherulitic felsites more or less silicified.
The Pliocene, on the other hand, is of freshwater origin, and contains silicified wood and numerous remains of Mammalia.
Impressions of plants and silicified stems are frequently found.
At Portrush, the Lower Lias is seen on the shore, crowded with ammonites, but silicified and metamorphosed by invading dolerite.