At some points the limestone has been dolomitized; near Bonsall it has been converted into a granular silicified rock.
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.
In other cases, especially near mineral veins, slates are filled with black needles of tourmaline or are bleached to pale grey and white colours, or are silicified and impregnated with mineral ores.
In the Kharga Oasis the upper portion consists of variously colored unfossiliferous clays with intercalated bands of sandstone containing fossil silicified woods (Nicolia Aegyptiaca and A raucarioxylon Aegypticum).