At Portrush, the Lower Lias is seen on the shore, crowded with ammonites, but silicified and metamorphosed by invading dolerite.
At some points the limestone has been dolomitized; near Bonsall it has been converted into a granular silicified rock.
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.
Calcified specimens are especially characteristic of the British Carboniferous formation; their preservation is equally perfect with that of the silicified fossils, and their investigation by Witham, Binney, Williamson and others has proved no less fertile.
Spicules of sponges and fragments of other organisms, such as molluscs, polyzoa, foraminifera and brachiopods, often occur in flint, and may be partly or wholly silicified with retention of their original structure.
At a more recent horizon, the silicified specimens of the Mesozoic Gymnosperms from Great Britain, France, and especially North America, are no less important.
Impressions of plants and silicified stems are frequently found.
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).
Volcanic activity may have extended into Miocene times; but the only fossiliferous relics of Cainozoic periods later than the Eocene are the pale clays and silicified lignites on the south shore of Lough Neagh, and the shelly gravels of pre-glacial age in county Wexford.
The Pliocene, on the other hand, is of freshwater origin, and contains silicified wood and numerous remains of Mammalia.