The group of rocks placed above the Primary, bears the general name of Silurian, from their being presented on the surface in a portion of Western England formerly occupied by a people whom the Roman historians call Silures. It is also developed very extensively in Northern Europe, and in North America. The group is divided by geologists into two distinct formations, the Lower and Upper.

Hitherto nothing has been said of the fossils which constitute so important a part of geological science. It is now to be observed that, from an early portion of the rock series to its close, the mineral masses are found to enclose remains of the organic beings (plants and animals) which flourished upon earth during the time when those were forming; and these organisms, or such parts of them as were of sufficient solidity, have been in many instances preserved with the utmost fidelity, although for the most part converted into the substance of the enclosing mineral. The rocks may be said thus to form an organic history of our globe, from perhaps near the commencement of life upon its surface to the present time. This is a piece of knowledge entirely new to man, and it may be safely said that he has never made a merely intellectual acquisition of a more interesting or remarkable nature. I am to trace this history as well as existing materials will permit.

The chronicles of this period consist of a series of beds, mostly calcareous, taking their general name (Oolite system) from a conspicuous member of them—the oolite—a limestone composed of an aggregation of small round grains or spherules, and so called from its fancied resemblance to a cluster of eggs, or the roe of a fish. This texture of stone is novel and striking. It is supposed to be of chemical origin, each spherule being an aggregation of particles round a central nucleus. The oolite system is largely developed in England, France, Westphalia, and Northern Italy; it appears in Northern India and Africa, and patches of it exist in Scotland, and in the vale of the Mississippi. It may of course be yet discovered in many other parts of the world.

The series, as shown in the neighbourhood of Bath, is (beginning with the lowest) as follows: 1. Lias, a set of strata variously composed of limestone, clay, marl, and shale, clay being predominant; 2. Lower oolitic formation, including, besides the great oolite bed of central England, fullers'-earth beds, forest marble, and cornbrash; 3. Middle oolitic formation, composed of two subgroups, the Oxford clay and coral rag, the latter being a mere layer of the works of the coral polype; 4. Upper oolitic formation, including what are called Kimmeridge clay and Portland oolite. In Yorkshire there is an additional group above the lias, and in Sutherlandshire there is another group above that again.