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.