We are convinced that the clayslates and graywackes which repose on the southern flank of the Grampians, as well as abundantly in the interior, will, upon strict examination, have their place assigned among the Silurian class. Mr. Nicol, who has done so much for the Lammermuir deposits, will find ample scope for his investigations, and all his ingenious speculations, in determining the true position of these argillaceous beds, which are of prodigious thickness and vast extent. This is not the place to enter into details, but in support of the view now advanced, the following among other reasons may be given. First of all, the clayslate of the Grampians resembles in its lithology the slates of Wales and Cumberland, admitted to be silurian. In hand specimens they cannot easily be distinguished from each other: practical men consider the slates of Dunkeld and Glenalmond as softer and less flinty than those of the south. They pass from extremely coarse into the finest grained varieties, when the graywacke character is entirely lost in the homogeneous mass. Their position in reference to the crystalline rocks, in the next place, is very distinct, never alternating with, nor lying conformable to, either the gneiss or mica-schists. They form the outer zone, from east to west, of the Grampian range, where feldspar, porphyries, and trappean rocks are along the whole line mixed up or associated with them. Then overlying the clayslate, precisely as in Cumberland, the old red sandstone is found in immediate succession and resting unconformably. Shall we add that, even in a topographical point of view, these beds will be admitted to vindicate their claim to Silurian origin, constituting, as they do, in extension, a portion of the great primary belt that encompasses the western shores of Great Britain, and beyond the channel, stretches through Brittany and Normandy? From considerations such as these there are sufficient grounds, we think, for constituting the clayslates and porphyries of the Grampians into a “physical group,” existing in a “separate physical region.” The absence of organic remains may be accounted for by the fact of the vast disturbance prevailing in the seas at the period, and indicated by the prodigious quantity of igneous matter spread repeatedly over their bottom. These causes would act in so far in preventing the existence and increase of living things, over all these parts, and most certainly in obliterating the traces of their remains, if any were deposited. But as future explorers may yet detect them in abundance we proceed to consider the nature and classes of fossils elsewhere discovered in the Silurian strata.