The group of rocks on which we next enter are termed fossiliferous, that is, there is contained in their hard stony substance the impressions and actual remains of organic bodies. As we proceed upward through the series in their ascending order, we will find different rocks distinguished by different classes of fossils, and characterized by distinct lithological appearances. They are in consequence divided into different formations, and called by particular names. Hence the origin of SYSTEMS, of which there are five or six recognized by geologists, separable into their respective groups of strata. Descending from the primary, the highest as well as lowest in the series of rocky combinations, the group which first invites attention is the SILURIAN; so denominated because the strata are widely spread over the districts in England and Wales, anciently inhabited by a people called the “Silures.” They are found in various quarters of the world, and occupy a large area on the southern frontiers of Scotland. The rocks of this class consist of a group of argillaceous, calcareous, and arenaceous deposits, varying in color and texture. They are of great thickness and severally impressed with their own written story, the fossil memoranda of the changes and events that occurred betwixt the formation of each. These are the transition rocks of Werner. The newly-adopted term of Silurian implies no peculiar theory as to their origin. It simply expresses the fact that in the district in question a complete succession of fossiliferous strata is interpolated between the oldest slaty crystalline rocks and the old red sandstone. The system is divided by their discoverer and historian, Sir R. I. Murchison, under the ascending series, into the Cambrian System, Llandeilo-flags, Caradoc Sandstone, Wenlock Shales and Limestones, and Lower and Upper Ludlow Rocks. Do the equivalents of all, or of any, of these groups exist in the Grampian range? Geologists for the most part have been answering these questions in the negative. Hitherto no true silurian deposits have been recognized as existing among the northern Scottish mountains, and no well-authenticated organism of the system has been detected in any of their localities. This, however, will hardly be taken as a conclusive argument after the admission into the family of the Skiddaw slate, in which the faintest traces of organized matter have only very recently been observed, while the over-lying series consisting of chlorite-slate, and alternating beds of porphyry and greenstone, from twenty to thirty thousand feet thick, have not yet been proved to contain a single fossil. “Good fossil groups,” Professor Sedgwick argues, “are the foundation of all geology; and are out of all comparison the most remarkable monuments of the past physical history of our globe, so far as it is made out in any separate physical region.”