We have seen that the mineral constitution of the Upper Laurentian affords evidence that in this age there were already land and water, and that the processes by which the land is being worn down, and its materials deposited on the sea-bottom, were in full operation; while the absence of any evidence of violent wave-action, and the presence of thick deposits of limestone, coaly matter, iron ore, and fine-grained beds of sediment, indicates a time of rest and quiescence. All these conditions were favourable to the presence of life, and we should expect to find in such a period some sign of its commencement. But here we are met by a formidable difficulty. If the beds of the Grenville series were originally deposits in a quiet sea, they are, as now existing in the old Laurentian hills and valleys, very much changed from their original condition. They have, in short, experienced the changes known to geologists by the formidable word metamorphism, whereby they have lost the more obvious characters of ordinary aqueous deposits, and have assumed new and strange forms. Dr. Adams, of Montreal, has taken the pains to collect a number of chemical analyses of the gneisses and schists or crystalline slates of the Grenville series, and finds that, however unlike to more modern shales and clays, they have substantially the same chemical composition. Now if they were originally such shales and clays, it has happened to them that the ingredients of the clays have rearranged themselves in new forms and become crystalline. We are familiar in a small way with such changes when brick clay, over-heated in the kiln, becomes fused into slag or vitrified; and if such slag were allowed to cool very slowly, it would present different kinds of crystalline minerals. We actually see changes of this kind in the substance of bricks which have been long exposed to intense heat in the walls of furnaces. Now in the crust of the earth, very old rocks, buried under newer deposits, and exposed to the heat of the interior molten rocks, experience such changes on a great scale; and there is one kind of influence present in the bowels of the earth which we in our experiments cannot easily imitate or understand, namely, the action of superheated water prevented by pressure from escaping as steam, and permeating the whole substance of deposits, which are thus baked at a high temperature in presence of water, instead of being exposed to mere dry heat, as in our kilns and furnaces. The study of the partial changes which have passed on later sediments where in contact with volcanic masses once intensely heated, enables us to understand the greater and more extensive metamorphism of the oldest rocks. Thus a mere mud becomes glorified by metamorphic crystallization into a micaceous schist. We have taken ordinary clay as an example; but under the same processes sand has been converted into a compact quartzite, ordinary limestone into crystalline marble,