Last of all we have the widely distributed Laurentian system of Logan, the oldest known to geologists, and which with the Huronian constitutes the great Archæan group of formations of Dana and others. In its lowest part this consists entirely of the stratified granitic rock known as gneiss, inter-bedded in some places with dark-coloured crystalline rocks or schists. This may be a part of the first-formed crust of our globe, produced under conditions different from those of any later rocks, and incompatible with the existence of life. The upper part of the Laurentian system, however, known in Canada as the "Grenville Series," shows evidence of ordinary marine deposition in quiet waters, which may have been not unfavourable to the lower forms of marine life; and though its beds have been greatly changed by heat and pressure, we can still to some extent realize the conditions of a time of comparative quiescence intervening between the underlying Lower Laurentian and the succeeding Huronian. This part of the system still contains gneisses, bedded diorites, and other rocks which may have been volcanic; but it has also quartzites and quartzose gneisses which must have been sandstones or shales, thick limestones, beds of carbon now in the state of graphite or plumbago, and large beds of iron ore. Such rocks were in all succeeding formations produced under water and by accumulations of the remains of plants and the hard parts of animals, in strictly sedimentary beds, usually formed slowly and without mechanical disturbance. Hence we may infer that aquatic life at least existed in this early period, and as there must have been land and water, shallows and deep seas, there may have been scope for various kinds of living beings. The Grenville period is, however, separated from the succeeding Huronian by a great interval, occupied mainly by volcanic ejections and earth-movements; so that our Grenville series, if it contains organic remains, may be supposed to afford species differing from those of the Huronian, and to form a sort of oasis in the desert of the early pre-Cambrian world. We find that the limestones of this age actually contain remains supposed to be of animal origin. They were first found in Canada, which contains the largest and best exposed area of these rocks in the world, and were brought under the notice of geologists by the late Sir William E. Logan, the first director of the Geological Survey of that country. In anticipation of details to be given later, the story of this discovery and its announcement may here be given in brief