As early as 1858, Sir William Logan had begun to suspect that certain laminated bodies found in the Laurentian limestones of the Grenville series might be of organic origin. The points which struck him were these: They differed from any known laminated concretions; they resembled the "Stromatoporæ" or layer-corals of the lower Palæozoic rocks next in succession to the Laurentian and Huronian; the forms were similar in all the specimens, while the mineralizing substances were different; they were found only in the limestone, and specially in one of the three great beds known in the formation, the upper limestone of the Grenville system. He exhibited specimens, and mentioned these probabilities at the meeting of the American Association in 1859. In 1862 it was suggested to Logan that the microscopic structure of some of the best preserved examples should be studied, and slices were accordingly prepared and submitted to the writer for examination. They revealed in the calcareous laminæ of the specimens complicated systems of canals or tubes filled with mineral matter, which appeared to be similar to those that Carpenter had recognised in the thickened parts of the shells of modern Foraminifera. This clew being followed, large numbers of slices of the supposed fossils and of the containing limestone and of similar limestones from other parts of the world were examined. The writer also visited the localities of "Eozoon," and studied its mode of occurrence _in situ_. The facts ascertained were communicated to the Geological Society of London, the name "Eozoon Canadense" being proposed for the species. Its description was accompanied by a paper on the geological conditions by Logan, and one on the chemical conditions by Sterry Hunt, while supplementary notes were added by the late Dr. Carpenter and Professor T. Rupert Jones. Thus launched on the scientific world, "Eozoon" at once became a fertile subject of discussion, and volumes of more or less controversial literature have appeared respecting it. It still has its friends and opponents, and this may long continue, as so few scientific men are sufficiently acquainted on the one hand with the possibilities and conditions of the preservation of fossils in crystalline rocks, and on the other hand with the structures of modern "Protozoa." Thus, few are in a position to form an independent judgment, and "Eozoon" has met with some scepticism on the part both of biological and mineralogical specialists. To aid us in forming an opinion, it will be necessary to consider the oldest known strata of the earth's crust, and the evidence which they afford of the condition of the world when they were deposited. As preliminary to this, we may look at the following table of pre-Cambrian formations in Canada. SUCCESSION OF PRE-CAMBRIAN ROCKS IN CANADA, AS UNDERSTOOD UP TO 1896. (_In descending order._)