ground for believing that the vast beds of Laurentian limestone have been originally organic in their origin, and primitively composed, in the main, of the calcareous skeletons of marine animals.  It would, in fact, be a matter of great difficulty to account for the formation of these great calcareous masses on any other hypothesis. (3) The occurrence of phosphate of lime in the Laurentian Rocks in great abundance, and sometimes in the form of irregular beds, may very possibly be connected with the former existence in the strata of the remains of marine animals of whose skeleton this mineral is a constituent. (4) The Laurentian Rocks contain a vast amount of carbon in the form of black-lead or graphite.  This mineral is especially abundant in the limestones, occurring in regular beds, in veins or strings, or disseminated through the body of the limestone in the shape of crystals, scales, or irregular masses.  The amount of graphite in some parts of the Lower Laurentian is so great that it has been calculated as equal to the quantity of carbon present in an equal thickness of the Coal-measures.  The general source of solid carbon in the crust of the earth is, however, plant-life; and it seems impossible to account for the Laurentian graphite, except upon the supposition that it is metamorphosed vegetable matter. (5) Lastly, the great beds of iron-ore (peroxide and magnetic oxide) which occur in the Laurentian series interstratified with the other rocks, point with great probability to the action of vegetable life; since similar deposits in later formations can commonly be shown to have been formed by the deoxidising power of vegetable matter in a state of decay.

In the words of Principal Dawson, “anyone of these reasons might, in itself, be held insufficient to prove so great and, at first sight, unlikely a conclusion as that of the existence of abundant animal and vegetable life in the Laurentian; but the concurrence of the whole in a series of deposits unquestionably marine, forms a chain of evidence so powerful that it might command belief even if no fragment of any organic or living form or structure had ever been recognised in these ancient rocks.”  Of late years, however, there have been discovered in the Laurentian Rocks certain bodies which are believed to be truly the remains of animals, and of which by far the most important is the structure known under the now celebrated name of Eozooen.  If truly organic, a very special and exceptional interest attaches itself to Eozooen, as being the most ancient fossil animal of which we have any knowledge; but there are some who regard it really a peculiar form of mineral structure, and a severe, protracted, and still unfinished controversy has been carried on as to its nature.  Into this controversy it is wholly unnecessary to enter here; and it will be sufficient to briefly explain the structure of Eozooen, as elucidated by the elaborate and masterly investigations of Carpenter and Dawson, from the standpoint that it is a genuine organism—­the balance of evidence up to this moment inclining decisively to this view.