Now when we find in the Grenvillian series, the first oceanic group of beds known to us, great and widely extended limestones, thousands of feet in thickness, and rivalling in magnitude those of any succeeding period, we naturally infer that marine life was at work. No doubt the primitive sea contained more lime and magnesia than the present ocean holds in solution; but while this might locally favour the accumulation of inorganic limestones, it cannot account for so great and extensive deposits. On the other hand, a sea rich in lime would have afforded the greatest facilities for the growth of those marine plants which accumulate lime, and through these for the nutrition of animals forming calcareous shells or corals. Thus we have presumptive evidence that there must have been in the Upper Laurentian sea something corresponding to our coral reefs and shell-beds, whatever this something may have been. These limestones, however, demand more particular notice (Fig. 19). One of the beds measured by the officers of the Geological Survey is stated to be 1,500 feet in thickness, another is 1,250 feet thick, and a third 750 feet; making an aggregate of 3,500 feet.[14] These beds may be traced, with more or less interruption, for hundreds of miles. Whatever the origin of such limestones, it is plain that they indicate causes equal in extent, and comparable in power and duration, with those which have produced the greatest limestones of the later geological periods. Now, in later formations, limestone is usually an organic rock, accumulated by the slow gathering from the sea-water, or its plants, of calcareous matter, by corals, foraminifera, or shell-fish, and the deposition of their skeletons, either entire or in fragments on the sea-bottom. The most friable chalk and the most crystalline limestones have alike been formed in this way. We know of no reason why it should be different in the Laurentian period. When, therefore, we find great and conformable beds of limestone, such as those described by Sir William Logan in the Laurentian of Canada, we naturally imagine a quiet sea-bottom, in which multitudes of animals of humble organization were accumulating limestone in their hard parts, and depositing this in gradually increasing thickness from age to age. Any attempts to account otherwise for these thick and greatly extended beds, regularly interstratified with other deposits, have so far been failures, and have arisen either from a want of comprehension of the nature and magnitude of the appearances to be explained, or from the error of mistaking the true bedded limestones for veins of calcareous spar. [Footnote 14: Logan: "Geology of Canada," p. 45.] [Illustration: Fig. 19A.--_Attitude of Limestone at Côte St. Pierre_ (see Map, p. 88). (_a_) Gneiss band in the Limestone, (_b_) Limestone with Eozoon. (_c_) Diorite and Gneiss.]