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.
[14] Logan: "Geology of Canada," [p. 45].
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.
Again, in the original molten world, it seems likely that most of the carbon present—at least, at the surface—was in the atmosphere in the gaseous form of carbon dioxide. This might be dissolved by the rain and other waters; but we know in the modern world no agency which can decompose this compound and reduce it to ordinary carbon or coal, except that of living plants, which are always carrying on this function to an enormous extent. We know that all our great beds of coal and peaty matter are composed of the remains of plants which took their carbon from the air and the waters in past times. We also know that this coaly vegetable matter may, under the influence of heat and pressure, when buried in the earth, be converted into anthracite and into graphite, and even into diamond. It is true that an eminent French chemist[15] has shown that graphite and hydrocarbons may be produced from some of the metallic compounds of carbon which may have been formed under intense heat in the interior of the earth, by the subsequent action of water on such compounds; but there is nothing to show that this can have occurred naturally, unless in very exceptional cases. Now in the Grenvillian system in Canada there is not only a vast quantity of carbon diffused through the limestones, and filling fissures in other rocks, into which it seems to have been originally introduced as liquid bitumen, but also in definite beds associated with earthy matter, and sometimes ten to twelve feet thick. The occurrence of this large amount of carbon warrants us in supposing that it represents a vast vegetable growth, either on the land or in the sea, or both.
[15] Henri Moissan, "Proceedings Royal Society," June, 1896,
In like manner, in later geological periods, beds of iron ore are generally accumulated as a consequence of the solvent action of acids produced by vegetable decay, as in the clay ironstones of the coal formation and the bog iron ores of later times. Thus the beds of magnetic iron occurring in the Upper Laurentian may be taken as evidences, not of vegetable accumulation, but of vegetable decay.
May not also the great quantity of calcium phosphate mined in the Grenville series in Canada, indicate, as similar accumulations do in later formations, the presence of organisms having skeletons of bone earth?
With reference to the carbon and iron ore of the Grenville series, I may quote the following from a paper published in the Journal of the Geological Society of London in 1870:—