LAURENTIAN GROUP.

In the course of the geological survey carried on under the direction of Sir W.E. Logan, it has been shown that, northward of the river St. Lawrence, there is a vast series of crystalline rocks of gneiss, mica-schist, quartzite, and limestone, more than 30,000 feet in thickness, which have been called Laurentian, and which are already known to occupy an area of about 200,000 square miles. They are not only more ancient than the fossiliferous Cambrian formations above described, but are older than the Huronian last mentioned, and had undergone great disturbing movements before the Potsdam sandstone and the other “primordial” or Cambrian rocks were formed. The older half of this Laurentian series is unconformable to the newer portion of the same.

Upper Laurentian or Labrador Series.—The Upper Group, more than 10,000 feet thick, consists of stratified crystalline rocks in which no organic remains have yet been found. They consist in great part of feldspars, which vary in composition from anorthite to andesine, or from those kinds in which there is less than one per cent of potash and soda to those in which there is more than seven per cent of these alkalies, the soda preponderating greatly. These feldsparites sometimes form mountain masses almost without any admixture of other minerals; but at other times they include augite, which passes into hypersthene. They are often granitoid in structure. One of the varieties is the same as the apolescent labradorite rock of Labrador. The Adirondack Mountains in the State of New York are referred to the same series, and it is conjectured that the hypersthene rocks of Skye, which resemble this formation in mineral character, may be of the same geological age.

Lower Laurentian.—This series, about 20,000 feet in thickness, is, as before stated, unconformable to that last mentioned; it consists in great part of gneiss of a reddish tint with orthoclase feldspar. Beds of nearly pure quartz, from 400 to 600 feet thick, occur in some places. Hornblendic and micaceous schists are often interstratified, and beds of limestone, usually crystalline. Beds of plumbago also occur. That this pure carbon may have been of organic origin before metamorphism has naturally been conjectured.

There are several of these limestones which have been traced to great distances, and one of them is from 700 to 1500 feet thick. In the most massive of them Sir W. Logan observed, in 1859, what he considered to be an organic body much resembling the Silurian fossil called Stromatopora rugosa. It had been obtained the year before by Mr. J. MacMullen at the Grand Calumet, on the river Ottawa. This fossil was examined in 1864 by Dr. Dawson of Montreal, who detected in it, by aid of the microscope, the distinct structure of a Rhizopod or Foraminifer. Dr. Carpenter and Professor T. Rupert Jones have since confirmed this opinion, comparing the structure to that of the well-known nummulite. It appears to have grown one layer over another, and to have formed reefs of limestone as do the living coral-building polyp animals. Parts of the original skeleton, consisting of carbonate of lime, are still preserved; while certain inter-spaces in the calcareous fossil have been filled up with serpentine and white augite. On this oldest of known organic remains Dr. Dawson has conferred the name of Eozoon Canadense (see Figs. 582, 583); its antiquity is such that the distance of time which separated it from the Upper Cambrian period, or that of the Potsdam sandstone, may, says Sir W. Logan, be equal to the time which elapsed between the Potsdam sandstone and the nummulitic limestones of the Tertiary period. The Laurentian and Huronian rocks united are about 50,000 feet in thickness, and the Lower Laurentian was disturbed before the newer series was deposited. We may naturally expect the other proofs of unconformability will hereafter be detected at more than one point in so vast a succession of strata.

Fig. 582. a. Chambers of lower tier communicating at +, and separated from adjoining chambers at O by an intervening septum, traversed by passages. b. Chambers of an upper tier. c. Walls of the chambers traversed by fine tubules. (These tubules pass with uniform parallelism from the inner to the outer surface, opening at regular distances from each other.) d. Intermediate skeleton, composed of homogeneous shell substance, traversed by f. Stoloniferous passages connecting the chambers of the two tiers. e. Canal system in intermediate skeleton, showing the arborescent saceodic prolongations. (Fig. 583 shows these bodies in a decalcified state.) f. Stoloniferous passages.
Fig. 583. Decalcified portion of natural rock, showing canal system and the several layers; the acuteness of the planes prevents more than one or two parallel tiers being observed.

The mineral character of the Upper Laurentian differs, as we have seen, from that of the Lower, and the pebbles of gneiss in the Huronian conglomerates are thought to prove that the Laurentian strata were already in a metamorphic state before they were broken up to supply materials for the Huronian. Even if we had not discovered the Eozoon, we might fairly have inferred from analogy that as the quartzites were once beds of sand, and the gneiss and mica-schist derived from shales and argillaceous sandstones, so the calcareous masses, from 400 to 1000 feet and more in thickness, were originally of organic origin. This is now generally believed to have been the case with the Silurian, Devonian, Carboniferous, Oolitic, and Cretaceous limestones and those nummulitic rocks of tertiary date which bear the closest affinity to the Eozoon reefs of the Lower Laurentian. The oldest stratified rock in Scotland is that called by Sir R. Murchison “the fundamental gneiss,” which is found in the north-west of Ross-shire, and in Sutherlandshire (see [Fig. 82]), and forms the whole of the adjoining island of Lewis, in the Hebrides. It has a strike from north-west to south-east, nearly at right angles to the metamorphic strata of the Grampians. On this Laurentian gneiss, in parts of the western Highlands, the Lower Cambrian and various metamorphic rocks rest unconformably. It seems highly probable that this ancient gneiss of Scotland may correspond in date with part of the great Laurentian group of North America.

[1] Quart. Geol. Journ., vol. iii, p. 156.

[2] This genus has been substituted for Barrande’s Conocephalus, as the latter term had been preoccupied by the entomologists.