Fig. 6. Gneiss and Limestone at St. Pierre.
(a.) Limestone. (b.) Gneiss and Diorite.
The limestone is here highly inclined and much contorted, and in all the excavations a thickness of about 100 feet of it may be exposed. It is white and crystalline, varying much however in coarseness in different bands. It is in some layers pure and white, in others it is traversed by many gray layers of gneissose and other matter, or by irregular bands and nodules of pyroxene and serpentine, and it contains subordinate beds of dolomite. In one layer only, and this but a few feet thick, does the Eozoon occur in any abundance in a perfect state, though fragments and imperfectly preserved specimens abound in other parts of the bed. It is a great mistake to suppose that it constitutes whole beds of rock in an uninterrupted mass. Its true mode of occurrence is best seen on the weathered surfaces of the rock, where the serpentinous specimens project in irregular patches of various sizes, sometimes twisted by the contortion of the beds, but often too small to suffer in this way. On such surfaces the projecting patches of the fossil exhibit laminæ of serpentine so precisely like the Stromatoporæ of the Silurian rocks, that any collector would pounce upon them at once as fossils. In some places these small weathered specimens can be easily chipped off from the crumbling surface of the limestone; and it is perhaps to be regretted that they have not been more extensively shown to palæontologists, with the cut slices which to many of them are so problematical. One of the original specimens, brought from the Calumet, and now in the Museum of the Geological Survey of Canada, was of this kind, and much finer specimens from Côte St. Pierre are now in that collection and in my own. A very fine example is represented, on a reduced scale, in [Plate. III.], which is taken from an original photograph.[C] In some of the layers are found other and more minute fossils than Eozoon, and these, together with its fragmental remains, as ingredients in the limestone, will be discussed in the sequel. We may merely notice here that the most abundant layer of Eozoon at this place, occurs near the base of the great limestone band, and that the upper layers in so far as seen are less rich in it. Further, there is no necessary connection between Eozoon and the occurrence of serpentine, for there are many layers full of bands and lenticular masses of that mineral without any Eozoon except occasional fragments, while the fossil is sometimes partially mineralized with pyroxene, dolomite, or common limestone. The section in [fig. 5] will serve to show the attitude of the limestone at this place, while the more general section, [fig. 3], taken from Sir William Logan, shows its relation to the other Laurentian rocks, and the sketch in [fig. 6] shows its appearance as a feature on the surface of the country.
[C] By Mr. Weston, of the Geological Survey of Canada.
NOTES TO CHAPTER II.
(A.) Sir William E. Logan on the Laurentian System.
[Journal of Geological Society of London, February, 1865.]
After stating the division of the Laurentian series into the two great groups of the Upper and Lower Laurentian, Sir William goes on to say:—