I regard these observations as of the utmost importance in reference to the relations of dolomite with fossiliferous limestones, and especially with those of the Grenville series. The waters of the Laurentian ocean must have been much richer in salts of magnesium than those of the present seas, and the temperature was probably higher, so that chemical changes now proceeding in limited lagoons might have occurred over much larger areas. If at that time there were, as in later periods, calcareous organisms composed of aragonite, these may have been destroyed by conversion into dolomite, while others more resisting were preserved, just as a modern Polytrema or Balanus might remain, when a coral to which it might be attached would be dolomitized, or might even be removed altogether by sea-water containing carbonic acid. There is reason to believe that this last change sometimes takes place in the deeper parts of the ocean at present. This would account for the persistence of Eozoon and its fragments, when other organisms may have perished, and also for the frequent filling of the canals and tubuli with the magnesian carbonate.
The main point here, however, for our present purpose is that, when a calcareous shell or skeleton has been thus infiltrated with a silicate, it becomes imperishable, so that any amount of alteration of the containing limestone short of its absolute fusion would not suffice to destroy an organism once injected with silicious matter. Thus the occasional persistence of silicified fossils in highly metamorphosed limestones is in no respect contradictory to the general fact, that when not preserved by silicious infiltration, they have perished, and this more especially in the case of those whose skeletons are composed of aragonite.
Carrying these facts with us, the next question is, What manner of fossil remains should we expect to find in the Upper Laurentian rocks, supposing that any such are therein preserved? The answer to this question follows at once from the facts as to the succession of life noticed above. Only the marine invertebrates have been traced as far back as the oldest Cambrian, and only Worms, Sponges, and Protozoa into the Huronian. We should therefore have no expectation of finding remains of any vertebrate animals or of any of the land invertebrates; and even allowing for the more favourable conditions, as compared with the Huronian, evidenced by the great limestones and the abundant carbon, we could scarcely expect anything higher than some of the lower types of invertebrate life, such as Worms, Hydroids, Corals and Protozoa. We have next to inquire what forms, possibly organic, have actually been found, and what information we can derive from them as to the beginnings of life. Since, however, such discoveries as have been made have been the result of much labour and scientific skill brought to bear on these old rocks, and are connected with the reputations of several eminent men, now deceased, we may first refer shortly to the history of the discovery of supposed fossils in the Laurentian rocks of Canada.
Fig. 25.—Nature-print of an etched Specimen of Eozoon.
Showing the laminæ, a part of the natural margin, near which passes a diagonal calcite vein, and at the upper right-hand corner, fragmental material with casts of Archæospherinæ. The dark lines represent the chambers filled with serpentine, the white the calcite wall.
THE HISTORY OF A DISCOVERY
VI
THE HISTORY OF A DISCOVERY
W
WHEN Mr. Logan, afterwards Sir William Logan, entered on the Geological Survey of Canada, in 1840, he found that vast and little-explored regions in the northern part of that country were occupied with gneissic rocks, similar to the oldest gneisses of Scotland and Scandinavia, and to which the name Azoic had been given by Murchison, as rocks destitute of fossils, while they had been the "fundamental granite" or ur-gneiss of most European geologists. They were unquestionably below and more ancient than the oldest fossiliferous Cambrian rocks both in Europe and North America, and geologists had for the most part contented themselves with regarding them as primitive rocks, destitute of any geological interest, much as some United States geologists of the present day call them the "Archæan complex," a name which the late Prof Dana has well characterized as a "term of despair."