river along whose valley they have been studied, while the upper is denominated the "Huronian," from the lake upon whose northern shores the upper members of the series are so finely exposed. The iron-bearing rocks of the northern peninsula of Michigan belong to the Huronian system, as well as those of Southeastern Missouri and Northern New York. CHAPTER VI. A RAY OF LIFE. URING the progress of that primeval age which witnessed the war of elements that I have already sketched, there was little opportunity for the unfolding of organic existence. The atmosphere was unfit for respiration; and the waters, if not too highly heated, were nevertheless charged with impurities destructive to both vegetable and animal life. It was a dreary and monotonous age, with nothing of that which now beautifies and diversifies the face of nature. The same sunlight fell upon the heaving waters of that tenantless and gloomy sea, and the same tide-wave performed its everlasting circuit round the globe. There was little diversity of weather or climate. The continents and mountain ridges, which give birth to oceanic and atmospheric currents, had not yet appeared above the wave. But there must have been a succession of seasons. The winter's sun, as now, went early to his couch, and his tardy rising belated the December mornings. His unequal favors to the different latitudes necessitated the trade winds and the great equalizing currents of the ocean. The higher density of the primeval atmosphere rendered it more retentive of the solar heat, and thus contributed greatly to diminish the rate of terrestrial cooling by radiation into space. Evaporation proceeded at a rapid rate, and condensation and precipitation were correspondingly copious. It was probably a stormy period, like the showery season which succeeds the protracted storm of the vernal equinox. It would seem almost inevitable that the temperature and constitution of the primeval sea should be incompatible equally with vegetable and animal life. It is true that both plants and animals are now known to flourish under conditions of heat and cold, and chemistry, which are entirely at variance with the general notions of organic adaptability. Certain plants, for instance, are reported as flourishing in the boiling geysers of Iceland and the hot springs of California. Others make their habitat upon the snows of Greenland, and impart the ruddy glow of warmth even in the undisputed empire of frost. The germs of vegetable, and even of animal life, populate every element and every locality; and only a temperature of some hundreds of degrees suffices to rid a fluid exposed to the air of all the vitalized germs that inhabit it. The egg of an insect, stuck in the crevice of the bark of an apple-tree, endures the rigors of a Canadian winter; and the organized chrysalis seems, in many cases, to possess equal powers of resisting cold. It is unsafe, then, to attempt to determine at what epoch the waters of the primeval sea became sufficiently cooled and purified to receive the first organic forms. There was, in all probability, an earliest epoch that was completely destitute of organic forms. But, to ascertain its beginning and its end, Geology must yet apply herself to a closer study of the monuments of the gneissic age. Reasoning deductively, it is equally presumable that vegetable life preceded animal life in order of appearance. Vegetable life is capable of enduring more extreme conditions. Vegetation could better tolerate the excess of carbonic acid in the atmosphere and the waters. Vegetation, moreover, is capable of drawing its sustenance from the mineral world, while animals rely exclusively upon organic food. The vegetable stands between the animal and the mineral, performing a sort of commissary function in behalf of the animal. The animal—even the carnivorous animal-implies the vegetable-requires the vegetable. All things considered, we are led to believe that plant life had a history upon our earth a full epoch before the existence of the lowest animals. There must have been a real AZOIC AGE. This deductive conclusion receives some support from inductive data. Petroleum, when existing in a state of wide or general distribution through a formation, is found to be traceable to vegetable organisms, generally marine plants, that have been reduced to a pulp and mingled with argillaceous mud before deposition. Petroleum is thus found in every formation, from the very latest down to the primeval gneiss. The actual presence of petroleum in gneissic strata affords a material prop to the doctrine of præzoic vegetation-a doctrine of no inconsiderable importance in establishing the harmony of the Mosaic and geologic records. But a few months since geologists were equally ignorant of the existence of vegetable and animal remains through the entire series of Laurentian and Huronian strata, unless, perchance, the so-called "Cambrian" rocks of the Old World be of the same age as the Huronian- -a conclusion which the eminent geologist, Dr. Bigsby, disinclines to accept. Geologists, it is true, drew the same inferences as now from the same data in reference to the existence of vegetal organization; but no actual or recognizable remains had been found, nor have they to this day. Greatly to the astonishment of the whole geological world, however, the abundant remains of animals have been discovered in strata which long antedate the most ancient in which a vegetable form has been descried. It was not by any means a rich fauna, but a single species, which populated the sea even in the Laurentian period. The faint tracery |