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modern geologists arrange the earth's crust into the following formations, or, referring to their fossils, into the following life-periods and rock-systems :

Life-Periods. Rock-Systems.

CAiNOzoic=REcraT. Life. { S0^TM"* or Recent* Tertiary.

/ Cretaceous or Chalk.

Mesozoio=Middle Life. 1 Oolitic or Jurassic.

\ Triassic or Upper New Eed.

(Permian or Lower New Eed. Carboniferous or Coal System. j Old Red Sandstone and Devonian. [_ Silurian. _ _ (Cambrian.

Eozoic=dawn Life. { Laurentian.

This arrangement is, of course, temporary or provisional, and may be altered as geologists become more intimately acquainted with rocks of other lands; but in the mean time it expresses our knowledge of the succession that prevails among the stratified formations, and may be received as the great chronological stages of the world's history. The technical terms are founded partly on mineral, partly on geographical, and partly on fossil distinctions, and may with a little exertion become intelligible to the least scientific. Thus " Cretaceous" and " Old Eed Sandstone " refer to the most prevalent rocks in these systems; "Laurentian " and "Cambrian" to districts where the systems are largely or typically displayed; and " Cainozoic" and "Palaeozoic" to the comparative recentness and antiquity of the fossil remains. A uniform system of nomenclature might have been preferable, but human knowledge progresses by slow degrees, and its terms and technicalities must be viewed as mere provisional expedients towards this advancement. Under the existing nomenclature geology has made bold and rapid progress, and any attempt to revolutionise, unless through the gradual increase of wider knowledge, would tend to obstruct rather than facilitate. It is to the oldest of these so-called systems that we now direct attention; to the conditions under which they seem to have been deposited; and to the kind of life that appears to have peopled the lands and waters of their respective periods. We have classed them as the "Primary Periods," because there is really a greater similarity between their rocks and fossils than there is between the rocks and fossils of any subsequent periods; because, so far as we know, their strata are all truly marine deposits; and further, because their fossil forms belong (speaking in general terms) to invertebrate types—zoophytes, shell-fish, crustacea, &c.—and are specially characterised by the absence of the vertebrates— the fishes, reptiles, birds, and mammals.

Beginning with the Laurentian, which term has been employed by Sir William Logan of the Canadian Geological Survey to designate the highly crystalline strata which belong especially to the valley of the St Lawrence, and constitute the great bulk of the Laurentide mountains, we may take Sir William's own description of the rocks which compose this oldest and deepest of sedimentary formations. "The rocks of this system," he says, "are almost without exception ancient sedimentary strata which have become highly crystalline. They have been very much disturbed, and form ranges of hills having a direction nearly north-east and south-west, rising to the height of 2000 or 3000 feet, and even higher. The rocks of this formation are the most ancient known on the American continent, and correspond probably to the oldest gneiss of .Finland and Scandinavia, and to some similar rocks in the north of Scotland. They consist, in great part, of crystalline schists (chiefly gneissoid or hornblende), associated with felspars, quartzites, and limestones, and are largely broken up by granites, syenites, and diorites, which form important intrusive masses. Among the economic minerals of the formation, the ores of iron are the most important, and are generally found associated with limestones." Interpreting Sir William's technicalities, it may be stated for the comprehension of the general reader that this old Laurentian formation, which is of vast thickness (some 30,000 feet or thereby), consists essentially of hard and crystalline strata like the gneiss, mica-schists, quartz-rocks, and marbles of the Scottish Highlands, or more, perhaps, like the still harder and more granitic-looking schists of the Scandinavian mountains. There are no sandstones, or shales, or limestones in the proper sense of the term. All these have been converted, long ages ago, by heat, pressure, and chemical action, into sparkling crystalline rocks; lines and layers of stratification are obscure and often altogether obliterated; veins and eruptive masses are frequent; and altogether the whole formation wears the aspect of a vast and venerable antiquity.

That the Laurentian system, like other stratified systems, was deposited in the form of sands, gravels, clays, muds, and other loose sediments, is beyond all question. Nature has no other mode of procedure. What is wasted from the lands is transferred to the waters; nothing is lost. It may change its form or place, but it is still in existence; and this incessant round of waste and reconstruction, as shown in a former Sketch (No. 2), is the ordained order of the universe. What a wonderful metamorphism these primeval sediments have undergone! Not mud, nor sand, nor gravel; not shale, nor sandstone, nor conglomerate; but glistening slates and crystalline schists — sandstones converted into flinty quartz-rocks, and limestones into variegated serpentines. And in the midst of all this metamorphism, the fossil organisms seem to have shared the same fate; for we cannot think of waters of deposition without associating with them some forms of life, however lowly. And yet, till within the last seven years, the search for traces of life in these primeval rocks was considered visionary, and azoic and hypozoic, or "lifeless " and "under-all-life," were the technicalities by which they were known. Nil desperandum, however, should be the geologist's motto, and especially of those who believe as we do that Life on this globe was coeval with the stratified rocks, and that the conditions which permitted the deposition of ordinary sediments must have been favourable at the same time to the manifestation of some form or other of vitality. And so it happens that traces of lowly organisation have recently been detected in these old Laurentian rocks—in the serpentinous limestones of Canada—thus holding out the hope that the primeval rocks of other regions will yet yield similar traces, and prove that the earliest waters were tenanted by their own forms of life, and were gladdened by the manifestations of sentient existence.

Indeed similar organisms have already been detected in the old (and probably contemporaneous) serpentines of Ireland and marbles of Bohemia; and as one form of life generally indicates the existence of another upon which it preyed or was in turn preyed upon, we may shortly expect important additions to this discovery. Since this was written in 1866, tracks, burrows, coral-like masses, and other forms of protozoa than those originally discovered by Sir W. Logan, have been found in a somewhat higher zone, and described by Principal Dawson of Montreal. The organism discovered, Eozoon Canadense, or "Dawn-animal of Canada," belongs to the lowest forms of life—lower than the infusory animalcules, and even still lower than the sponges. It is one of the Foraminifera, mere animated specks, which have nevertheless the power of secreting lime from the waters, and enveloping themselves in elegant and variously-formed cases. These calcareous cases are mere microscopic points, and yet they are perforated by numerous pores {foramina, hence the name of the order), through which the creature procures its food and holds intercourse with the outer world of waters. Individually minute, they live in colonies, and only become conspicuous by their aggregations, which in the instance of the eozoon vary from a few inches to a foot or two in diameter. The white calcareous mud which covers so much of the Atlantic sea-bed is a similar foraminiferal accumulation; so is the nummulitic limestone which stretches eastward in a great zone through Europe and Asia; so also is a large proportion of the chalkhills of England; and so backward in time through other limestones, till we reach the oldest and earliest Laurentian marble. With a little manipulation, the organisms constituting the existing sea-muds or the chalk are readily revealed; but the old eozoon has to be polished, cut into microscopic slices, and treated with acids before the peculiarities of its structure can be rendered intelligible. So great is the change produced by the mineralisation of ages.* Strange that the minutest of organisms should be capable of piling up such stupendous rock-masses; strange and suggestive

* We are aware some geologists have called in question the organic nature of this Canadian eozolin—regarding it merely as a peculiar mineral structure mimetic of the organic, examples of such simulative structures being well known in other formations. The majority of competent observers, however, maintain its organic nature; and from a conversation we had, in the autumn of 1865, with Principal Dawson of Montreal, who has examined the rocks in, situ, with their unobliterated lines of deposition and layers of organic growth (brought out more clearly by weathering), we share the conviction that the EozoHn Canadense, whatever be its zoological affinities, is of animal and not of mineral aggregation. To the practised eye external appearances are often conclusive of organic structure, and these, in the present case, had been observed and accepted before the microscope was called into court to complete the evidence. Those interested in this matter may refer to the papers by Dr Carpenter, Professor King, and others, in the Journals of the Geological Society for 1865 and 1866.

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