Part 7

THE FOUNDATIONS OF THE CONTINENTS, AND THEIR GENERAL TESTIMONY AS TO LIFE

That the reader may be enabled better to understand the relation of the old foundations or pillars of the earth to the beginning of life, and the preservation of the remains of the earliest animals, it may be well to reverse the method we have hitherto followed, and to present a theoretical or ideal historical sketch of the early history of the earth, beginning with that stage in which it may be supposed to have been a liquid mass, considerably larger than it is at present, and intensely heated, and surrounded by a vast vaporous envelope composed of all the substances capable of being resolved by its heat into a gaseous condition--a smooth and shining spheroid, invested with an enormous atmosphere.

In such a condition its denser materials, such as the heavier metals, would settle toward the centre, and the surface would consist of lighter material composed of the less dense and more oxidizable substances combined with oxygen, and similar in character and appearance to the slag which forms on the surface of some ores in the process of smelting. Of this slaggy material there might, however, be different layers more or less dense in proceeding from the interior to the surface. This molten surface would, of course, radiate heat into space; and as it would naturally consist of the least fusible matters, these would begin to form a solid crust. We may imagine this crust at first to be smooth and unbroken, though such a condition could scarcely exist for any length of time, as the hardened crust would certainly be disturbed by ascending currents from within, and by tidal movements without. Still, it might remain for ages as a spheroidal crust, presenting little difference of elevation or depression in comparison with its extent. When it became sufficiently thick and cool to allow water to lie on its surface, new changes would begin. The water so condensed would be charged with acid substances which would begin to corrode the rocky surface. Penetrating into crevices and flashing into steam as it reached the heated interior, it would blow up masses and fragments of stone, and would perhaps force out and cause to flow over the surface beds of molten material from below the crust, and differing somewhat from it in their composition. All this aqueous work would accelerate the cooling and thickening of the crust, and at length a universal or almost universal heated ocean would envelope the globe, and so far as its surface was concerned, the reign of water would replace that of fire. We may pause here to consider the probable nature of the earth's crust in this condition.

The substance most likely to predominate would be silica or quartz, one of the lighter and most infusible materials of the crust; but which, heated in contact with alumina, lime, potash, and other earths and alkalis, forms fusible slags, enamels and glasses. One of these, composed of silica, alumina, and potash, or soda, was long ago named by the German miners felspar, a name which it still retains, though now several distinct kinds of it are distinguished by different names. Another is a compound of silica with magnesia and lime, forming the mineral known as Amphibole or Hornblende, and by several other names, according to its colour and crystalline form. In many deep-seated rocks these minerals are formed together, and having crystallized out separately give a spotted and granular character to the mass. Naturally colourless, all these minerals, and especially the felspar and hornblende, are liable to be coloured with different oxides of iron, the felspar usually taking a reddish, and the hornblende a greenish or blackish hue. Now, if we examine a fragment of the oldest or fundamental gneiss or granite, we shall see glassy grains of quartz, reddish or white flat-surfaced crystals of felspar, and dark-coloured prisms of hornblende. When destitute of any arrangement in layers, the rock is granite; when arranged more or less in flakes or laminæ, it is gneiss, the structure of which may arise either from its having been formed in successive beds, or from its having been flattened or drawn out by pressure. These structures can be seen more or less distinctly in any ordinary coarse-grained granite, or with the lens or microscope in finer varieties.

The Lower Laurentian rocks of our section consist essentially of the materials above described, with a vast variety in the proportions and arrangements of the constituent minerals. There is, there-fore, nothing to prevent us from supposing that these rocks are really remains of the lower portions of the original crust which first formed on the surface of our cooling planet, though the details of their consolidation and the possible interactions of heat and heated water may admit of much discussion and difference of opinion.

But after the formation of a crust and its covering in whole or in part with heated water, other changes must occur, in order to fit the earth for the abode of life. These proceeded from the tensions set up by the contraction and expansion of the interior heated nucleus and the solid crust--a complicated and difficult question, when we consider its laws and their mode of operation, but which resulted in the folding and fracturing of the crust along long lines which are parts of great circles of the earth, running in N.E. and S.W. and N.W. and S.E. directions; and these ridges, which in the earliest Archæan period must have attained to great height and very rugged outlines, formed the first rudiments of our mountain chains and continents. Those constituting the Laurentian nucleus of North America--a very simply outlined continent--form a case in point (Fig. 18).

The elevation of these mountain ridges forced the waters to recede into the lower levels. As the old psalm of creation has it,--

"The mountains ascend, the valleys descend into the place Thou hast founded for them,"

and so sea-basins and land were produced.

Milton merely paraphrases this when he says,--

"The mountains huge appear Emergent, and their broad, bare backs upheave Into the clouds; their tops ascend the sky. So high as heaved the tumid hills, so low Down sunk a hollow bottom wide and deep. Capacious bed of waters."

Englishmen have been accused of taking their ideas of creation from Milton rather than from nature or the Bible. Milton had not the guidance of modern geology. His cosmology is entirely that of a close student of the Biblical narrative of creation. He is in many respects the best commentator on the early chapters of Genesis, because he had a very clear conception of the mind of the writer, and the power of expressing the ideas he derived from the old record. For the same reason he is the greatest bard of creation and primitive man, and surprisingly accurate and true to nature.

Showing the protaxis or nucleus of the continent.]

Then began the great processes of denudation and sedimentation to which we owe the succeeding rock formations. The rains descended on the mountain steeps, and washed the decaying rocks as sand, gravel and mud into the rivers and the sea. The sea itself raged against the coasts, and cut deeply into their softer parts; and all the detritus thus produced by atmospheric and marine denudation was spread out by the tides and currents in the bed of the ocean, and its gulfs and seas, forming the first aqueous deposits, while the original land must have been correspondingly reduced.

The sea might still be warm, and it held in solution or suspension somewhat different substances from those now present in it, and the land was at first a mere chaos of rocky crags and pinnacles. But so soon as the temperature of the waters fell somewhat below the boiling point, and as even a little soil formed in the valleys and hollows of the land, there was scope for life, provided that its germs could be introduced.

On a small scale there was something of this same kind in the sea and land of Java, after the great eruption of Krakatoa, in 1883. The bare and arid mountain left after the eruption, began, in the course of a year, to be occupied by low forms of vegetable life, gradually followed by others, and verdure was soon restored. The once thickly peopled sea-bottom, so prolific of life in these warm seas, but buried under many feet of volcanic ashes and stones, soon began to be re-peopled, and is now probably as populous as before. But in this case there were plenty of spores of lichens, mosses, and other humble plants to be wafted to the desolate cone, and multitudes of eggs and free-swimming germs of hundreds of kinds of marine animals to re-people the sea-bottom. Whence were such things to come from to occupy the old Archæan hills and sea-basins? and all our knowledge of nature gives us no answer to the question, except that a creative power must have intervened; but in what manner we know not. That this actually occurred, we can, however, be assured by the next succeeding geological formation. We have seen that the granitic and gneissic ridges could furnish pebbles, sand, and clay, and these once deposited in the sea-bottom could be hardened into conglomerate, sandstone and slate. But beside these we have in the next succeeding or Upper Laurentian formation rocks of a very different character. We have great beds of limestone and iron ore, and deposits, of carbon or coaly matter, now in the peculiar state of graphite or plumbago, and it is necessary for us to inquire how these could originate independently of life. In modern seas limestone is forming in coral reefs, in shell beds, and in oceanic chalky ooze composed of minute microscopic shells; but only in rare and exceptional instances is it formed in any other way; and when we interrogate the old limestones and marbles which form parts of the land, they give us evidence that they also are made up of calcareous skeletons of marine animals or fragments of these.

Scale of Map 7 miles to one inch. See also Dr. Bonney's paper, Geol. Mag., July, 1895.

Dotted area: Limestone. Horizontal lines: Upper gneiss (fourth gneiss of Logan). Vertical lines: Lower gneiss (third gneiss of Logan). Diagonal lines: Overlying Cambrian and Cambro-Silurian (Ordovician). (See also Fig. 19A.)

Now when we find in the Grenvillian series, the first oceanic group of beds known to us, great and widely extended limestones, thousands of feet in thickness, and rivalling in magnitude those of any succeeding period, we naturally infer that marine life was at work. No doubt the primitive sea contained more lime and magnesia than the present ocean holds in solution; but while this might locally favour the accumulation of inorganic limestones, it cannot account for so great and extensive deposits. On the other hand, a sea rich in lime would have afforded the greatest facilities for the growth of those marine plants which accumulate lime, and through these for the nutrition of animals forming calcareous shells or corals. Thus we have presumptive evidence that there must have been in the Upper Laurentian sea something corresponding to our coral reefs and shell-beds, whatever this something may have been.

These limestones, however, demand more particular notice (Fig. 19).

One of the beds measured by the officers of the Geological Survey is stated to be 1,500 feet in thickness, another is 1,250 feet thick, and a third 750 feet; making an aggregate of 3,500 feet. These beds may be traced, with more or less interruption, for hundreds of miles. Whatever the origin of such limestones, it is plain that they indicate causes equal in extent, and comparable in power and duration, with those which have produced the greatest limestones of the later geological periods. Now, in later formations, limestone is usually an organic rock, accumulated by the slow gathering from the sea-water, or its plants, of calcareous matter, by corals, foraminifera, or shell-fish, and the deposition of their skeletons, either entire or in fragments on the sea-bottom. The most friable chalk and the most crystalline limestones have alike been formed in this way. We know of no reason why it should be different in the Laurentian period. When, therefore, we find great and conformable beds of limestone, such as those described by Sir William Logan in the Laurentian of Canada, we naturally imagine a quiet sea-bottom, in which multitudes of animals of humble organization were accumulating limestone in their hard parts, and depositing this in gradually increasing thickness from age to age. Any attempts to account otherwise for these thick and greatly extended beds, regularly interstratified with other deposits, have so far been failures, and have arisen either from a want of comprehension of the nature and magnitude of the appearances to be explained, or from the error of mistaking the true bedded limestones for veins of calcareous spar.

(a) Gneiss band in the Limestone, (b) Limestone with Eozoon. (c) Diorite and Gneiss.]

Again, in the original molten world, it seems likely that most of the carbon present--at least, at the surface--was in the atmosphere in the gaseous form of carbon dioxide. This might be dissolved by the rain and other waters; but we know in the modern world no agency which can decompose this compound and reduce it to ordinary carbon or coal, except that of living plants, which are always carrying on this function to an enormous extent. We know that all our great beds of coal and peaty matter are composed of the remains of plants which took their carbon from the air and the waters in past times. We also know that this coaly vegetable matter may, under the influence of heat and pressure, when buried in the earth, be converted into anthracite and into graphite, and even into diamond. It is true that an eminent French chemist has shown that graphite and hydrocarbons may be produced from some of the metallic compounds of carbon which may have been formed under intense heat in the interior of the earth, by the subsequent action of water on such compounds; but there is nothing to show that this can have occurred naturally, unless in very exceptional cases. Now in the Grenvillian system in Canada there is not only a vast quantity of carbon diffused through the limestones, and filling fissures in other rocks, into which it seems to have been originally introduced as liquid bitumen, but also in definite beds associated with earthy matter, and sometimes ten to twelve feet thick. The occurrence of this large amount of carbon warrants us in supposing that it represents a vast vegetable growth, either on the land or in the sea, or both.

In like manner, in later geological periods, beds of iron ore are generally accumulated as a consequence of the solvent action of acids produced by vegetable decay, as in the clay ironstones of the coal formation and the bog iron ores of later times. Thus the beds of magnetic iron occurring in the Upper Laurentian may be taken as evidences, not of vegetable accumulation, but of vegetable decay.

May not also the great quantity of calcium phosphate mined in the Grenville series in Canada, indicate, as similar accumulations do in later formations, the presence of organisms having skeletons of bone earth?

With reference to the carbon and iron ore of the Grenville series, I may quote the following from a paper published in the Journal of the Geological Society of London in 1870:--

"The quantity of graphite in the Upper Laurentian series is enormous. In a recent visit to the township of Buckingham, on the Ottawa River, I examined a band of limestone believed to be a continuation of that described by Sir W. E. Logan as the Green Lake Limestone. It was estimated to amount, with some thin interstratified bands of gneiss, to a thickness of 600 feet or more, and was found to be filled with disseminated crystals of graphite and veins of the mineral to such an extent as to constitute in some places one-fourth of the whole; and making every allowance for the poorer portions, this band cannot contain in all a less vertical thickness of pure graphite than from twenty to thirty feet. In the adjoining township of Lochaber Sir W. E. Logan notices a band from twenty-five to thirty feet thick, reticulated with graphite veins to such an extent as to be mined with profit for the mineral. At another place in the same district a bed of graphite from ten to twelve feet thick, and yielding twenty per cent, of the pure material, is worked. When it is considered that graphite occurs in similar abundance at several other horizons, in beds of limestone which have been ascertained by Sir W. E. Logan to have an aggregate thickness of 3,500 feet, it is scarcely an exaggeration to maintain that the quantity of carbon in the Laurentian is equal to that in similar areas of the Carboniferous system. It is also to be observed that an immense area in Canada appears to be occupied by these graphitic and Eozoon limestones, and that rich graphitic deposits exist in the continuation of this system in the State of New York; while in rocks believed to be of this age near St. John, New Brunswick, there is a very thick bed of graphitic limestone, and associated with it three regular beds of graphite, having an aggregate thickness of about five feet.

"It may fairly be assumed that in the present world, and in those geological periods with whose organic remains we are more familiar than with those of the Laurentian, there is no other source of unoxidized carbon in rocks than that furnished by organic matter, and that this has obtained its carbon in all cases, in the first instance, from the deoxidation of carbonic acid by living plants. No other source of carbon can, I believe, be imagined in the Laurentian period. We may, however, suppose either that the graphitic matter of the Laurentian has been accumulated in beds like those of coal, or that it has consisted of diffused bituminous matter similar to that in more modern bituminous shales and bituminous and oil-bearing limestones. The beds of graphite near St. John, some of those in the gneiss at Ticonderoga in New York, and at Lochaber and Buckingham and elsewhere in Canada, are so pure and regular that one might fairly compare them with the graphitic coal of Rhode Island. These instances, however, are exceptional, and the greater part of the disseminated and vein graphite might rather be compared in its mode of occurrence to the bituminous matter in bituminous shales and limestones.