The Ancient Life History of the Earth Part 4

PART II

HISTORICAL PALaeONTOLOGY

CHAPTER VII.

THE LAURENTIAN AND HURONIAN PERIODS.

The _Laurentian Rocks_ const.i.tute the base of the entire stratified series, and are, therefore, the oldest sediments of which we have as yet any knowledge. They are more largely and more typically developed in North America, and especially in Canada, than in any known part of the world, and they derive their t.i.tle from the range of hills which the old French geographers named the "Laurentides." These hills are composed of Laurentian Rocks, and form the watershed between the valley of the St Lawrence river on the one hand, and the great plains which stretch northwards to Hudson Bay on the other hand. The main area of these ancient deposits forms a great belt of rugged and undulating country, which extends from Labrador westwards to Lake Superior, and then bends northwards towards the Arctic Sea. Throughout this extensive area the Laurentian Rocks for the most part present themselves in the form of low, rounded, ice-worn hills, which, if generally wanting in actual sublimity, have a certain geological grandeur from the fact that they "have endured the battles and the storms of time longer than any other mountains" (Dawson). In some places, however, the Laurentian Rocks produce scenery of the most magnificent character, as in the great gorge cut through them by the river Saguenay, where they rise at times into vertical precipices 1500 feet in height. In the famous group of the Adirondack mountains, also, in the state of New York, they form elevations no less than 6000 feet above the level of the sea. As a general rule, the character of the Laurentian region is that of a rugged, rocky, rolling country, often densely timbered, but rarely well fitted for agriculture, and chiefly attractive to the hunter and the miner.

As regards its mineral characters, the Laurentian series is composed throughout of metamorphic and highly crystalline rocks, which are in a high degree crumpled, folded, and faulted. By the late Sir William Logan the entire series was divided into two great groups, the _Lower Laurentian_ and the _Upper Laurentian_, of which the latter rests unconformably upon the truncated edges of the former, and is in turn unconformably overlaid by strata of Huronian and Cambrian age (fig. 20).

[Ill.u.s.tration: Fig. 20.--Diagrammatic section of the Laurentian Rocks in Lower Canada. a Lower Laurentian; b Upper Laurentian, resting unconformably upon the lower series; c Cambrian strata (Potsdam Sandstone), resting unconformably on the Upper Laurentian.]

The _Lower Laurentian_ series attains the enormous thickness of over 20,000 feet, and is composed mainly of great beds of gneiss, altered sandstones (quartzites), mica-schist, hornblende-schist, magnetic iron-ore, and haemat.i.te, together with ma.s.ses of limestone.

The limestones are especially interesting, and have an extraordinary development--three princ.i.p.al beds being known, of which one is not less than 1500 feet thick; the collective thickness of the whole being about 3500 feet.

The _Upper Laurentian_ series, as before said, reposes unconformably upon the Lower Laurentian, and attains a thickness of at least 10,000 feet. Like the preceding, it is wholly metamorphic, and is composed partly of ma.s.ses of gneiss and quartzite; but it is especially distinguished by the possession of great beds of felspathic rock, consisting princ.i.p.ally of "Labrador felspar."

Though typically developed in the great Canadian area already spoken of, the Laurentian Rocks occur in other localities, both in America and in the Old World. In Britain, the so-called "fundamental gneiss" of the Hebrides and of Sutherlands.h.i.+re is probably of Lower Laurentian age, and the "hypersthene rocks"

of the Isle of Skye may, with great probability, be regarded as referable to the Upper Laurentian. In other localities in Great Britain (as in St David's, South Wales; the Malvern Hills; and the North of Ireland) occur ancient metamorphic deposits which also are probably referable to the Laurentian series. The so-called "primitive gneiss" of Norway appears to belong to the Laurentian, and the ancient metamorphic rocks of Bohemia and Bavaria may be regarded as being approximately of the same age.

[Ill.u.s.tration: Fig. 21.--Section of Lower Laurentian Limestone from Hull, Ottawa; enlarged five diameters. The rock is very highly crystalline, and contains mica and other minerals. The irregular black ma.s.ses in it are graphite. (Original.)]

By some geological writers the ancient and highly metamorphosed sediments of the Laurentian and the succeeding Huronian series have been spoken of as the "Azoic rocks" (Gr. _a_, without; _zoe_, life); but even if we were wholly dest.i.tute of any evidence of life during these periods, this name would be objectionable upon theoretical grounds. If a general name be needed, that of "Eozoic"

(Gr. _eos_, dawn; _zoe_, life), proposed by Princ.i.p.al Dawson, is the most appropriate. Owing to their metamorphic condition, geologists long despaired of ever detecting any traces of life in the vast pile of strata which const.i.tute the Laurentian System. Even before any direct traces were discovered, it was, however, pointed out that there were good reasons for believing that the Laurentian seas had been tenanted by an abundance of living beings. These reasons are briefly as follows:--(1) Firstly, the Laurentian series consists, beyond question, of marine sediments which originally differed in no essential respect from those which were subsequently laid down in the Cambrian or Silurian periods. (2) In all formations later than the Laurentian, any limestones which are present can be shown, with few exceptions, to be _organic_ rocks, and to be more or less largely made up of the comminuted debris of marine or fresh-water animals. The Laurentian limestones, in consequence of the metamorphism to which they have been subjected, are so highly crystalline (fig. 21) that the microscope fails to detect any organic structure in the rock, and no fossils beyond those which will be spoken of immediately have as yet been discovered in them. We know, however, of numerous cases in which limestones, of later age, and undoubtedly organic to begin with, have been rendered so intensely crystalline by metamorphic action that all traces of organic structure have been obliterated. We have therefore, by a.n.a.logy, the strongest possible ground for believing that the vast beds of Laurentian limestone have been originally organic in their origin, and primitively composed, in the main, of the calcareous skeletons of marine animals. It would, in fact, be a matter of great difficulty to account for the formation of these great calcareous ma.s.ses on any other hypothesis. (3) The occurrence of phosphate of lime in the Laurentian Rocks in great abundance, and sometimes in the form of irregular beds, may very possibly be connected with the former existence in the strata of the remains of marine animals of whose skeleton this mineral is a const.i.tuent. (4) The Laurentian Rocks contain a vast amount of carbon in the form of black-lead or _graphite_.

This mineral is especially abundant in the limestones, occurring in regular beds, in veins or strings, or disseminated through the body of the limestone in the shape of crystals, scales, or irregular ma.s.ses. The amount of graphite in some parts of the Lower Laurentian is so great that it has been calculated as equal to the quant.i.ty of carbon present in an equal thickness of the Coal-measures. The general source of solid carbon in the crust of the earth is, however, plant-life; and it seems impossible to account for the Laurentian graphite, except upon the supposition that it is metamorphosed vegetable matter. (5) Lastly, the great beds of iron-ore (peroxide and magnetic oxide) which occur in the Laurentian series interstratified with the other rocks, point with great probability to the action of vegetable life; since similar deposits in later formations can commonly be shown to have been formed by the deoxidising power of vegetable matter in a state of decay.

In the words of Princ.i.p.al Dawson, "anyone of these reasons might, in itself, be held insufficient to prove so great and, at first sight, unlikely a conclusion as that of the existence of abundant animal and vegetable life in the Laurentian; but the concurrence of the whole in a series of deposits unquestionably marine, forms a chain of evidence so powerful that it might command belief even if no fragment of any organic or living form or structure had ever been recognised in these ancient rocks." Of late years, however, there have been discovered in the Laurentian Rocks certain bodies which are believed to be truly the remains of animals, and of which by far the most important is the structure known under the now celebrated name of _Eozoon_. If truly organic, a very special and exceptional interest attaches itself to _Eozoon_, as being the most ancient fossil animal of which we have any knowledge; but there are some who regard it really a peculiar form of mineral structure, and a severe, protracted, and still unfinished controversy has been carried on as to its nature. Into this controversy it is wholly unnecessary to enter here; and it will be sufficient to briefly explain the structure of _Eozoon_, as elucidated by the elaborate and masterly investigations of Carpenter and Dawson, from the standpoint that it is a genuine organism--the balance of evidence up to this moment inclining decisively to this view.

[Ill.u.s.tration: Fig. 22.--Fragment of _Eozoon_, of the natural size, showing alternate laminae of loganite and dolomite. (After Dawson.)]

The structure known as _Eozoon_ is found in various localities in the Lower Laurentian limestones of Canada, in the form of isolated ma.s.ses or spreading layers, which are composed of thin alternating laminae, arranged more or less concentrically (fig.

22). The laminae of these ma.s.ses are usually of different colours and composition; one series being white, and composed of carbonate of lime--whilst the laminae of the second series alternate with the preceding, are green in colour, and are found by chemical a.n.a.lysis to consist of some silicate, generally serpentine or the closely-related "loganite." In some instances, however, all the laminae are calcareous, the concentric arrangement still remaining visible in consequence of the fact that the laminae are composed alternately of lighter and darker coloured limestone.

When first discovered, the ma.s.ses of _Eozoon_ were supposed to be of a mineral nature; but their striking general resemblance to the undoubted fossils which will be subsequently spoken of under the name of _Stromatopora_ was recognised by Sir William Logan, and specimens were submitted for minute examination, first to Princ.i.p.al Dawson, and subsequently to Dr W. B. Carpenter.

After a careful microscopic examination, these two distinguished observers came to the conclusion that _Eozoon_ was truly organic, and in this opinion they were afterwards corroborated by other high authorities (Mr W. K. Parker, Professor Rupert Jones, Mr H.

B. Brady, Professor Gumbel, &c.) Stated briefly, the structure of _Eozoon_, as exhibited by the microscope, is as follows:--

[Ill.u.s.tration: Fig. 23.--Diagram of a portion of _Eozoon_ cut vertically. A, B, C, Three tiers of chambers communicating with one another by slightly constricted apertures: _a a_, The true sh.e.l.l-wall, perforated by numerous delicate tubes; _b b_. The main calcareous skeleton ("intermediate skeleton"); c, Pa.s.sage of communication ("stolon-pa.s.sage") from one tier of chambers to another; d, Ramifying tubes in the calcareous skeleton.

(After Carpenter.)]

The concentrically-laminated ma.s.s of _Eozoon_ is composed of numerous calcareous layers, representing the original skeleton of the organism (fig. 23, b). These calcareous layers serve to separate and define a series of chambers arranged in successive tiers, one above the other (fig. 23, A, B, C); and they are perforated not only by pa.s.sages (fig. 23, c), which serve to place successive tiers of chambers in communication, but also by a system of delicate branching ca.n.a.ls (fig. 23, d). Moreover, the central and princ.i.p.al portion of each calcareous layer, with the ramified ca.n.a.l-system just spoken of, is bounded both above and below by a thin lamina which has a structure of its own, and which may be regarded as the proper sh.e.l.l-wall (fig. 23, a a).

This proper wall forms the actual lining of the chambers, as well as the outer surface of the whole ma.s.s; and it is perforated with numerous fine vertical tubes (fig. 24, a a), opening into the chambers and on to the surface by corresponding fine pores. From the resemblance of this tubulated layer to similar structures in the sh.e.l.l of the Nummulite, it is often spoken of as the "Nummuline layer." The chambers are sometimes piled up one above the other in an irregular manner; but they are more commonly arranged in regular tiers, the separate chambers being marked off from one another by projections of the wall in the form of part.i.tions, which are so far imperfect as to allow of a free communication between contiguous chambers. In the original condition of the organism, all these chambers, of course, must have been filled with living-matter; but they are found in the present state of the fossil to be generally filled with some silicate, such as serpentine, which not only fills the actual chambers, but has also penetrated the minute tubes of the proper wall and the branching ca.n.a.ls of the intermediate skeleton. In some cases the chambers are simply filled with crystalline carbonate of lime. When the originally porous fossil has been permeated by a silicate, it is possible to dissolve away the whole of the calcareous skeleton by means of acids, leaving an accurate and beautiful cast of the chambers and the tubes connected with them in the insoluble silicate.

[Ill.u.s.tration: Fig. 24.--Portion of one of the calcareous layers of _Eozoon_, magnified 100 diameters. a a, The proper wall ("Nummuline layer") of one of the chambers, showing the fine vertical tubuli with which it is penetrated, and which are slightly bent along the line a' a'. c c, The intermediate skeleton, with numerous branched ca.n.a.ls. The oblique lines are the cleavage planes of the carbonate of lime, extending across both the intermediate skeleton and the proper wall. (After Carpenter.)]

The above are the actual appearances presented by _Eozoon_ when examined microscopically, and it remains to see how far they enable us to decide upon its true position in the animal kingdom.

Those who wish to study this interesting subject in detail must consult the admirable memoirs by Dr W. B. Carpenter and Princ.i.p.al Dawson: it will be enough here to indicate the results which have been arrived at. The only animals at the present day which possess a continuous calcareous skeleton, perforated by pores and penetrated by ca.n.a.ls, are certain organisms belonging to the group of the _Foraminifera_. We have had occasion before to speak of these animals, and as they are not conspicuous or commonly-known forms of life, it may be well to say a few words as to the structure of the living representatives of the group.

The _Foraminifera_ are all inhabitants of the sea, and are mostly of small or even microscopic dimensions. Their bodies are composed of an apparently structureless animal substance of an alb.u.minous nature ("sarcode"), of a gelatinous consistence, transparent, and exhibiting numerous minute granules or rounded particles. The body-substance cannot be said in itself to possess any definite form, except in so far as it may be bounded by a sh.e.l.l; but it has the power, wherever it may be exposed, of emitting long thread-like filaments ("pseudopodia"), which interlace with one another to form a network (fig. 25, b). These filaments can be thrown out at will, and to considerable distances, and can be again retracted into the soft ma.s.s of the general body-substance, and they are the agents by which the animal obtains its food.

The soft bodies of the _Foraminifera_ are protected by a sh.e.l.l, which is usually calcareous, but may be composed of sand-grains cemented together; and it may consist of a single chamber (fig.

26, a), or of many chambers arranged in different ways (fig.

26, _b-f_). Sometimes the sh.e.l.l has but one large opening into it--the mouth; and then it is from this aperture that the animal protrudes the delicate net of filaments with which it seeks its food. In other cases the entire sh.e.l.l is perforated with minute pores (fig. 26, e), through which the soft body-substance gains the exterior, covering the whole sh.e.l.l with a gelatinous film of animal matter, from which filaments can be emitted at any point. When the sh.e.l.l consists of many chambers, all of these are placed in direct communication with one another, and the actual substance of the sh.e.l.l is often traversed by minute ca.n.a.ls filled with living matter (e.g., in _Calcarina_ and _Nummulina_). The sh.e.l.l, therefore, may be regarded, in such cases, as a more or less completely porous calcareous structure, filled to its minutest internal recesses with the substance of the living animal, and covered externally with a layer of the same substance, giving off a network of interlacing filaments.

[Ill.u.s.tration: Fig. 25.--The animal of _Nonionina_, one of the _Foraminifera_, after the sh.e.l.l has been removed by a weak acid; b, _Gromia_, a single-chambered Foraminifer (after Schultze), showing the sh.e.l.l surrounded by a network of filaments derived from the body substance.]

[Ill.u.s.tration: Fig 26.--Sh.e.l.ls of living _Foraminifera_. a, _Orbulina universa_, in its perfect condition, showing the tubular spines which radiate from the surface of the sh.e.l.l; b, _Globigerina bulloides_, in its ordinary condition, the thin hollow spines which are attached to the sh.e.l.l when perfect having been broken off; c, Textularia variabilis; d, Peneroplis planatus; e, Rotalia concamerata; f, _Cristellaria subarcuatula._ [Fig. a is after Wyville Thomson; the others are after Williamson. All the figures are greatly enlarged.]]

Such, in brief, is the structure of the living _Foraminifera_; and it is believed that in _Eozoon_ we have an extinct example of the same group, not only of special interest from its immemorial antiquity, but hardly less striking from its gigantic dimensions.

In its original condition, the entire chamber-system of _Eozoon_ is believed to have been filled with soft structureless living matter, which pa.s.sed from chamber to chamber through the wide apertures connecting these cavities, and from tier to tier by means of the tubuli in the sh.e.l.l-wall and the branching ca.n.a.ls in the intermediate skeleton. Through the perforated sh.e.l.l-wall covering the outer surface the soft body-substance flowed out, forming a gelatinous investment, from every point of which radiated an interlacing net of delicate filaments, providing nourishment for the entire colony. In its present state, as before said, all the cavities originally occupied by the body-substance have been filled with some mineral substance, generally with one of the silicates of magnesia; and it has been a.s.serted that this fact militates strongly against the organic nature of _Eozoon_, if not absolutely disproving it. As a matter of fact, however--as previously noticed--it is by no means very uncommon at the present day to find the sh.e.l.ls of living species of _Foraminifera_ in which all the cavities primitively occupied by the body-substance, down to the minutest pores and ca.n.a.ls, have been similarly injected by some a.n.a.logous silicate, such as glauconite.

Those, then, whose opinions on such a subject deservedly carry the greatest weight, are decisively of opinion that we are presented in the _Eozoon_ of the Laurentian Rocks of Canada with an ancient, colossal, and in some respects abnormal type of the _Foraminifera_.

In the words of Dr Carpenter, it is not pretended that "the doctrine of the Foraminiferal nature of _Eozoon_ can be _proved_ in the demonstrative sense;" but it may be affirmed "that the _convergence of a number of separate and independent probabilities_, all accordant with that hypothesis, while a separate explanation must be invented for each of them on any other hypothesis, gives it that _high probability_ on which we rest in the ordinary affairs of life, in the verdicts of juries, and in the interpretation of geological phenomena generally."

It only remains to be added, that whilst _Eozoon_ is by far the most important organic body hitherto found in the Laurentian, and has been here treated at proportionate length, other traces of life have been detected, which may subsequently prove of great interest and importance. Thus, Princ.i.p.al Dawson has recently described under the name of _Archoeosph.o.e.rinoe_ certain singular rounded bodies which he has discovered in the Laurentian limestones, and which he believes to be casts of the sh.e.l.ls of _Foraminifera_ possibly somewhat allied to the existing _Globigerinoe_. The same eminent palaeontologist has also described undoubted worm-burrows from rocks probably of Laurentian age. Further and more extended researches, we may reasonably hope, will probably bring to light other actual remains of organisms in these ancient deposits.

THE HURONIAN PERIOD.

The so-called _Huronian Rocks_, like the Laurentian, have their typical development in Canada, and derive their name from the fact that they occupy an extensive area on the borders of Lake Huron. They are wholly metamorphic, and consist princ.i.p.ally of altered sandstones or quartzites, siliceous, felspathic, or talcose slates, conglomerates, and limestones. They are largely developed on the north sh.o.r.e of Lake Superior, and give rise to a broken and hilly country, very like that occupied by the Laurentians, with an abundance of timber, but rarely with sufficient soil of good quality for agricultural purposes. They are, however, largely intersected by mineral veins, containing silver, gold, and other metals, and they will ultimately doubtless yield a rich harvest to the miner. The Huronian Rocks have been identified, with greater or less certainty, in other parts of North America, and also in the Old World.

The total thickness of the Huronian Rocks in Canada is estimated as being not less than 18,000 feet, but there is considerable doubt as to their precise geological position. In their typical area they rest unconformably on the edges of strata of _Lower_ Laurentian age; but they have never been seen in direct contact with the _Upper_ Laurentian, and their exact relations to this series are therefore doubtful. It is thus open to question whether the Huronian Rocks const.i.tute a distinct formation, to be intercalated in point of time between the Laurentian and the Cambrian groups; or whether, rather, they should not be considered as the metamorphosed representatives of the Lower Cambrian Rocks of other regions.

As regards the fossils of the Huronian Rocks, little can be said.

Some of the specimens of _Eozoon Canadense_ which have been discovered in Canada are thought to come from rocks which are probably of Huronian age. In Bavaria, Dr Gumbel has described a species of _Eozoon_ under the name of _Eozoon Bavaric.u.m_, from certain metamorphic limestones which he refers to the Huronian formation. Lastly, the late Mr Billings described, from rocks in Newfoundland apparently referable to the Huronian, certain problematical limpet-shaped fossils, to which he gave the name of _Aspidella_.

LITERATURE.

Amongst the works and memoirs which the student may consult with regard to the Laurentian and Huronian deposits may be mentioned the following:[10]--

(1) 'Report of Progress of the Geological Survey of Canada from its Commencement to 1863,' pp. 38-49, and pp. 50-66.

(2) 'Manual of Geology.' Dana. 2d Ed. 1875.

(3) 'The Dawn of Life.' J. W, Dawson. 1876.

(4) "On the Occurrence of Organic Remains in the Laurentian Rocks of Canada." Sir W. E. Logan. 'Quart. Journ. Geol. Soc.,'

xxi. 45-50.'

(5) "On the Structure of Certain Organic Remains in the Laurentian Limestones of Canada." J. W. Dawson. 'Quart. Journ. Geol.

Soc.,' xxi. 51-59.

(6) "Additional Note on the Structure and Affinities of Eozoon Canadense." W. B, Carpenter. 'Quart. Journ. Geol. Soc.,' xxi.

59-66.

(7) "Supplemental Notes on the Structure and Affinities of Eozoon'

Canadense," W. B. Carpenter, 'Quart. Journ. Geol. Soc.,'

xxii. 219-228.

(8) "On the So-Called Eozoonal Rocks." King & Rowney. 'Quart.