Part 10

(1) In regard to the first of these questions, I may quote here, with some slight alteration, from a recent memoir of my own:--

In recent years I have been disposed to attach more importance than formerly to the general form of Eozoon. The earlier examples studied were, for the most part, imbedded in the limestone in such a manner as to give little definite information as to external form; and at a later date, when Sir William Logan employed one of his assistants, Mr. Lowe, to quarry large specimens at Grenville and Côte St. Pierre, the attempt was made to secure the most massive blocks possible, in order to provide large slabs for showy museum specimens.

More recently, when collections have been made from the eroded and crumbling surfaces of the limestone in its wider exposures, it was found that specimens of moderate size had been weathered out, and could, either naturally or by treatment with acid, be entirely separated from the matrix. Such specimens sometimes showed, either on the surfaces or on the sides of "funnels" and tubes penetrating the mass (Figs. 33, 34), a confluence of the laminæ, constituting a porous cortex or limiting structure. Specimens of this kind were figured in 1888, and I was enabled to add to the characters of the species that the original and proper form was "broadly turbinate with a depression or cavity above, and occasionally with oscula or pits penetrating the mass." The great flattened masses thus seemed to represent confluent or overgrown individuals, often contorted by the folding of the enclosing beds.

This specimen shows an oscuilform, cylindrical funnel, cut in such a manner as to show its reticulated wall and the descent of the laminæ toward it. Two-thirds of natural size. From a photograph. Col. Carpenter, also in Redpath Museum.

There are also in well-preserved specimens certain constant properties of the calcite and serpentine layers. The former are continuous, and connected at intervals, so that if the silicious filling of the chambers could be removed, the calcareous portion would form a continuous skeleton, while the serpentine filling the chambers, when the calcareous plates are dissolved out by an acid, forms a continuous cast of the animal matter filling the chambers (Fig. 36). This cast of the sarcodous material, when thus separated, is very uniformly and beautifully mammillated on the surfaces of the laminæ, and this tuberculation gradually passes upward into smaller chambers having amœboid outlines, and finally into rounded chamberlets. It is also a very constant point of structure that the lower laminæ of calcite are thicker than those above, and have the canal-systems larger and coarser. There is thus in the more perfect specimens a definite plan of macroscopical structure (Fig. 35).

1. Natural size. 2. Acervuline cells of upper part. 3. Group of the same coalescing into a lamina with tuberculated surface. 4. Laminæ with tuberculated surfaces in section. (See also Fig. 36.)

The normal mode of mineralization at Côte St. Pierre and Grenville is that the laminæ of the test remain as calcite, while the chambers and larger canals are filled with serpentine of a light green or olive colour, and the finer tubuli are injected with dolomite. It may also be observed that the serpentine in the larger cavities often shows a banded structure, as if it had been deposited in successive coats, and the canals are sometimes lined with a tubular film of serpentine, with a core or axis of dolomite, which also extends into the finer tubuli of the surfaces of the laminæ. This, on the theory of animal origin, is the most perfect state of preservation, and it equals anything I have seen in calcareous organisms of later periods. This state of perfection is, however, naturally of infrequent occurrence.

The finer tubuli are rarely perfect or fully infiltrated. Even the coarser canals are not infrequently imperfect, while the laminæ themselves are sometimes crumpled, crushed, faulted, or penetrated with veins of chrysotile or of calcite. In some instances the calcareous laminæ are replaced by dolomite, in which case the canal-systems are always imperfect or obsolete. The laminæ of the test itself are also in some cases replaced by serpentine in a flocculent form. At the opposite extreme are specimens, or portions of specimens, in which the chambers are obliterated by pressure, or occupied only with calcite. In such cases the general structure is entirely lost to view, and scarcely appears in weathering. It can be detected only by microscopic examination of slices, in parts where the granular structure or the tubulation of the calcite layers has been preserved. All palæontologists who have studied silicified fossils in the older rocks are familiar with such appearances.

It has been alleged by Möbius and others that the canal-systems and tubes present no organic regularity. This difficulty, however, arises solely from imperfect specimens or inattention to the necessary results of slicing any system of ramifying canals. In Eozoon the canals form ramifying groups in the middle planes of the laminæ, and proceed at first almost horizontally, dividing into smaller branches, which ultimately give off brushes of minute tubuli running nearly at right angles to the surfaces of the lamina, and forming the extremely fine tubulation which Dr. Carpenter regarded as the proper wall (Figs. 38, 39).

In my earlier description I did not distinguish this from the canal-system, with which its tubuli are inwardly continuous. Dr. Carpenter, however, understood this arrangement, and has represented it in his figures (see also Fig. 28). It is evident that in a structure like this a transverse or oblique section will show truncated portions of the larger tubes apparently intermixed with others much finer and not continuous with them, except very rarely. Good specimens and many slices and decalcified portions are necessary to understand the arrangement This consideration alone, I think, entirely invalidates the criticisms of Möbius, and renders his large and costly figures of little value, though his memoir is, as I have elsewhere shown, liable to other and fatal objections.

It has been pretended that the veins of chrysotile, when parallel to the laminæ, cannot be distinguished from the minute tubuli terminating on the surfaces of the laminæ. I feel confident, however, that no microscopist who has seen both, under proper conditions of preservation and study, could confound them. The fibres of chrysotile are closely appressed parallel prisms, with the optical properties of serpentine. The best preserved specimens of the "proper wall" contain no serpentine, but are composed of calcite with extremely minute parallel cylinders of dolomite about five to ten microms. in diameter, and separated by spaces greater than their own diameter (Figs. 40, 41). In the rare cases where the cylinders are filled with serpentine, they are, of course, still more distinct and beautiful. At the same time, I do not doubt that observers who have not seen the true tubulation may have been misled by chrysotile veins when these fringe the laminæ. Möbius, for instance, figures the true and false structure as if they were the same.

(From camera tracings.)]

Fig. 42.

Figures selected from Möbius, to show the resemblance of structures of Eozoon to those of modern Foraminifera.

Protest should here be made against that mode of treating ancient fossils which regards the most obscure or defaced specimens as typical, and those better preserved as mere accidents, of mineral structure. In Tertiary Nummulites injected with glauconite it is rare to find the tubuli perfectly filled, except in tufts here and there; yet no one doubts that these patches represent a continuous structure.

I have remarked on previous occasions that the calcite constituting the laminæ of Eozoon often has a minutely granular appearance, different from that of the surrounding limestone. Under a high power it resolves itself into extremely minute dots or flocculi, somewhat uniformly diffused. Whether these dots are particles of carbon, iron, apatite, or silicious matter, or the remains of a porous structure, I do not know; but similar appearances occur in the calcareous fossils contained in altered limestones of later date. Wherever they occur in crystalline limestones, supposed to be organic, the microscopist should examine them with care. I have sometimes by this appearance detected fragments of Eozoon which afterward revealed their canals.

(2) The second question requires us to consider the nature and origin of the substances constituting the specimens. Reference has already been made to these in our fifth chapter, but they may be more particularly noticed here in connection with the forms as above described.

The calcareous laminæ are usually composed of clear translucent calcite or calcium carbonate, though, as in the case of many later fossils, sometimes replaced by dolomite. It often has the fine granular appearance above referred to, but is nearly always crystalline, and traversed by cleavage planes visible under the microscope. This crystalline structure, as every student of fossils knows, is very common in calcareous fossils of all geological ages. In the thicker laminæ the canals traversing them and branching out in their substance are usually visible under a low power, except when they are filled with calcite similar to that of the laminæ themselves. In this case they can be seen only by very careful management of an oblique and subdued light. When occupied with serpentine, this presents, in a thin slice under transmitted light, a yellowish or brownish colour, and in a specimen decalcified with an acid an opaque white appearance. In some of the larger threads of serpentine, as already stated, this mineral forms a thin outer cylinder with a core of calcite or dolomite within; but this appearance is not common. Here and there, especially in the lower layers, a portion of a tube is filled with the harder mineral pyroxene, which is in some respects similar to serpentine, except that it contains lime as well as magnesia, and is destitute of water as an ingredient The finer tubuli into which the canals ramify are most usually filled with dolomite or magnesian limestone, which has a glossy appearance and higher lustre than the surrounding calcite, and so may be distinguished even in a transparent slice; but these fine dolomite threads are best seen when the surface of a slice is treated with a dilute acid in the cold, in which circumstances the calcite is dissolved, while the dolomite remains as tufts of delicate cylindrical hairs, presenting often a very beautiful appearance under the microscope. Thus, as in many other fossils, what are supposed to have been tubes and tubuli are found not empty, but filled with matter even harder and more resisting than the shell itself.

Serpentine is a mineral which has been produced in different ways. Some igneous or volcanic rocks consist largely of compounds of silica and magnesia (olivine, etc.). When these rocks have become cold and are exposed to the action of water, they sometimes absorb this and become hydrated, thus passing into a kind of serpentine. When such rocks are pulverized and dispersed as volcanic ash, this falling into the sea may be there hydrated, and may form serpentinous layers, or in a fine paste or in solution may pass into the pores and cavities of shells and other organic things, acting, as we have seen, in the same manner with ordinary glauconite. In like manner serpentine of this origin may form nodules or grains in limestones, in consequence of its particles being aggregated together by concretionary attraction. We have already seen that some comparatively modern so-called glauconites are essentially of the nature of serpentine, and we know that in the old Laurentian sea, salts of magnesia and magnesian minerals were abundant, so that serpentinous minerals might play a greater part than they do in the modern seas. Loganite, the mineralizing substance of the Burgess Eozoon, is different from serpentine, yet closely allied to the glauconites. The presence of pyroxene may be explained in a similar way. It is a frequent constituent of bedded volcanic rocks and of volcanic ashes, and beds of it occur in the Grenville series which once, no doubt, were ash-beds. Layers of it also occasionally occur from a similar cause in the limestone, and crystals of it have been deposited by water in the veins passing through the limestones and schists. Dr. Johnston-Lavis has described in the July number of the Geological Magazine for 1895 the aqueous deposition at ordinary temperature of crystals of pyroxene and hornblende, in cavities and crevices of bones included in an ash-bed of recent date, and in presence of calcite, apatite, and fluoride of calcium, as in the Grenville series. This is a modern instance analogous to that suggested above. Hence all these minerals filling the cavities and canals of Eozoon may have been deposited by water at ordinary temperatures, and have no connection with the alteration to which the beds have been subsequently subjected.

I may add here that a Tertiary glauconite from the Calcaire Grossier of Paris analysed by Berthier is essentially a serpentine composed of silicate of iron and magnesia, that Loganite as analysed by Hunt contains thirty-one per cent, of magnesia, and that Hoskins has shown that modern glauconites often contain large proportions of magnesia and equivalent bases.

It is also to be observed that independently of volcanic debris the reports of the Challenger expedition show that in the deep seas the decay of organic matter causes an alkaline condition of the sediments leading to the formation of alkaline silicates, while the presence of decaying volcanic dust furnishes the basis, whether of iron, alumina, or magnesia, necessary for the making up of glauconite. I have also suggested that the assimilation by Protozoa making calcareous skeletons, of the matter of Diatoms or humble plants having soluble silica in their organization or of silicious Protozoa, and sponge germs, must set free much soluble silica as a rejected or excrementitious matter which may contribute to the same result.

It is much more likely that the serpentine of the Laurentian limestones was produced in these ways than that it resulted from the hydration of magnesian minerals after the rock was consolidated. In the former case it would be in the most favourable conditions for mineralizing organisms as glauconites do in the modern seas. In the latter it would cause disturbances and changes of volume of which we have no evidence.

We thus find that the chemistry of the modern seas and that relating to the preservation of fossils of various ages by silicious infiltrations lends great probability to the belief that serpentine played this role in the oldest seas, though it would seem that dolomite was more suitable to the filling of the extremities of the minute tubes and their finer terminations.

(3) Our third question leads to the inquiry in what modern or ancient marine animals we can find structures akin to those of our supposed Laurentian fossil. The first analogy which suggested itself to Sir W. Logan, and a very natural one, was that to the so-called layer-corals (Figs. 43 to 45) that abound in the Silurian, Ordovician, and Cambrian rocks, and which though undoubtedly fossil animals, have proved very difficult to interpret or to assign to any known group. At first vaguely associated with the true corals, they were subsequently regarded as probably of more simple character, and as gigantic Protozoa; and later strong reasons have been assigned for giving them an intermediate place, as allied to those curious communities of humble animals possessing simple stomachs and prehensile tentacles (Hydroids) which form some of the simpler corals (Millepores, etc.), and the crusts (Hydractiniæ) which cover dead shells and other bodies in the sea. When examined microscopically, however, they differ very much among themselves, and it may be that some of them were Hydroids and some Protozoa.

(a) Portion of oblique section, (b) Wall with pores, and coated with crystals of quartz, (c) Thickened portion of wall with canals, (d) Laminæ and pillars.]

The oldest that we at present know, and consequently the nearest in time to Eozoon, impress us rather with the latter affinity. They are the fossils of the genus Cryptozoon of Hall (Fig. 7), which form great masses filling certain beds of Upper Cambrian age, and which, when sliced and studied microscopically, are found to consist of concentric thin laminæ filled in between with a porous mass of calcareous matter penetrated by an infinity of tortuous tubes. Forms of this kind have been traced downward into pre-Cambrian beds in Colorado, and as we shall find in New Brunswick, into the Upper Laurentian itself.

They present, however, structural differences from Eozoon, which rather conforms to the arrangements found in some Protozoa of smaller size, and which, under the name of Foraminifera, have abounded in all geological periods, and are excessively abundant in the modern ocean. They may be defined as animals composed of a soft and apparently homogeneous animal jelly known as protoplasm or sarcode. When carefully examined, however, it is found to have a granular texture and to be divisible into two layers, an outer and an inner, while it possesses a little hollow vessel capable of expanding and absorbing the liquid matter of the enclosing protoplasm, and of contracting so as to expel its contents. This seems to be the only organ of circulation and excretion. There are, however, small cells or reproductive bodies in the interior, varying in number, size, and development in different forms. The most remarkable property of these creatures is that of stretching out from the surface of the body threads or projections of the protoplasm, often of considerable length, and which serve at once as organs of locomotion and prehension.

From original sketches.