WE have seen that the mineral constitution of the Upper Laurentian affords evidence that in this age there were already land and water, and that the processes by which the land is being worn down, and its materials deposited on the sea-bottom, were in full operation; while the absence of any evidence of violent wave-action, and the presence of thick deposits of limestone, coaly matter, iron ore, and fine-grained beds of sediment, indicates a time of rest and quiescence. All these conditions were favourable to the presence of life, and we should expect to find in such a period some sign of its commencement.
But here we are met by a formidable difficulty. If the beds of the Grenville series were originally deposits in a quiet sea, they are, as now existing in the old Laurentian hills and valleys, very much changed from their original condition. They have, in short, experienced the changes known to geologists by the formidable word metamorphism, whereby they have lost the more obvious characters of ordinary aqueous deposits, and have assumed new and strange forms. Dr. Adams, of Montreal, has taken the pains to collect a number of chemical analyses of the gneisses and schists or crystalline slates of the Grenville series, and finds that, however unlike to more modern shales and clays, they have substantially the same chemical composition. Now if they were originally such shales and clays, it has happened to them that the ingredients of the clays have rearranged themselves in new forms and become crystalline. We are familiar in a small way with such changes when brick clay, over-heated in the kiln, becomes fused into slag or vitrified; and if such slag were allowed to cool very slowly, it would present different kinds of crystalline minerals. We actually see changes of this kind in the substance of bricks which have been long exposed to intense heat in the walls of furnaces. Now in the crust of the earth, very old rocks, buried under newer deposits, and exposed to the heat of the interior molten rocks, experience such changes on a great scale; and there is one kind of influence present in the bowels of the earth which we in our experiments cannot easily imitate or understand, namely, the action of superheated water prevented by pressure from escaping as steam, and permeating the whole substance of deposits, which are thus baked at a high temperature in presence of water, instead of being exposed to mere dry heat, as in our kilns and furnaces. The study of the partial changes which have passed on later sediments where in contact with volcanic masses once intensely heated, enables us to understand the greater and more extensive metamorphism of the oldest rocks. Thus a mere mud becomes glorified by metamorphic crystallization into a micaceous schist. We have taken ordinary clay as an example; but under the same processes sand has been converted into a compact quartzite, ordinary limestone into crystalline marble, clay-ironstone into magnetic iron ore, coal into graphite, and lavas or volcanic ashes into hard crystalline granites, gneisses, or pyroxene rocks or hornblendic schists, according to their original composition. There may exist portions of these old rocks which have been exempt from such alteration, but hitherto we have not been able to find them, and they are probably under the ocean bed, or deeply burled beneath later rocks, while the parts exposed are precisely those which have by their crumpling and pressure, and the influence of internal heat, become most hardened and altered, and have therefore best resisted denudation. We need not therefore be astonished if any organic remains originally present in such rocks should have perished, or should have been subjected to such changes of composition and form as to have altogether lost their original characters. The searcher for fossils in such rocks has to expect that these can have been preserved only under very rare and exceptional circumstances. We have now to consider what these circumstances are, and for simplicity may suppose that we are endeavouring to discover in a crystalline limestone the remains of animals having a skeleton of limestone, as is the case with most shell-fishes and corals, and with many Protozoa and marine worms. In regard to these, we have to consider what may happen to them when they are imbedded in calcareous marl or ooze, or the limestone which results from the hardening of such materials; and we have to bear in mind that such organisms usually consist of hard, stony walls or partitions, enclosing cavities originally filled with the soft parts of the animal which may be supposed to have disappeared by decay before or during the mineralization of its skeleton.
So long as the imbedding mass continues soft and incoherent, shells, corals, etc., can be recovered in a condition similar to that of recent specimens, except that they may have become bleached in colour and brittle in texture, owing to the removal of organic matter intimately associated with the lime, and that their cavities may have been filled with sand or silt washed into them, or with calcite or calcareous spar introduced in solution in water. But if the containing mass has become a hard stone, the material filling the interior of our shell or coral has experienced a similar change; and when we break open the stone, we may obtain the specimen, now hard, solid, and heavy, but still showing more or less of its outer surface and markings, and possibly to some extent also its internal structure when it is sliced and studied under the microscope. But if the whole mass has been metamorphosed, and has become crystalline, the contained fossil and its contents may have experienced a similar change, and may have so coalesced with the containing matrix that it is no longer separable from it. Even in this case, however, if the whole is reduced to a thin transparent slice and examined microscopically, some traces may be found of the external and internal limiting lines of the fossil, and even of its minute structures, which often cause it to present an appearance granular, cellular, or otherwise different from that of the enclosing matrix. It requires, however, both skill and care to detect organic remains in such circumstances, and they may often escape observation, except when, as in many old crystalline limestones, the fossils are darkened in whole or in part with coaly matter derived from the decay of their own organic substance. The crystalline Trenton limestone of Montreal, used there as a building stone, is an excellent example ([Fig. 22]).
Fig. 22.—Section of "Trenton Limestone" (magnified).
Showing its composition of fragments of calcareous fossils.
Fig. 23.—Diagram of different States of Fossilization of the Cell of a Tubulate Coral.
(a) Natural condition, (b) Cell filled with calcite. (c) Walls calcite, filling silica. (d) Walls silica, filling calcite. (e) Both walls and calcite silica. All these conditions are found in the fossil corals of the corniferous Limestone of Canada—Middle Permian.
It is otherwise, however, when the calcareous fossils have been filled or injected with some mineral matter different from the matrix, as, for example, silica or some silicate, oxide or sulphide of iron. In this case the texture, colour, or hardness of the filling appear different from those of the limestone, and may be seen in a fresh fracture or polished slice; or when the rock is weathered, the hard mineralizing substance may project from the surface of the specimens, or may be disclosed by treating the surface with a weak acid. The figures here given may suffice to show some of these conditions of mineralization in ordinary limestones, and the effects which they produce ([Fig. 23]).
The mineral matters which thus aid in preserving fossils are of various kinds, and the whole subject is a very curious one; but for the present we may content ourselves with two kinds of mineralization—that by silicates and that by magnesian limestone or dolomite.
From the bottom of modern seas the dredge often brings up multitudes of minute shells, especially those of the simple gelatinous Protozoa, known as Foraminifera, whose internal cavities and pores have been filled with a greenish mineral composed of silica, iron and potash, combined with water (or, chemically speaking, a hydrous silicate of iron and potassium), which is named glauconite from its bluish-green colour—a name which we shall do well to remember. In such compounds, bases of similar chemical properties often replace one another, so that various glauconites differ somewhat in composition, the iron being in part often replaced by alumina or magnesia, and the potash by soda. The combined water also differs somewhat in its percentage. When minute shells fossilized in this way are treated with an acid so as to remove the calcareous shell itself, the enclosed silicate remains as a beautiful cast or core, representing all the forms of the interior, and any pores that may have penetrated the walls, and also perfectly representing the soft gelatinous body of the animal which once tenanted the shells ([Fig. 24]). (See also [Fig. 25] at end of chapter.)