The Ancient Life History of the Earth. Henry Alleyne Nicholson

from the Silurian upwards, as well as the common phenomenon of the silicification of organic bodies (such as corals and shells) which are known with certainty to have been originally calcareous.]

      In addition to deposits formed of flint itself, there are other siliceous deposits formed by certain silicates, and also of organic origin. It has been shown, namely—by observations carried out in our present seas—that the shells of Foraminifera are liable to become completely infiltrated by silicates (such as "glauconite," or silicate of iron and potash). Should the actual calcareous shell become dissolved away subsequent to this infiltration—as is also liable to occur—then, in place of the shells of the Foraminifera, we get a corresponding number of green sandy grains of glauconite, each grain being the cast of a single shell. It has thus been shown that the green sand found covering the sea-bottom in certain localities (as found by the Challenger expedition along the line of the Agulhas current) is really organic, and is composed of casts of the shells of Foraminifera. Long before these observations had been made, it had been shown by Professor Ehrenberg that the green sands of various geological formations are composed mainly of the internal casts of the shells of Foraminifera, and we have thus another and a very interesting example how rock-deposits of considerable extent and of geological importance can be built up by the operation of the minutest living beings.

      As regards argillaceous deposits, containing alumina or clay as their essential ingredient, it cannot be said that any of these have been actually shown to be of organic origin. A recent observation by Sir Wyville Thomson would, however, render it not improbable that some of the great argillaceous accumulations of past geological periods may be really organic. This distinguished observer, during the cruise of the Challenger, showed that the calcareous ooze which has been already spoken of as covering large areas of the floor of the Atlantic and Pacific at great depths, and which consists almost wholly of the shells of Foraminifera, gave place at still greater depths to a red ooze consisting of impalpable clayey mud, coloured by oxide of iron, and devoid of traces of organic bodies. As the existence of this widely-diffused red ooze, in mid-ocean, and at such great depths, cannot be explained on the supposition that it is a sediment brought down into the sea by rivers, Sir Wyville Thomson came to the conclusion that it was probably formed by the action of the sea-water upon the shells of Foraminifera. These shells, though mainly consisting of lime, also contain a certain proportion of alumina, the former being soluble in the carbonic acid dissolved in the sea-water, whilst the latter is insoluble. There would further appear to be grounds for believing that the solvent power of the sea-water over lime is considerably increased at great depths. If, therefore, we suppose the shells of Foraminifera to be in course of deposition over the floor of the Pacific, at certain depths they would remain unchanged, and would accumulate to form a calcareous ooze; but at greater depths they would be acted upon by the water, their lime would be dissolved out, their form would disappear, and we should simply have left the small amount of alumina which they previously contained. In process of time this alumina would accumulate to form a bed of clay; and as this clay had been directly derived from the decomposition of the shells of animals, it would be fairly entitled to be considered an organic deposit. Though not finally established, the hypothesis of Sir Wyville Thomson on this subject is of the greatest interest to the palæontologist, as possibly serving to explain the occurrence, especially in the older formations, of great deposits of argillaceous matter which are entirely destitute of traces of life.

      It only remains, in this connection, to shortly consider the rock-deposits in which carbon is found to be present in greater or less quantity. In the great majority of cases where rocks are found to contain carbon or carbonaceous matter, it can be stated with certainty that this substance is of organic origin, though it is not necessarily derived from vegetables. Carbon derived from the decomposition of animal bodies is not uncommon; though it never occurs in such quantity from this source as it may do when it is derived from plants. Thus, many limestones are more or less highly bituminous; the celebrated siliceous flags or so-called "bituminous schists" of Caithness are impregnated with oily matter apparently derived from the decomposition of the numerous fishes embedded in them; Silurian shales containing Graptolites, but destitute of plants, are not uncommonly "anthracitic," and contain a small percentage of carbon derived from the decay of these zoophytes; whilst the petroleum so largely worked in North America has not improbably an animal origin. That the fatty compounds present in animal bodies should more or less extensively impregnate fossiliferous rock-masses, is only what might be expected; but the great bulk of the carbon which exists stored up in the earth's crust is derived from plants; and the form in which it principally presents itself is that of coal. We shall have to speak again, and at greater length, of coal, and it is sufficient to say here that all the true coals, anthracites, and lignites, are of organic origin, and consist principally of the remains of plants in a more or less altered condition. The bituminous shales which are found so commonly associated with beds of coal also derive their carbon primarily from plants; and the same is certainly, or probably, the case with similar shales which are known to occur in formations younger than the Carboniferous. Lastly, carbon may occur as a conspicuous constituent of rock-masses in the form of graphite or black-lead. In this form, it occurs in the shape of detached scales, of veins or strings, or sometimes of regular layers;[7] and there can be little doubt that in many instances it has an organic origin, though this is not capable of direct proof. When present, at any rate, in quantity, and in the form of layers associated with stratified rocks, as is often the case in the Laurentian formation, there can be little hesitation in regarding it as of vegetable origin, and as an altered coal.

      [Footnote 7: In the Huronian formation at Steel River, on the north shore of Lake Superior, there exists a bed of carbonaceous matter which is regularly interstratified with the surrounding rocks, and has a thickness of from 30 to 40 feet. This bed is shown by chemical analysis to contain about 50 per cent of carbon, partly in the form of graphite, partly in the form of anthracite; and there can be little doubt but that it is really a stratum of "metamorphic" coal.]

CHAPTER III.

       Table of Contents

      CHRONOLOGICAL SUCCESSION OF THE FOSSILIFEROUS ROCKS.

      The physical geologist, who deals with rocks simply as rocks, and who does not necessarily trouble himself about what fossils they may contain, finds that the stratified deposits which form so large a portion of the visible part of the earth's crust are not promiscuously heaped together, but that they have a certain definite arrangement. In each country that he examines, he finds that certain groups of strata lie above certain other groups; and in comparing different countries with one another, he finds that, in the main, the same groups of rocks are always found in the same relative position to each other. It is possible, therefore, for the physical geologist to arrange the known stratified rocks into a successive series of groups, or "formations," having a certain definite order. The establishment of this physical order amongst the rocks introduces, however, at once the element of time, and the physical succession of the strata can be converted directly into a historical or chronological succession. This is obvious, when we reflect that any bed or set of beds of sedimentary origin is clearly and necessarily younger than all the strata upon which it rests, and older than all those by which it is surmounted.

      It is possible, then, by an appeal to the rocks alone, to determine in each country the general physical succession of the strata, and this "stratigraphical" arrangement, when once determined, gives us the relative ages of the successive groups. The task, however, of the physical geologist in this matter is immensely lightened when he calls in palæontology to his aid, and studies the evidence of the fossils embedded in the rocks. Not only is it thus much easier to determine the order of succession of the strata in any given region, but it becomes now for the first time possible to compare, with certainty and precision, the order of succession in one region with that which exists in other regions far distant. The value of fossils as tests of the relative ages of the sedimentary rocks depends on the fact that they are not indefinitely or promiscuously scattered through the crust of the earth—as it is conceivable that they might be. On the contrary, the first and most firmly established law of Palæontology is, that