the vast series of formations so designated comprises a total thickness of strata equal to at least three-fourths of all the known fossiliferous deposits. It is marked off from the far smaller and less chronologically important groups of the Secondary and Tertiary rocks, not because it covers an approximately equal lapse of geological time, but simply and solely for convenience of stratigraphical classification. As we shall see more fully hereafter, the Paleozoic period probably occupied double or treble as long a time as the Secondary and Tertiary periods put together. So that any argument based upon the occurrence or non-occurrence of particular plants or animals in the Palæozoic system is utterly futile, unless it specifies distinctly whether it refers to the earliest Cambrian or the latest Carboniferous deposits-the relics of two periods apparently separated from one another very much more widely than we ourselves are separated from the days of the Chalk and the Blue Lias. It is, in short, as though one were first arbitrarily to divide English history into three epochs-the Primitive period, including all times between the landing of Hengst and the reign of Elizabeth, the Stuart period, and the Hanoverian period-and then to argue that English literature can never have undergone any progressive development, because in the primitive or very earliest of these three epochs it had already produced Chaucer, Shakespeare, Bacon, and Spenser. Absurdities and incongruities not less ridiculous than these have been gravely put forward as solemn refutations of Darwinism by more than one distinguished but ungeological writer.

Again, while it is perfectly true that we do find the remains of vertebrates somewhere about the middle of the Paleozoic seriesthat is to say, in the Upper Silurian, underlaid by forty thousand feet of previous fossiliferous rocks-the statement is once more very misleading by its studious generality and dishonest avoidance of detail. For the vertebrates whose remains we thus discover are fishes, the very lowest and simplest class of all their group. The amphibians do not appear with certainty before the Carboniferous period, at the very close of the great Palæozoic series, in comparatively recent times. The first true reptiles are found in the Permian, and they attained their highest development in the all but modern Secondary epoch. Birds are not known till the Jurassic times-the day-before-yesterday of geology. And mammals have never yet made their appearance before the new red sandstone, while it is only in the still soft and claylike mud of the very recent Tertiary epoch that their most important and familiar forms find a full development. The geological record bears out in minute detail the very smallest particulars of the Darwinian theory.

Finally, it is quite forgotten by those who argue in this superficial fashion that we have still under our eyes the sedimentary deposits of a vast and very ancient epoch, the Laurentian, underlying all our known fossiliferous strata, and testifying to an immense lapse of primæval time in which the traces of animal and vegetable life are few and very doubtful. The vast ages thus unaccounted for would be amply sufficient, as we hope to show, for the development of the primitive fauna and flora up to the point at which we find it when the book of paleontology abruptly opens its first chapter with the teeming and diversely peopled seas of the Cambrian age.

Where such wide misconceptions exist or such strange misrepresentations are made, it may be worth while to meet them by a definite statement of the real facts, so far as they concern the evolutionist hypothesis. I propose, accordingly, in the present paper to give an approximate chronology of geological time, based upon such indications as the various strata have afforded to the greatest investigators of our own or recent days. Geologists, as a rule, it is true, have avoided making any definite statements as to the exact number of years which any particular deposit may be supposed to represent; and they have done so on very good and sufficient grounds. There is always a danger that such calculations, however vague, may be upset by further discoveries; and scientific men generally refrain from inferences which new facts may at any moment invalidate. But when the absence of such approximate chronological tables is made the ground for fallacious arguments by the unscientific, who twist aside geological terms so as to give countenance to very dubious reasoning, it is well to step aside somewhat from this wholesome principle, and to place the question at issue before the general public in its most vivid, graphic, and definite light. For this purpose, I propose here to estimate roughly the time occupied by the deposition of the best known formations, and then to point out the relative date of the first remains which mark the earliest known appearance of the chief animal or vegetable groups upon our earth. Such a chronology, extending over unknown millions of years, must of course be highly conjectural and full of acknowledged lacunæ ; but it will at least serve to place the subject before the reader in a clear and comprehensible form, while it will correct numerous intentional or accidental misrepresentations which occur only too often in the pages of controversial authors.

We must begin by fixing upon some arbitrary period of so many millions of years, representing the total of geological time, which we may divide out proportionately among the various formations ac

cording to their probable relative duration. How long we suppose this arbitrary period to be, viewed absolutely, is a matter of small importance, since at best it can only be a happy guess; but the serious point for our consideration is the relative amount of the total sum which we must allot to each geological epoch. However, we must to some extent be guided by physical and astronomical data, as well as by those supplied to us by the history of our own earth. Now, it happens that on physical grounds alone Sir William Thomson has made a calculation which may serve us as a basis for our chronological system. He holds that the sun has almost certainly illumined the earth for a less period than five hundred million years, and probably for not more than one hundred million. The reasons given for this calculation, being based upon deductions from the still infant theory of energy, may not perhaps be so certain as many persons are willing to believe; but at least we shall probably keep on the safe side if we do not exceed Sir William Thomson's smaller estimate of a hundred million years. Let us call it, for the sake of simplification, a million centuries, and we shall be dealing with a number more readily grasped by the human intelligence. How, then, are we to distribute these million centuries in due proportions among the various geological formations from the very ancient Laurentian to the quite recent Quaternary?

Professor Huxley has pointed out a simple and effective method of roughly making the distribution. Let us take one hundred thousand feet as an average estimate for the total thickness of the stratified rocks containing more or less certain traces of life. Then, if we suppose the strata to have been uniformly deposited at the rate of a thousandth of a foot (or one eighty-third of an inch) per annum, the whole thickness would take just a million centuries for its deposition This arbitrary figure represents on the whole a very good conjectural rate of growth. Of course, some strata would originally take much longer to form up to a foot's thickness than others. Among very early limestone rocks, again, pressed close together by ages of crushing under sea and mountains, until sometimes all traces of their original structure are completely obscured, a foot, doubtless, represents a far greater lapse of time than among loose modern ooze or mud formations, like the chalk and the red crag. Moreover, the oldest strata, being produced by the wear and tear of the elements on solid igneous rocks, hard as porphyry, quartz, or trachyte, must necessarily have taken longer to deposit than the more modern strata, which were in turn made up from the detritus of the earlier and comparatively soft sedimentary rocks so produced. On the

other hand, the rapid growth of peat in marshy bogs would permit of very quick deposition of coal. But all these objections are really so much gain for the evolutionist, inasmuch as they show still more clearly the enormously long time which must have been occupied by the deposition of the earliest and least-known formations. The fact is, that as the greater part of England and Wales, and especially of the most populous districts, lies upon the Secondary or Tertiary systems, while the least populous parts lie upon the Primary, there has arisen a very general, though vague, misconception, favoured by the nature of the words themselves, that the Secondary and Tertiary systems are each of them equivalent in duration to the Primary. The newer strata contain more, as well as more interesting, fossils; they compose all the most striking and popularly known deposits; they fill up the larger half of geological treatises; they are, as it were, brought home to everybody's door throughout all southern or eastern England; and so they naturally engage far more of ordinary attention than do the incalculably more important Primary rocks. Occupying the largest space in our minds, our geological maps, and our paleontological works, they come as a matter of course to occupy in imagination the largest space in cosmical time. It is only professional geologists, as a rule, who are able to translate the hieroglyphics of nature, given in terms of thickness, so as to be realisable to their intelligence in the terms of actual duration, which they dimly symbolise. This task we must now endeavour tentatively to perform.

As soon as our earth ceased to be incandescent, and became covered in large part by water, it commenced its depositions of submarine sediments. The oldest known sedimentary rocks, comprising the Laurentian and Huronian systems of Canada, have a total average thickness which cannot certainly be estimated at anything less than 30,000 feet. Sir William Logan, indeed, the greatest authority upon these primæval formations, considered the measurable thickness of his Upper and Lower Laurentian alone to amount respectively to 20,000 and 10,000 feet, while he set down the Huronian system as reaching some 18,000 more. But as doubts have been raised whether the Huronian series are not really the metamorphosed representatives of the Upper Laurentian, we will omit them altogether from our calculation, so as to avoid any possible cause of offence. The great Cambrian system, the next in order of time, has a thickness which has been fairly estimated at from 25,000 to 30,000 feet. We will adopt the smaller figure. The Silurian is pretty certainly known to number 6,000 feet. The Old Red Sand

stone, with its doubtful contemporary, the Devonian, cannot be put down for less than 10,000. The Carboniferous series amount to at least 12,000 feet, the Coal-Measures alone sometimes attaining to fully that thickness. Thus the whole Primary group, including the so-called azoic rocks, has a total vertical extent of not less than 83,000 feet. By the side of these enormous thicknesses, we can only allow 10,000 feet for the whole of the Secondary formations, from the Permian to the Chalk inclusive, while we shall be generous if we assign 1,000 feet to the little group of the Tertiary and PostTertiary deposits. This gives us a total thickness for the whole geological series of 94,000 feet. Let us allow 6,000 more for the breaks between each of these main divisions, or the unrepresented strata, and we have the round number with which we started, 100,000 feet.

A tabular statement will make these relations clear, and will allow us to translate our known thicknesses into conjectural but relatively ascertained dates, upon the system already explained.

Once more, let us construct a second or chronological table, distributing the margin of six million years equally between all the strata, and adopting the old-fashioned letters A. M. (Anno Mundi) in a new sense as marking the lapse of time from the beginning of sedimentary deposits upon our earth. We shall then get a definite chronology in round numbers as follows: