of the geographic forms. The agencies of change, which have been fully discussed in Chapter I., cannot but act slowly; accumulating stresses may take place, and it is conceivable that catastrophes might occur; but there is really no evidence of them in the earth's geologic history, except on a minor scale.

I have shown that sedimentation and mountain building are in the nature of cause and effect. Whatever be the fluctuations in the rate of denudation and deposition, such fluctuations are limited by meteorological conditions; and even if the largest coefficient of denudation be granted, the time taken up in providing materials for the evolution of a mountain chain must be enormous.

Therefore it appears that, the evolution of a continent being dependent upon two factorsfirstly, the alteration in volume of enormous sections of the earth's mass, and, secondly, the deposition of sediment and its lateral folding into bordering mountain chains-the rate of continentmaking must necessarily be extremely slow. To this is attributable the comparative permanence of land conditions, which has led some naturalists to infer that continents have, speaking generally, been in the same relative position throughout all geologic time.

CHAPTER III

CONTINENTAL GROWTH AND GEOLOGICAL PERIODS 1

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N the early days of geology a period indicated a certain section of the earth's history distinctly marked off from that which preceded and that which followed. Each period had a fauna and flora peculiar to itself, by which it could be recognised through the fossil remains found embedded in its rocks. There is no doubt that the Mosaic account of the Creation gave traditional support to the notion of distinct breaks in the geological chain. Each period represented, not unnaturally, to early thinkers a separate creation, followed by complete destruction. What first attracted attention were the salient differences between the fossil contents of strata geologically far apart, such as the reptiles of the Lias and the fishes of the Chalk. Ingenious men found then, as Mr. Gladstone did later, a parallelism in the order of creation between the Mosaic account and the record of the rocks.

When, however, the record was further searched, interesting links were discovered, which, if they did

This chapter is the reproduction of an article that I contributed to Natural Science in 1894.

not actually bridge over the differences, led men to think that, could all be restored, the earth's history would be found to be one continuous record, unbroken by cataclysmic collapses and successive repairs. Curiously enough, the cataclysmal ideas held their ground in face of the uniformitarian theory of the earth given to the world by Hutton at the close of the last century. Lyell, following on the same lines as Hutton, with a wealth of illustration and rare literary skill, showed that a true interpretation of Nature in the past was to be sought in the action of present causes. The science of geology was thus put on a stable base, and men were taught to arrive at their opinions by reasoning upon facts, instead of merely giving free scope to their imaginations. The doctrine of uniformity may not be theoretically correct; indeed, uniformitarianism is a misnomer, even as applied to Lyell's conception of the history of the earth, which might, with quite as much justice, be called development. The popular conception, however, of uniformity was that things are now as they have been in the past and as they will be in the future. From this it resulted that 'periods' came to be looked upon only as arbitrary divisions of geological history, which enabled one to grasp the sequence of geological events. Indeed, it is but fair to say that there is not in the whole of Lyell's 'Principles,' for which no one has a profounder regard than myself, any indication of how the distinctive geological and physical features of the

several periods came about, or, indeed, why they should exist at all. As a student of Lyell from my earliest dabblings in geology, this was long a mystery to me. In putting forward the following suggestions as to how the periods were evolved, I do so with all humility, looking upon them as a development of the great master's work.

Sedimentation and Land-making. It is a wellknown axiom in geology that the land is being lowered at an average rate of about 1 foot in 4,000 years by meteoric action, by rain and rivers, or all those chemical and mechanical forces that come under the general term sub-aërial denudation. To this is to be added the mechanical abrasion of coasts. The matter carried into the ocean in solution in river waters is, I have shown, on the average of many years, about one-third that in suspension. This seems at first blush a large proportion, but when we consider that the matter in solution is a much more constant quantity than the matter in suspension, our surprise is modified.

The sedimentary matter-which is chiefly silica, either in the form of grains of sand or in a much finer state of comminution, as flour of rock mixed with the decomposition products of various rocks, notably felspathic, forming what when deposited we call clay-is laid down in a more restricted area than the matter in solution. Mixed with these products of denudation are calcareous particles, mica, and other minerals, which all go to make up one or other of the various sedi

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mentary strata. These are, as a whole, often looked upon as the effects of mechanical erosion, but, so far as this is an expression of dynamic action, they are only partially so. Chemical forces have, in my opinion, much more to do with loosening the bonds of the rocky particles than has mere pounding of the boulders along shores and river-beds; they also effect the separation of the rock-masses. To be impressed with this fact one has only to look at some of the enormous masses of rock in mountain districts in Wales, moved to their present positions during the last phase of the Ice Age. Although they have been exposed to nothing more than meteorological influences since, they are frequently split up into many separate blocks, and are much weathered. Likewise, in granite districts enormous masses of granitic sand are, as we may say, liberated by the decomposition of granite. To these sediments must be added boulders and pebbles which go to form conglomerates. Boulders are, however, seldom delivered into the ocean by large rivers. They remain in the higher reaches or mountain tributaries, so that the boulder-beds found in the sea are either the products of coast erosion, the dynamic undermining of cliffs by wave action assisted by meteorological influences, or are formed by mountain torrents with swift gradients, or are carried by glacial agency or by floating ice.

These mechanical products of denudation are sifted out and arranged by the ocean waves, tides,