entombed in the earth's crust is to show that in the domain of geological evolution all has been systematic and orderly. It becomes more and more apparent that the features of the earth's surface are the progressive result of the interaction. of internal and external forces. If there be one thing more than another that impresses itself on the mind, it is the slowness with which these vast changes come about and the enormous length of geological time.

In the early stages of investigation the relations of one set of phenomena to those of another were naturally not apparent.

For instance, it was not known that mountainbuilding is always preceded by great sedimentation. This was a generalisation that could not be made until a vast amount of geological mapping had been done.

Furthermore, these sediments-the comminuted débris of earlier lands-cannot have been laid down otherwise than very slowly. Even if we assume that the area of deposit is but one-tenth that of the area of denudation, the accumulation of a thickness of deposit comparable with that exhibited in a large mountain range must have taken millions of years. But it is proved that there are rocks existing in which the accumulation proceeded much more slowly than the ordinary denudation of the land. In a most interesting paper by Professors Edgeworth David and E. F. Pitman it has been shown that there exist in New South Wales

Paleozoic Radiolarian rocks reaching a thickness of 9,000 feet, inclusive of submarine tufts, and composed chiefly of tests of radiolaria, which are present in the bulk of these rocks in the proportion of one million to the cubic inch. The rocks are estimated to extend a distance of 285 miles north and south. Dr. Hinde in a separate paper on the same subject considers that the rocks are of Devonian age, and he observes: For the formation. of so great a thickness of rock composed principally of extremely fine calcareous or clayey materials filled with the remains of these microscopic organisms, an enormous period of quiet sedimentation must be conceded.' 1

Thus we see that the regenerative forces of the earth wait upon the crumbling and destruction of the old lands. The one is consequent upon the other. A vivid conception of the slow pace at which these operations proceed is presented to us when we consider the extreme comminution to which the sedimentary particles composing the rocky envelope have been reduced. The chapter on slaty-cleavage emphasises this, as it does the slowness with which the dynamic forces brought into play by internal heat and pressure act in imposing new structures upon the rocks, and, figuratively speaking, provide the marble from which Nature's chisel carves out those varied features which make the beauty of the landscape. Q. J. G. S., pp. 16-63.

The value of Time as a geological factor in the regeneration and reconstruction of the face of the earth, though it may not at first be apparent, assumes overwhelming importance with the increase of our knowledge of the mysterious yet systematic and purposeful ways of Nature.

CHAPTER XXIII

BEARING OF THE INVESTIGATIONS ON THE SUPPOSED PERMANENCE OF OCEANS AND CONTINENTS

IT will be of interest to consider in what way the

physical and experimental investigations recorded in the preceding chapters affect the question of the permanence of the great oceans and the continents.

But it will be first necessary to ascertain what meaning is attached by the leading geologists who have written upon the subject to the term 'permanence.'

A study of the various views that have from time to time been put forward leaves an impression upon the mind that the controversialists have not all been discussing the same thing.

Dana seems to have originated the idea that continents have been built up about certain nuclei -portions of the crust of the earth that have first hardened--and that they have developed by a process of inorganic growth to their present form. Inferentially this would constitute permanency of the great land masses and the great oceans, but it was not formally stated in that way. Darwin favoured the view that the land masses have been

substantially in their present position since Cambrian times, and very properly condemned the readiness of some naturalists to build land-bridges across the oceans to account for every little difficulty that beset them in the way of geographical distribution of plants and animals.

The naturalist who gave the most definite form and fixity to these speculations on the permanence of the great land areas is undoubtedly Wallace, though in his later speculations he appears willing to admit that a larger area of the globe has been subject to fluctuating continental movements than he at first supposed.

As regards the great depths of the ocean he is still firmly of opinion that they have never been land during the geological periods we are acquainted with. Formerly, for practical purposes, he took the 1,000-fathom line as generally and roughly indicating the separation between the oceanic and continental areas. Further information has led him to consider that the 1,500- or perhaps in a few cases the 2,000-fathom line marks out the deeper and unchangeable portions of the oceanic basins.1

Intermediate between the extreme views held by Wallace on the permanence of continental and oceanic conditions and those of Lyell and the older geologists who considered that every part of the present oceans had once been land and every part

1 "The Permanence of the Great Oceanic Basins' (Natural Science, vol. i. p. 419, 1892).

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