will only change when the internal forces of the earth, reacting on the sedimentary mantle, develop expansion therein, and by lateral and vertical pressures and movements, oftentimes renewed, develop those ridges of the earth called mountain chains, and so diversify the planet's surface by sketching out new land surfaces where now is sea, thus modifying the form and conditions of the old

continents.

GENERAL CONCLUSIONS

The result of our reflections upon the group of facts which it has been my object to bring together in systematic order tends to show that the growth of land areas of the globe is governed by certain laws of development. The records of the rocks tell us pretty plainly that there must, throughout geologic time, have existed land areas on the globe comparable in extent with those now existing. It is not my intention in this chapter to touch upon that vexed question, the permanence of ocean basins, or to sketch out the lines of former land extension. We have seen that land areas grow by accretion from existing land. The ruins of former continents have added to their extent, so that by process of accretion their outlines and physiographic features have altered; therefore the present continents, though the outgrowth of earlier ones, may be vastly different in form, position, and orography from their predecessors. That the land areas should have been preserved through geologic time,

considering that their mean heights are so little above the water, has always presented itself to my mind as a geological crux. We now see that the waste of the land and the collection of the resultant sediments in the bordering seas nature's means of renewal; and we further gather that continuity of land areas throughout geologic time, so necessary for the preservation of terrestrial life, is in this way secured. The origin of mountain ranges and the growth and decay of continents are thus closely related. New lands are the consequents of sedimentary loading and recurrent expansion, acting through a chain of events which I have dealt with in the 'Origin of Mountain Ranges,' and which seem to me to be the explanation which brings together all the hitherto isolated facts of geology into one comprehensible whole. The history of our planet is not one of fortuitous accident, but of orderly development, the principles of which it is the aim of this work to investigate and establish.

CHAPTER IV

OCEANOGRAPHY

SUB-OCEANIC CONFIGURATION OF THE EARTH'S CRUST

NTRODUCTION. The evolution of the

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graphical forms of the land areas of the globe having been considered, it will be necessary in relation thereto to further study the ocean basins and the complex questions included under the term 'oceanography.'

It is only within the last five-and-twenty years that anything of an accurate nature has been known of the form of the ocean bottom. Surveys and soundings there had been, but these were mostly in shallow depths and near to land.

The Challenger expedition enlarged our knowledge on a systematic scale; but these soundings in the deep seas were necessarily taken at such distances apart that, except as giving a rough idea of the average depth of the ocean on the tracks that the ship followed, little could be inferred.1

With the extension of commerce and the laying of deep-sea cables came the necessity for more

It is necessary to acknowledge the work of our Navy of late years in Pacific deep-sea soundings, as well as the surveys of the U.S. Fish Commission in the Atlantic and the Steamer Blake in the Gulf of Mexico and the Pacific.

accurate knowledge of the form of the ocean bottom; but even now it is only in exceptional cases that anything like a complete set of soundings, sufficient to enable a section to be plotted, is taken. The result of these more complete surveys is, generally, to show that the contours of the ocean bottoms are much more diversified than was formerly inferred. I have pointed out in a former paper that if the present continents were submerged to the depth of some of the great seas, a series of soundings taken as far apart as those of the Challenger would show no more variety in depth than do the Challenger's, and that the inference that the ocean basins are little more than vast plains or saucer-shaped depressions, with here and there volcanic cones protruding above the surface of the waters or stopping short at various depths below, was not based on sufficient data.

The Sub-Oceanic Margin of Spain and West Africa.-Soundings from the Bay of Biscay along the West Coast of Africa reveal greatly diversified submerged ground, which appears to have a considerable effect in inducing deep-sea currents.

In relation to these soundings and the nature of the sea-bottom deposits, Mr. Stallibrass, in a paper on 'Deep-sea Soundings in connection with Submarine Telegraphy,' says, 'Of all bottoms these oozes are to be preferred. The fact of their being found shows that no currents exist in these parts, and

1 'Oceans and Continents,' Geo. Mag., vol. vii. 1880, p. 388.

they are so soft that the cable sinks far down into them. The old idea that currents do not exist at any great depth has long since been rejected. Currents may exist at almost any depth. Between the Canary Isles there are strong currents 1,000 fathoms below the sea face, and their scouring action may be clearly detected.' Mr. J. Y. Buchanan, in the discussion of the paper above referred to, expressed similar views, and added, 'If we find hard ground we know that there must be something to prevent the accumulation of sediment. Now, the only thing that prevents the accumulation of sediment is a current; and one help that telegraph soundings have given to geographical science is the indication that tidal currents exist even at very great depths in the open ocean.' Mr. W. H. Huddleston, in a paper on 'The Eastern Margin of the North Atlantic Basin,' gives a considerable amount of information as to the form of the ocean floor and the gradients of what he terms the sub-oceanic slopes, which, as he shows, are exceptionally steep in the Bay of Biscay off the coast of Spain. Off Rivadeo the 100-fathoms platform is of the usual width for this coastnearly 30 miles, in fact; from the edge of the platform a depth of 1,130 fathoms is obtained in the next 5 miles, giving an incline of about 1 in 4, or about 30°. About 30 miles to the north of Bilbao is a channel from 1,000 to 1,500 fathoms

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1 Journ. of the Soc. of Telegraph Engineers, p. 509 (1887). 2 Geo. Mag., March and April 1899.

3 P. 153.