side Ireland, at a distance of about 50 miles from its western coast, thence it trends north-eastward, outside the Hebrides and the Shetland Islands. Outside these limits the sea-bottom plunges rapidly down to the 500 and 1,000 fathom lines, and it is therefore supposed that the European continent at one time extended westward as far as the boundary of this submarine platform, and that this platform was formed during periods of subsidence, when the sea ate its way farther and farther across the land, planing down its prominences and filling up its hollows until the movement ceased and the present configuration of land and water was produced. If now upheaved the platform would form an undulating plateau, like a Siberian steppe, only diversified by occasional lakes on the sites of the deeper holes which exist on the seabottom. Marine Erosion compared with Subaerial Detrition. In any comparison between the relative amount of erosion performed by the sea, and that accomplished by the other detritive agencies at work upon the surface of the land, the above considerations must be borne in mind. At the present time, where the land is stationary or rising, the sea can only act on the edge of the land, so that the extent of marine erosion must be comparatively small as compared with that effected by the subaerial agencies which act upon every foot of its surface. Dr. Geikie has calculated that if the sea eats away the edge of a continent at the rate of 10 feet in a century, and subaërial agents remove a layer one foot thick from its general surface in 6,000 years, the whole continent of Europe would be entirely worn away by the latter agencies before the sea could pare off more than a mere marginal strip of land 150 miles in breadth. But, as he truly says, "In estimating the amount of influence to be attributed to each of the denuding agents in past times, we require to take into account the complicated effects which would arise from the upheaval or depression of the earth's crust. If frequent risings of the land or 1 Geikie, "Text Book of Geology," third edition, p. 448. elevations of the sea-floor into land had not taken place in the geological past, no great thickness of stratified rocks could have been formed, for the first continents must soon have been washed away." It also follows that the areas upheaved must have been more considerable than the areas depressed, and therefore, even in past times, the amount of detrition effected by rain, frost, springs and rivers, must have always been greater than that accomplished by the sea. CHAPTER III. THE ORIGIN OF SOME SPECIAL PHYSICAL FEATURES. IN N the first chapter we discussed some of the conditions which have guided the erosive agencies in selecting certain lines of action, and in developing certain general features of the earth's surface; we now propose to pursue the subject and to explain the origin of some of the special features and local peculiarities which meet the eye of the observer among mountains, highlands, and table-lands. Mountain peaks may be defined as the sharpest forms of watersheds, produced by the continuous scarping of hard rocks at altitudes where the atmospheric agencies work with the greatest power. Their peculiar shapes depend chiefly on the kind of material out of which they are carved. Almost every kind of rock produces its own special kind of mountain scenery, and the outlines presented by peaks of granite, quartzite, schist, slate, sandstone and limestone differ so greatly from one another that an experienced geologist can often make a good guess at the kind of rock which forms a mountain peak long before he reaches it. Granite hills are always broad, round-shouldered heights, only here and there forming anything which can be called a peak, these consisting of loose weathered blocks, piled one on another, and often resembling masses of ruined masonry. Hills of gneiss are similar, but often have steeper slopes. Quartzite hills always form sharp triangular ridges or conical peaks, which often have some resemblance to volcanoes, but generally have steeper and straighter outlines. Schiehallien, in Perthshire, is a notable instance of a quartzite hill; seen from the south-east it appears as a rocky ridge with a long gentle slope on the eastern side, and a steep descent toward the west, but viewed from the north-west it appears as a perfect cone, "raising its gleaming peak of snowy quartzite to a height of 3,547 feet.' But "nowhere in the Highlands," says Sir Arch. Geikie,2 "can the whole of the distinctive features of quartzite scenery be seen on so grand a scale as among the mountains of Islay and Jura. In the latter island the quartz-rock rises into the group of lofty cones known as the Paps of Jura, 2,571 feet above the sea, which almost washes their base. The prevailing colour is grey, save here and there where a mass glistens white as if it were snow; and as the vegetation is exceedingly scanty, the character of the rock and its influence on the landscape can be seen to every advantage." Hard sandstones and grits, when nearly horizontal, have also a tendency to form huge single pyramidal mountains like Morven and Maida Pap in Caithness, and the wonderful groups of isolated sandstone mountains which are such conspicuous objects in the west of Sutherland and Ross. To quote again from Sir Arch. Geikie's admirable descriptions of Scottish scenery, "these huge pyramids, rising to heights of between 2,000 and 4,000 feet, consist of dark red strata so little inclined that their edges can be traced by the eye in long level bars on the steeper hillsides and precipices, like lines of masonry. . . . . These huge isolated cones are among the most striking memorials of denudation to be seen in the British Islands. Quenaig, Canisp, Suilven, Coulmore, and the hills of Coygach, Dundonald, Loch Maree and Torridon are merely detached patches of a formation, not less than 7,000 or 8,000 feet thick, which once spread over the north-west of Scotland" (op. cit., p. 203). When the sandstone strata have a decided dip or inclination the conical form of peak is more or less modified, such hills generally breaking into a steep precipitous slope on the outcrop side, and often forming what is termed 1 "The Scenery of Scotland," second edition, p. 205. a mural escarpment that is so steep as to resemble a wall, while the opposite side of the ridge falls away in a longer slope, which tends to become a dip-slope, that is, the surface of the ground often coincides with the surface of a particular bed for some distance. Hard and massive limestones generally develop tall columnar or turret-like peaks, the forms depending partly on the horizontality or inclination of the beds, but more especially on the lines of jointing. Sometimes the general trend of a limestone mountain is determined by the direction of one set of joints, while its separation into blocks and peaks is determined by another set having a transverse direction. This is well illustrated in the case of the Drei Zinnen, an excellent view of which forms the frontispiece to this volume.' These are peaks of hard Dolomite, and form part of the region of the Dolomite Mountains in the Tyrol. The view from this very spot has been thus briefly described:-"From Landro, Monte Cristallo, shooting up in towers between its glaciers, is the dominating object. Not far from the post-house, however, through a gorge opening eastward there is an apparition of three splintered spires (the Drei Zinnen, or three battlements) than which in the way of mountains I know nothing stranger.' The hard crystalline schists form mountain crests which often present a sharp and serrated array of peaks. Such serrated ridges occur among the gnarled schists which tower into the mountain heights of Inverness; and the rough, rugged, and craggy outline which their summits exhibit may be ascribed partly to the influence of foliation and jointing, and partly to the unequal weathering due to the differences of hardness and texture in the rock material. Fig. 200 is a view in the Graian Alps, borrowed from Professor Bonney's "Alpine Regions," and shows a peak composed of chloritic and micaceous schists which belong to the upper part of the crystalline series of the Alps. Many of the eminences, which are termed aiguilles in Swit This plate is borrowed from Professor Bonney's " Alpine Regions," and was drawn by Mr. Whymper from a sketch by Professor Bonney. 2 "The Dolomite Mountains." by Messrs. Gilbert and Churchill. |