close relation that often exists between bending and fracture. The force which produced the anticlinal uplift must have been continued until the strain of bending was relieved by actual fracture and displacement.

4. Mountain Chains.-Mountain ranges, as explained on p. 459, have been formed by successive uplifts, and in many cases there is no evidence that the range has ever been completely submerged since the first uplift took place; nay, if, as seems probable, successive uprisings are actually due to the restoration of hydrostatic equilibrium upon the lightening of the crust by the removal of material, then we may conclude that the process of denudation has been in continual operation ever since the first upheaval of the chain. We should consequently expect to find that a greater thickness of rock had been removed from the summits of mountain ranges than from any other parts of the earth's crust.

It can often be proved that the total amount of rock removed from the central axis of a mountain range is enormous. Captain Dutton has recently estimated the amount of rock removed from the Rocky Mountains of the Colorado region since their first upheaval, and believes it to have averaged 10,000 feet in thickness over an area of 13,000 to 15,000 square miles in extent. The Alps date from a still more recent period than the Rocky Mountains; but the compression to which the rocks composing them have been subjected seems to have been much greater, and therefore the total amount of vertical upheaval along their axis has probably been greater than in the case of the American range; but as their average height is about the same, the amount of denudation must have been greater in the case of the Alps. This inference is borne out by the facts, for the thickness of the sedimentary deposits affected by the upheaval of the Alps, and exposed by denudation, has been estimated at about 50,000 feet; and this thickness may be regarded as the amount of rock removed from the summits of the Alps since their first upheaval.

It is necessary to guard the student against supposing that the removal of 50,000 feet of rock from a mountain range means that the range must have been 50,000 feet high at some period of its history. It need never have

been much higher than it now is, because the two processes of upheaval and degradation have gone on together, and it was only in consequence of the repeated upheavals that so great a thickness of rock was removed. The process may be compared to the uprising of an iceberg out of the water, in proportion as the top is melted away by the sun, or to a piece of machinery, one part of which raises a block of wood or metal, while another part planes off the surface of the material as fast as it is brought within its reach.

Mountain ranges, therefore, differ from other hills as being mountains notwithstanding and in spite of the immense amount of detrition and denudation which has taken place over and around them.

General Conclusions.-From what has been said in this and preceding chapters the reader will be prepared to accept the statement that almost all the external features of the land, hills, glens, valleys, and plains have been produced by erosion. Their form is a sculptured form, carved out of the rocks by the operation of all those agencies which have been described in the earlier chapters of this work. These agents combine to form a great surface-carving machinery, and the disturbing forces which result in the upheaval of land are chiefly efficient in bringing the rock within the influence of the erosive agents, and are seldom productive of any direct effect upon the form of the ground.

Three distinct processes may generally be traced in the development of those physical features by which the surface of the land is diversified.

1. The block or mass of land out of which these features are subsequently carved is upheaved from beneath the sea.

2. Every part of this block as it reaches the sea-level suffers from the erosion of the waves, which prepare the surfaces of planation or marine denudation.

3. The inequalities of hill and vale are then carved out of these surfaces by the atmospheric and fluviatile agencies already described. The only exceptions to the rule that the form of the ground is due to detrition and erosion, are to be found in the alluvial flats and deltas of rivers, in

497 hills of blown sand, and in the cones of active volcanoes on which the rain has not yet produced any appreciable effect.

It must be remembered, however, that in the case of mountain chains, though the forms of the peaks and valleys are the work of erosive agencies, yet the existence of the range itself is due to the force which has caused its elevation. Fig. 149 has been given as a simplified section across such a range, and though, of course, their structure is in reality much more complicated, still the diagram will serve to explain the process of their formation and to show what is meant by hills of upheaval compared with hills of circum-detrition, which have been amply illustrated by the preceding figs. 151, 152, 154. Detrition may have been equally active in both cases, and indeed the amount of material removed may have been greater in the former than in the latter case: but the comparative elevation of the ground is in the latter chiefly caused by the removal of the surrounding parts, while in the former the hills would have been more lofty if the detritive agents had been less active.

To sum up, therefore, the principal features of the earth's surface may be thus classified according to their mode of origin.

Valleys are of one kind only; they have all been formed by the erosive action of rain, springs, and rivers.

Plains are of two kinds: 1, plains of formation; 2, plains of denudation.

Plains of formation are such as are formed by the undisturbed extension of beds which retain the original horizontality of deposition. River deltas, and alluvial flats, fens, marshes, and silted-up lakes are instances of such plains. In this class may also be included the slightly inclined or undulating surfaces sometimes produced by the elevation of a shallow sea-bottom which special local causes have exempted from the action of marine erosion; such are the Pampas of South America, the Tundras of Siberia, large tracts in Russia and Poland, and perhaps also the great Sahara Desert in North Africa.

Plains of denudation are those which have been formed by marine erosion across the edges and outcrops of strata

K K

without reference to their inclination, flexures, or fractures. They are surfaces of planation formed by the march of the sea across the country. The limestone plains of central Ireland may be cited as an instance, the country bordering the rivers Rhine and Moselle is another, and the plains of Cheshire is a third. When elevated to a considerable height above the sea-level, they form what are known as table-lands.

Hills are of three kinds: 1, hills of accumulation; 2, hills of circum-detrition; 3, hills of upheaval.

Hills of accumulation are such as have been formed by the piling-up of materials upon the surface of the ground. Volcanoes are the most important instances; these are conical piles formed by the ejection of ashes and lava streams (see fig. 1). Sand hills heaped up by the action of wind on drifting sand come under the same category.

Hills of circum-detrition are such as have been left by the removal of the surrounding rocks, or are isolated by valleys of erosion. They have been illustrated by figs. 151, 154.

Hills of upheaval are those formed by the elevation of a central axis faster than it can be worn down by detritive agencies, see fig. 149.