is to be seen in high mountains, whose tops are riven peaks, and whose sides lie shivered where they fell."1 Captain Beechey in his "Voyage towards the North Pole" describes the amount of this action as very great in Spitzbergen. He found that the mountain sides were undergoing rapid disintegration from the absorption of wet during summer and its expansion by frost in winter. "Masses of rock were in consequence repeatedly detached from the hills, accompanied by a loud report, and falling from a great height were shattered to fragments at the base of the mountain, there to undergo a more active disintegration." 2 In describing the various erosive agencies now acting upon the slopes and summits of the Rocky Mountains, Mr. Clarence King thus describes the work done by frost and changes of temperature on the summit peaks: "The whole peak region is seen to be riven with innumerable cracks, which are due to unequal expansion and contraction in a region alternately chilled by radiation and warmed by the sun. Upon the steep slopes and sharp blade-like ridges, the results of such fissuring, together with the leverage of expanding ice, have the effect to dislodge large fragments of rock, and produce immense slopes of débris." He observes that this action takes place at all hours of the day, "but especially when a sudden chill (as during the hour after sunset) has the effect of congealing the percolating waters;" and he adds-"Upon the summits of the Rocky Mountains, the Uinta, and Wahsatch, and at very many points of the ranges near the Pacific coast, I have heard during the day thousands of blocks dislodge themselves and bound down the slopes. The resulting accumulations of débris form a very conspicuous feature throughout the Cordilleras. In many instances they must amount to fully 1,000 feet in thickness. In the Sierra Nevada, where all these phenomena are on a grander scale, I have seen débris-slopes measuring 4,000 feet from top to bottom." The summits of the Alps present similar appearances, those which are not covered with perpetual snow termi"Frost and Fire," vol. ii. p. 181. 2 (6 Exploration of the 40th Parallel," vol. i. p. 471. nating in jagged peaks and narrow points which are sometimes called aiguilles or needles. On these exposed peaks disintegration proceeds at a rapid rate, and showers of stones fall continually night and day, but especially in the afternoon when the sun's rays are hottest and the expanding force is consequently greatest. The accompanying sketch, fig. 17, representing the forms of some of these peaks, is taken, by permission, from Prof. Bonney's "Alpine Regions." General Results of Atmospheric Detrition.-Of all the disintegrating agencies above mentioned, rain is the most important and universal; there are very few districts on the face of the earth which are absolutely rainless, and wherever it falls it operates both chemically and mechanically over large areas at once. There are, therefore, few rock-surfaces in the world which are not affected by this agency, assisted in most cases either by extreme heat or extreme cold, or by both combined. G The consequence of the erosive action of these atmospheric agencies is the production of a superficial covering of disintegrated rock or soil; but this soil is being continually removed from all sloping surfaces by the mechanical action of rain, and is thus constantly travelling to lower and lower levels, while fresh rock-surfaces above are being as constantly exposed to further disintegration by the weather. The general result of this continual action is that all land surfaces are being slowly, but surely, wasted away, and the soil, particle by particle, is being carried away into the rivers, and by them into the sea. It is clear, therefore, that were there no counteracting influences of upheaval, such as have been mentioned in Chapter IV., the continents of the primeval world would have been long ago destroyed, and a universal but shallow ocean would now envelope the surface of the globe. This decay and wasting of the surface is, however, by no means uniform; it is most rapid in countries which have a large rainfall, and least appreciable in those which enjoy a dry and equable climate, like that of Egypt. Moreover, it is always greater in mountainous regions than on plains and gentle slopes. Again, as we have seen, some rocks yield much more rapidly than others, and this difference in the rate of waste produces inequalities in the surface, which are especially noticeable where two rocks of very different constitution are brought into conjunction with one another at the surface; one will always be worn down to a lower level than the other. It should also be remarked that the rapidity with which different rocks yield to the weathering agencies has no relation to their hardness. Where other conditions are equal, soft clay resists their action better than the hardest limestone, because the latter yields so readily to the chemical action of carbonated waters which have no effect upon the former. We have seen, too, that even such hard rocks as granite and gneiss are often decomposed and disintegrated in situ to an astonishing depth. CHAPTER VI. UNDERGROUND CIRCULATION OF WATER. A LARGE proportion of the rain-water which falls upon the earth sinks beneath the surface of the soil, and after percolating through the rocks below for a greater or less distance, is thrown out again in the form of springs. The depth to which it sinks depends on the nature of the rocks which it encounters in its downward passage. Some rocks are much more pervious than others; sand and sandstone are pervious by reason of their loose and porous nature; limestone and other hard rocks by reason of the numerous cracks and joints by which they are traversed; while all kinds of clay and shale are comparatively impervious to water, though their constant wetness shows that they are not absolutely impermeable. Čauses of Springs.-The position of springs is therefore always determined by the nature and relative position of the rock-beds, and is generally due to one of the following causes. The underground water is brought again to the surface 1. By meeting with an impervious stratum. 2. By meeting with a fault or line of displacement. 1. If rain falls on a pervious rock it descends vertically, but if it reaches a more impervious stratum below, it flows laterally along the top of this bed and breaks out in springs along the line where this comes to the surface of the ground. The strength of the resulting springs will depend upon the relation between the slope of the ground and the slope of the rock-beds beneath. Fig. 18 is an imaginary cut or section through a hill composed of sand or soft sandstone (a, a), underlaid by a stratum of clay (c, c), the surface of which slopes (or dips) slightly towards the west. It is clear that the rain which falls on this hill will pass down through the sand and flowing outwards along the surface of the clay will issue in springs along a line on the western side of the hill, s', being a point on this line. That there should also be a spring at s'on the eastern side is not at first sight so obvious, since the fall of the beds is towards, s', and water cannot run up hill: it must be remembered, however, that in passing downwards and outwards through the sandstone, the water will have to overcome a considerable amount of friction, and that this friction will tend to hold the water up under the hill. If, therefore, the rainfall be tolerably constant, the water will be accumulated in the lower portion of the sandstone more quickly than it Fig. 18. Origin of Springs. The line of saturation is indicated by the dotted line. can escape by the springs, and consequently a certain thickness of the rock will be in a state of constant saturation. The thickness of the saturated portion will decrease towards the sides where the water finds egress, and its upper limit will form a curved line; this is called the line of saturation, and when this line rises above the level of the point s2, the hydrostatic pressure will force the water outwards, and cause a spring at that point as well as at s'. The former, however, will of course be a weaker spring, and will be the first to fail in a dry season, when the line of saturation under the hill is gradually depressed. 2. The instance above given is a very simple case, though it is one of frequent occurrence in nature. In other cases, however, strong springs do not occur where they might be expected on the above principle, the regularity of the underground waterflow being interfered with by the existence of |