WATERFALLS. In the article VALLEYS we have arranged a general view of the main features of the earth's surface, and a series of inferences touching the forces whereby the diversified forms of hills and valleys have been occasioned. But these forms, though on a large scale they appear permanent, because the great modifying agencies which produced them have passed away, are really undergoing continual change from causes in daily operation. The most solid stone is wasted by the feeble but unceasing power of decomposition possessed by the atmosphere. Rain washes away the disintegrations occasioned by varying temperature and chemical processes; the hills lose and the valleys gain, and the balance of decay and renewal of land is only finally adjusted on the shores and in the basin of the sea. Among the phenomena which show this mutability of the supposed solid land with most distinctness, are interruptions to the general uniformity of the inclinations of valleys and the even slopes of hills : for these changes of slope are.pointe of variation of the intensity of the agencies excited by the slope. These interruptions of uniformity are all referrible to the unequal power of resistance which rocks of different hardness, or dissimilar position, or unequal thickness, or unlike modes of association present to external agencies. Thus have been formed round the high limestone hills of the northern counties a series of rocky terraces, not leis regular than the escarpments made by military art ; and thus the oolitic ranges of the Cotswold show horizontal mounds of sand and cliffs of stone above the broad plains of has clays and red marls which margin the Severn and the Avon. On these grand features of the earth's surface the action of the atmosphere (including chemical and mechanical operations) produces only slight modifications ; but when the terraced slopes in their flexures round the bills turn into the valleys, anew agency is_brought to work upon them. Rivulets, however small in quantity, and torrents, even such as are of only temporary energy, exert a positive influence in wasting and trans porting away earthy materials; and these effects rise to a maximum wherever, from any of the causes already alluded to, the surface of the earth presents successive points of leas and greater power to resist the action of running water. Wherever, in a valley whose slope is con siderable, the rocky mane successively crossed by the stream arc of very unequal hardness, as, for example, when solid limestone is found resting un soft shale or feebly indurated sandstone, a more than ordinarily rapid current is occasioned over the lower beds of the lime stone into the upper beds of the shale. This difference of slope in the running water is of a nature to increase continually to a certain point, depending on the relative firmness and thickness of the hard and soft rocks, the inclination of the valley, the magnitude of the stream, and other less important particulars. Thus rapids and cataracts are formed;
and where the conditions combine in the must favourable degree water falls, to use that term in a more specific sense, are produced.
The character of these varies according to the disposition of the yielding and resisting portions of the rocks. Wherever stratification is absent, as in granite, or concealed, as in some metamorphic slates, the main features of the waterfall are determined by the direction of the natural joints in the stone. Hence the picturesque character of the falls of the Bruer (Highlands), Lodore (Cumberland), and the ltteciddial (North Wales). In memo camas these natural joints yield in parallel lines, and give a deep narrow passage to the water. Scale Force, in Cumberland, is an example. But the most interesting, if not the most picturesque, class of waterfalls, is occasioned by the stratified rocks; and the most curious of them are observed where bard lime stones ur gritstones rest upon yielding shales or soft clays. By the continual action of the stream, the shales, kept constantly damp, crumble and fall away even at considerable heights and distances from the points where they are touched by the water. Thus, a hollow space is formed beneath the limestone which crowns the precipice; and this proceeds so far as to reach at last some of the natural joints which divide the ruck. nun the limestone falls down, the waterfall recedes, and the process of removal and destruction is renewed. Thus, on the aides of the hills, in the limestone dales of the north in counties of England, the waterfalls ale daily receding up tho streams, and thus are the Falls of Niagara forced continually farther up the river. The pro cess is by no means slow. Beneath Hardrow Force, in Yorkshire (a fall of Pa feet), the effect since the general valley of the Yoro was exca 'rated by other forces has been to produce a sinuous glen within steep vertical walls of rock, at the foot of which yet lie great heap. of fallen materials, which the feeble stream that formed the glen has not been powerful enouill to remove. For an account of the natural processes by which the halls of Niagara have been displaced, and are still under going change, the volumes of Sir C. Lyell (' Principles of Geology ;' Travels in North Americn ; ' the former of which are instructive on all points connected with the operations of running water) may be consulted. Exactly such effects as are hero attributed to running streams happen on the seacoasts where rocks of a particular nature occur under analogous circumstances.