In all cases, and in every country, it appears that, notwithstanding the operation of later agencies, the main features of the surface of the land are due to the positions in which subterranean movements left the displaced masses of rocks. But the operation of subsequent agencies is distinctly traceable in modifications of these features on the sea-coast, and in the interior of every country.
The surface of the land has been scooted, and as the various stony and earthy masses which come to the surface have uneqnal compact ness, and are unequally capable of resisting the chemical and mechanical agencies which originate in the varying heat and moisture of the atmo sphere, we find in consequence a multitude of irregularities, both on a large and small scale, directly related to the properties of the rocks. On the sea-coaat souse parts are known to be wasted (as the coasts of Sheppey, Dunwich, and Bridlington) even rapidly, one or several yards annually on the average ; others seem almost unchanged by a thousand years of storm and tempest, as the " Worm's }lead ; " and some considerable tracts of new land have been added to the shores of Lincolnshire, along the banks of the Thames, and by the side of the Severn.
The line of meat from the Tyne to the Humber is instructive in this respect. The prominences on the Durham coast, ending with Hartlepool, are guarded by magnesian limestone, and the eatuary the Tees is excavated in red marl and has clays. The peak near Robin Hood's Bay, Scarborough Castle Hill, Filey Brig, and Flamborough Head, are all promontories of hard rocks ; but Robin Hood's Bay, Scarborough Bay, Filey Bay, and Bridlington Bay, are all excavated and wasted in clays of the liassic, oolitic, and diluvial periods. The interior of the country shows similar effects on a grander scale. The great vales and plains of England, in parts the least influenced by subterranean disturbances, are by no means the excavated paths of rivers, nor are the great ranges of hills the separating summits between such rivers. The plains and vales are lines of soft and perishable strata, and the crests which divide these vales are ranges of harder rocks. A transverse section of the English strata shows always, both on a large and small scale, this important fact (see So. 1), and every well-shaded topographical map, coloured geologically, strates the extent of its application in explaining the irregularity of surface. The chalk hills, oolitic hills, kc., alternating with vales of clay, in all the southern and eastern parts of England, give to those parts characters far more important than the undulations connected with river channels.
Similarly, hills and valleys, in which rocks of unequal power of resisting watery action appear, show the force and continuity of this action by the prominence of the hard rocks and the excavated surfaces of the softer masses. Thus, in fig. 2, we see on the breast and edges of a hill composed of limestones, sandstones, and shales, the especial prominence of the limestones ; and where these cross a valley, each limestone edge is the place of a waterfall. By studying in such valleys the manlier in which the actual stream wastes the rocks, we can easily assure ourselves of the truth of the general explanation offered above. In fig. 3 is a section of a waterfall, showing the edge of limestone (a) over which the water falls, and under it a bed of sandstone (b) little wasted, but at the bottom a body of shale which has perished by the dampness and spray, and is excavated in a remarkable manner.
Just such an action is observable on similar cliffs by the sea, and in each case the same effect follows : the falling of the hard rock at top from want of support below. Thus the situation of a waterfall is daily displaced, and is moving up the stream, as the Falls of Niagara and Hardrow Force are known to have done. (Lyell,` Principles of Geology.') Into all these effects of waste on the earth's surface rain enters for something important. Few surfaces of rock are altogether exempt from chemical changes dependent on atmospheric variations ; all are more or less liable to perish with rain, frost, and watery movements ; and thus the individual features of hills and valleys, the ranges of high and low ground, and the outlines of land and sea, appear to be effects impressed by subterranean movements and fractures of the earth's crust, modified by the action of the sea on materials of unequal resisting power, while they were below, and while they were rising through its waters, and by the subsequent mechanical agencies of rivers, rains, and chemical forces excited by atmospheric variations. [GEOLOGY, in NAT. HIST. Div.] Mr. G. Poulett Scrope, M.P., F.R.S., the author of some precious contributions to igneous geology, of the most original kind, but which, after the lapse of more than the third part of a century, are only beginning to be duly appreciated, has urged, In Geology and Volatnee of Central France,' first published In 1S26, and in a paper on the excavation of certain valleys, communicated to the Geological Society a few years after, the conclusive nature of the evidence of the immense changes in the surface of the earth which have been effected by the action of " meteoric agents"—that is, of the atmosphere, its depositions, and its ever-varying temperature,—together with that of the streams they produce, upon the different rocks and materials eon stituting the land, and which, in his judgment, entirely supersede the necessity for referring them to the agency of mighty overwhelming flooda and currents, however occasioned. Describing the volcanic
formations of the Velay, and in particular the position and actual state of a current of lava, which has spread itself to the width of five, seven, and nine miles, covering an extensive and elevated table-land formerly called the Coiron, which consists of Jurassic or oolitic strata, he shows that there cannot be the least doubt but that the whole of this lofty tract of secondary strata has been solely preserved from destruction by its volcanic capping. The remainder of the oolitic formation around, forming the secondary district of the Rhone valley, has been eaten into in all directions by various mountain torrents, and gnawed down by meteoric abrasion to a far lower level. The immense quantity of matter which must have been thus abstracted from the oolitic mass since the epoch at which this lava was emitted from the now extinct volcano of the Mout Mezen—an epoch proved to be but recent by the fact that, beneath the basalt which it has become, a vegetable soil is found, containing terrestrial shells of a species still existing in the same country—" cannot but strike us with astonishment. There can be no doubt," Mr. Scrope continues, "that the surface on which the b waft of the Coiron rests was at that period the lowest of the neigh bouring levels, or these repeated currents of liquid [flowing] matter could not have flowed in its direction ; yet at present this same surface vastly overtop, every other height of the same formation, and ranges upwards of a thousand feet above the average level of the valley-basins of the Anleche and IthOne on either side. That a considerable pro portion of these was excavated by 'rain and rivers,'—in other words, by meteoric agency such as is still in operation, and not by any diluvial or general flood,—is susceptible of direct proof. To attribute, there fore, the remainder to any other cause of an hypothetical nature unsupported by evidence, would seem to be contrary to the rules of analogy. But the conclusion that the greater portion of the valley of the Rhone has been so recently excavated, and by such agency alone, involves important consequences, since the same agents must have been at work everywhere else, and produced results as stupendous during the same comparatively recent period." (' G. & V. of Central France,' ch. viii.) In the second edition of this work, published in 1858 (ch. ix., pp. 207-208, note), the author expresses his opinion that neither Sir C. Lyell (the most comprehensive advocate for the sufficiency of existing causes) nor the bulk of geologists are " even yet sufficiently impressed with the immense amount of excavation or denudation effected on supra-marine land by the erosive force of the pluvial and fluvial waters." "That story," be adds, "is yet to be written." There are, however, limitations to these changes in the present condition of the earth's surface, some of which are ultimately imposed upon them by their own operation. They were acutely urged by the late Rev. Dr. W. D. Conybeare, a consummate geologist of his time, in his introduction to the 'Outlines of the Geology of England and Wales,' by himself and the late Mr. W. Phillips, and in some papers in the ' Philosophical Magazine' consequent upon the first appearance of Sir C. Lyell's Principles of Geology.' From thirty to forty years have elapsed since the publication of his views, which have been almost forgotten, on which account we cite, from the publication first men tioned, some of the moat apposite :—" Over a great part of the earth's surface the influence of these wasting causes is absolutely null, the mantle of greensward that invests it being an effectual protection. The burrows of the aboriginal Britons, after a lapse of certainly little less, and in many instances probably more, than two decades of centuries, retain very generally all the pristine sharpness of their out line; nor is the slight fosse that sometimes surrounds them in any degree filled up. Causes, then, which in two thousand years have not affected in any perceptible manner these small tumuli, so often scattered in very exposed situations over the crests of our hills, can have exerted no very great influence on the mass of these hills them selves in any assignable portion of time which even the imagination of a theorist can allow itself to conceive ; and where circumstances are favourable to a greater degree of waste, still there is often a tendency to approach a maximum at which farther waste will be checked : the abrupt cliff will at last become a slope, and that slope become defended by its grassy coat of proof. It should appear that even the action of the sea, certainly the most powerful and important of all those we have surveyed, has a similar tendency to impose a limit to its own ravagssi." Dr. Conybeare also considers, in relation to the permanence of the general surface of the globe, the possible effect on the level of the sea of the quantity of materials carried into it, which lie conclude, to be absolutely imperceptible—a conclusion amply justified by the recent investigation of Mr. A. Tyler, as noticed in the article SEA, col. 418.