Whatever influence the geological constitution and lithological nature of the strata and rock-masses in which a valley has been ex cavated, may have upon its form, that form is still independent of what may be termed the geometrical disposition and configuration of those strata and rock-masses. This is equally true of mountains as of valleys. It seems natural to suppose that when a group of strata inclining upwards towards the same line from opposite points form what is denominated an anticlinal, a hill or a mountain should be pro duced; and that when a synclinal is formed by the similar meeting in one lino of a group of strata inclining downwards from opposite points, a valley should be the result ; and in many cases such is the fact. But in many cases also the reverse occurs, valleys being situated en anticlinal arches, and mountains consisting geologically of syn clinal troughs of strata. (Lycll, ' Manual,' p. 57.) Thus it has been recently shown by Sir R. I. Murchison and Mr. Geikie (` Quart. Journ. of Geol. Soc.,' May, 1861), that the enormous mass of Ben Lawers, " like many other mountains in Scotland; as well as elsewhere," actually occupies a synclinal trough, while the deep valley of Loch Tay, like that of the Great Glen through which the Caledonian canal extends, runs along an anticlinal arch. In this manner the geogra phical configuration of the land is often quite different from its geological configuration, or that of its geological elements. In con nection with this subject, it may be useful to advert to the manner in which the terms in question have come to be employed in modern physical geography,' and which has already led to erroneous inferences. When a geographer speaks of an anticlinal line, ridge, or axis, he simply means the ridge formed by the meeting of an upward slope and counterslope, or the imaginary line drawn through or along it; and by a synclinal, in like manner, he merely understands a linear Ilepreamion or valley, formed by the meeting of a downward slope and tseintendope; without reference to the geological constitution of the land having such figures. Thus geographical antielination and syncli (derivatlem employed, we believe, fur the first time by Mr. Brayley). are quite different things from the disposition of groups of strata, designates! by the geologists an anticlinal and synclinal. Thus, also, the slope and counter-slope may consist of strata, either meeting or sloping away from each other ; a difference quite unim portant in geography, but of great moment in geology. Geographers are often not geologists, and borrow appellations from the science of the latter, which they use in a sense, quite correct, of their own, but at the same time quite different from its original one.
Another and frequently occurring case of the position of a valley on an anticlinal axis is the following. In certain dome-shaped hills, or elevated regions, which all consider an having been thrust up by a force from below, there is often an elliptical cavity at the summit, due partly to the fracture of the upraised rocks, but still more to aqueous denudation, as they rose out of the sea. The central cavity is called a ralley of &ration. It exposes to view the subjacent strata or rocks, and the incumbent stratified mass, the central portion of which has thus been removed, dips away on all sides from the axis. The structure and the theory of the production of such valleys were first recognised, and the appellation given to them, by the late Rev. Dr. Buckland, who described a remarkable instance in the valley of Kingselere, in Berkshire, together with others, all presenting the same features of a valley, circumscribed on all sides by an escarpment, whose component strata dip outwards from an anticlinal line, running along the central axis of the valley. The most symmetrical valley of elevation in the
British isles occurs in the Woolhope district in Herefordshire, con sisting of two concentric narrow ranges of hills, almost continuously surrounding a broad, nearly elliptical dome; the lowest and most ancient strata forming the dome, the incumbent strata the including hills. (Buckland, 'Trans. Geol. Soc.,' series II., vol. ii. ; Murchison, Siluria ; J. Phillips, in 3letn. of Geol. Survey,' vol. H.; Lycll, ' Principles,' 1853, p. 421,) The perusal of the article PARALLEL ROADS in the NATURAL HISTORY DIVISION of this work is referred to above (col. 541), as desirable for the student of the history of valleys. Subsequently to the investiga tions of the eminent observers referred to in that article, and also to those of Professor Agassiz, Sir G. S. Mackenzie, Mr. R. Chambers, and others, the structure of the parallel roads of Lochaber has been care fully studied by a distinguished American geologist, Professor henry D. Rogers, F.ILS., who occupies, greatly to the advantage of science, the chair of Natural History iu the University of Glasgow. The results of this study he stated in a discourse delivered at the Royal Institution, in London, on March 22nd of the present year 0861), of which an abstract appSars in tho ' Proceedings ; ' and in which he states that lie has been led by it to reject all the hypotheses hitherto offered in explanation of the terraces, as inadequate, and to recognise in certain phenomena discovered by him, but not before noticed or theoretically considered, a key to the solution of the problem. Of these we will briefly notice the principal, omitting the details, and subjoin the con clusion at which Professor Rogers has arrived.
These parallel roads are apparently level, and therefore parallel, " but further instrumental measurements," Professor Rogers remarks, "are necessary before the question of their absolute horizontality can be regarded as satisfactorily settled ; " on which point, therefore, lie seems to be at variance with Mr. Darwin, as well as with Sir T. L. Dick and Mr. Maclean, by whom, as stated in the former article, they were carefully levelled.
Each " road," " shelf," or " terrace," according to Professor Rogers, is a nearly level, wide, deep groove, in the easily eroded boulder-drift OF diluvium iSURFACE OF THE EARTH] which, to a greater or leas thick ness, everyw iero clothes the sides of the mountains exhibiting them. They vary greatly in their relative distinctness. With scarcely an exception, each terrace or shelf is most deeply imprinted in the hill side, and is broadest, where the surface thus grooved has its aspect down the glen, or towards the Atlantic ; and is faintest where the ground fronts towards the head of the valley, or the German Ocean. While conspicuous on the open sides and the westward sloping shoulders of the hills, the terraces disappear altogether in the recesses or deeper corries which scollop the flanks of the mountains. Each grows usually more and more distinct as it approaches the head of its own special glen, until those of the two opposite sides meet in a round spoon-like point. Each again coincides accurately in level with some " water shed " (1) or notch in the hills leading out from its glen into some other glen or valley adjoining.