Valleys which like these twist about in the same regular curves as the channel of a brook meandering through a meadow, can only be accounted for by the slow and long continued erosion of the streams that still flow in them, increased at intervals by wintry floods. To attribute them to a transient and tremendous rush of water in the main direction of the valley appears to be impossible. Whilst these valleys were slowly excavated, other rivers, during the same protracted period, will have produced likewise an amount of excavation propor tioned to their volume and velocity. and the nature of the rocks they flowed over. In the examples cited above, the rocks are mostly hard strata, yet the valleys are wide and deep. Where softer strata, as sands, clays, and marls, were the materials worked upon, the valleys excavated may be expected, as they are found to be, far wider in pro portion to the volume of water flowing through them. The comparative softness of the materials also, by accelerating the lateral erosion of the stream, will have multiplied the shiftings of its channel, and reduced their sum with greater certainty to one average direction. Hence the deeply sinuous valleys, such as those particularised, are only found penetrating the more solid rock formations. Mr. Scrope con cludes by a confirmation of his opinion that extreme curvature of channel can only be produced by a slow and comparatively tranquil process of excavation, by a reference to mountainous districts, in which, where the torrents and rivers are most rapid, their course is nearly straight ; from which a legitimate converse induction arises, that a certain subdued velocity in the stream is necessary to produce the former result.
It may deserve investigation, whether the spiral course of the rivers and valleys of eastern Africa, noticed in the geographical portion of this article, may not have originated in the windings of the rivers in a former and very different geological condition of the country, of which the actual curves are the remains, after the elevation of the land into its present form.
In a paper on the elevation and denudation of the lake district of Cumberland and Westmoreland (` Quart. Journ. of Geol. Soc.,' vol. iv., pp. 70-98), Mr. Hopkins has investigated and apparently solved the problem of the origin and process of formation of the lake valleys; throwing great light on the history of similar valleys in other countries. The probable origin of the lakes, it may be remarked, that is, in the first instance, of their valleys, in diverging dislocations, was first suggested by l'rofessor Sedgwick, but Mr. Hopkins has placed the argument in its favour, for the first time, in a determinate and demon strative form.
It would appear impossible not to ascribe the origin of the lakes of Coniston and Windermere, for example, to the disloca tions of strata, with which they are so immediately associated. Mr. Hopkins has described an enormous dislocation of the band of lime stone interstratified with the older palxozoic rocks seen just above Coniston Water, producing a horizontal displacement of about a mile. The direction of the fault, as determined by a line joining the extremi ties of the dislocated portion of the limestone band, passes exactly down the lake. Another fault ranges down the valley of Troutbeck,
as indicated by a dislocation of the limestone band, and a great hori sontal displacement. It ranges accurately with that part of the lake of 1Vindermere which lies to the south of the embouchure of the valley. On the east of Troutbeck also, there are dislocations evidently connected with the formation of the two striking valleys of Troutbeck and Kentmere. The lino of dislocation would seem to pass exactly along that part of the latter valley in which the mere is situated. Indeed, according to the geologist whose views we are reciting, the existence of any of the larger lakes cannot be accounted for in dependently of similar dislocations. Taking Wastwater, for instance, its depth is found to be forty-five fathoms, so that its bottom is pro bably almost a hundred feet lower than the level of the sea. It is evident that such a basin could not be scooped out by the action of water; nor is its depth increased by an accumulation of detritus at the mouth of the valley, for the river by which its surplus water is discharged cuts into the solid rock. The lake (and its including valley), could only be formed. therefore, by a relative subsidence of its bottom. the strata being relatively displaced on opposite aides of the fault. If. in any such case, this relative subsidence do not extend to the mouth of the valley, or be less there than in the upper part of it, a lake will necessarily be formed. This general explanation will apply to all the lakes of the district.
The lakes, Mr. Hopkins concludes, are thus only "the secondary and accidental consequences of the faults with which they are associated, the primary effects being the valleys in which those lakes are situated ; for, whatever may have been the agency by which the masses once occupying those valleys have been removed, it is easy to see that it would act more efficiently along lines of dislocation than elsewhere; and since the existence of dislocations along the lake valleys may be considered as established, it would seem impossible to avoid the conclusion, that those valleys must themselves have originated in such dislocations. We are thus led to conclude that a disloca tion was produced before the valley began to be formed ; that this led to the formation of the valley by denuding causes ; and that the subsidence which caused the lake was one of the last of that series of repeated disturbances which might occur during the long interval of time which was probably necessary for the completion of the valley. . . . . This view of the origin of valleys of this kind must be con sidered as applicable principally in places nearest the centres or axes of elevation. In other cases they may have arisen altogether from aqueous action ; or, when they originated in dislocations, they may have had their directions so altered, and their character so modified by denuding causes, as to retain no distinct traces of their origin." From these illustrations, derived from local phenomena, we return to a part of the general history of the subject, in which again physical geography and geology are united, but remain distinct..