BRITISH POSTGIACIAL &° RECENT DEPOSITS. 409 



Every one knows that while with each flood a stream adds by 

 new deposition to the height of its flood-plain, it continues at 

 the same time to excavate the bed in which it flows, so that by 

 and by, when its channel is sufficiently deepened, some portions 

 of the alluvial flats cease to be subject to floods. But the river 

 sooner or later undermines these heightened terraces, as it winds 

 about, and is ever forming newer flats at lower levels. And 

 should it continue to flow with the same volume and under the 

 same conditions, the newer flats would eventually come to occupy 

 as broad a space as that formerly covered by the older terraces, 

 the latter in fact would be entirely demolished. The fact that 

 the high-level postglacial terraces have not been denuded away, 

 but still in many places form more or less continuous plains on 

 one or both sides of a valley, sufficiently proves that the streams 

 had formerly a much larger sectional area. And the generally 

 coarser character of the older river-deposits is in complete accord 

 with this view. 1 



As we follow the rivers down to the lowlands we shall find 

 the contrast between the modern alluvial flats and the older 

 terraces becoming more and more pronounced. The former 

 have clearly been excavated out of the latter. So extensively 

 and approximately level are many of the more ancient terraces 

 that they have frequently been described as old sea-beaches, 

 notwithstanding that they are manifestly related to the river 

 valleys, that the carry of the gravel-stones is invariably down 

 the valleys, that the arrangement of the stones and the character 

 of the bedding point to the passage of a current of water con- 

 tinuously in the same direction, and that while the deposits 

 have now and then yielded remains of land-plants and molluscs, 



1 The geological reader will understand that the relative levels of the inland- 

 and mountain-districts are assumed to have remained practically unchanged since 

 glacial times. There is not the slightest evidence to show that the inland or hilly- 

 districts, during the Glacial Period, stood at a higher or lower level relatively to 

 the surrounding low grounds than they do now. Such oscillations of sea-level as 

 are proved to have taken place in Postglacial times appear to have affected the 

 country en masse, although there are good grounds for believing that in the 

 extreme north of Scotland there has been a recent submergence which does not 

 seem to have extended far to the south. 



