272 
ME. PRESTWICH ON THE GEOLOGY OF THE DEPOSITS 
shows the Testacea to have generally lived on the spot; and the sharp and entire state of the 
greater part of the fossil bones shows that they have rarely been rolled far or subjected 
to much violence*. The deposits we have noticed might be, it is true, lake deposits so 
far as these organic remains are concerned ; but the continuity and the distant transport 
of the gravel, and its uniform relative wear, point to water with a course, in each case, in 
one direction of commensurate length ; while the absence of the remains of any inde- 
pendent basins, depressions, or barriers in the high-level valley, referable to old lakes, 
indicate that even the oldest beds are due rather to early river-action — the rivers, how- 
ever, having been necessarily broader at some places than at others. 
§ 4. THE BEIGE-EARTH OR LOESS. 
All the valleys of the South-east of England contain brick-earth forming banks on 
their sides and often beds at various heights on their slopes. This is particularly appa- 
rent in the valleys of the Stour, the Medway, and the Thames. In France it is seen on a 
larger scale in the valleys of the Somme and the Seine and of their tributaries. This 
deposit contains occasionally some land-, and rarely a few freshwater shells, and it is in 
almost every respect identical with the Loess of the valley of the Rhine. This well-known 
formation has been described as an independent deposit by various geologists, amongst 
others by Sir Charles Lyell f, who concluded that it was a fluviatile silt deposited by 
that river after its hydrographical basin had acquired very nearly its present outline of 
hill and valley; and he explained the height (near Basle 1200 feet above the sea) to 
which it rises above the valley by supposing a filling up of the main and lateral valleys 
by river silt during a period of subsidence, and a re-excavation in part during a subse- 
quent upheaval. In many respects the analogy established by this distinguished authority 
between this ancient river-deposit and that of the plain of the Mississippi, and his argu- 
ment in favour of its fluviatile character, are satisfactory and conclusive. 
The difficulty I have felt in applying this hypothesis in its entirety to certain valleys 
in the South of England, and more especially to the valleys of the Somme and the 
Seine, has been the great height to which the Loess rises above the levels of these 
rivers, and the impossibility, for reasons assigned at p. 265, of admitting such a deposition 
of silt to have taken place since their present basins had been formed ; nor would it be 
easy to conceive pre-existing depressions, of the depth we shall have to speak of, to have 
been filled up without an influx of the sea further inland than at present, and a conversion 
of the river-valleys into narrow estuaries far beyond existing tidal influences. But the 
evidence is rather in a contrary direction. The quaternary deposits adjoining the 
estuary of the Thames show conditions more purely freshwater than those which now 
obtain in the adjoining waters. The Loess at Menchecourt contains no trace of Qiarine 
organisms ; and in the valley of the Seine we have recently found freshwater shells in the 
low-level valley-gravels of Rouen. At the same time a subsidence so regulated as to be 
* There are, however, a sufficient number of rolled fragments to show wear in river-shingle. 
t Manual of Elem. Geol., 5th edit. p. 122-5. By some it has been considered a glacier mud. 
