34 6 
Superficial Gravels and Clays. 
[July, 
and sometimes one-half of the whole mass seemed to be 
shells. There were three species of Unio and the little 
Neritina fluviatilis especially abundant. When gravel from 
the bed of the Thames is now dredged up, it also contains 
many shells. In the sections exposed in 1875, during the 
extension of the embankment west of the Houses of Par- 
liament, the bed of the river was seen to be crowded with 
shells, principally Unio pictorum and Bithynia tentaculata. 
Now, it is another objection to Prof. Prestwich’s theory that 
the gravels that he supposes were deposited in the old river 
do not contain river-shells. I never saw even the fragment 
of a recent species of river mollusk in all the sections I 
have examined around Ealing, and none have been recorded 
by others. At Brentford, when we get down nearly to the 
level of the present river, drifted shells are often met with 
in the gravel, hut they have evidently been obtained from 
the older sands of the pre-diluvial river, and do not belong 
to the time of the deposition of the gravels. 
The brick-clay that overlies the gravel is considered by 
Prof. Prestwich to be inundation mud. We have silty de- 
posits now forming or lately formed on the low flats adjoining 
the Thames : they consist of dark blue clay, with remains 
of vegetation and land and fresh-water shells. Sometimes 
peat beds occur. The brick-clay, on the other hand, is a 
homogeneous unstratified brown clay, without any organic 
remains. We have thus the formation of two deposits, the 
valley gravels and the brick-clays, ascribed to river aCtion, 
and yet, in all the sections exposed, differing in most im- 
portant particulars from the sediments now being deposited 
by rivers. 
The absence of these remains from some of the beds 
might be explained, but it seems impossible to believe that 
the excavation of the valley could have progressed during 
several thousands of years, and the river everywhere have 
left behind it thick deposits of gravel and clay, without any 
river shells. The preservation of the fragmentary shells at 
Brentford, derived from the older sands, proves that their 
absence is not due to their destruction since the outspread 
of the gravels. 
The structure of the deposits is opposed to the origin 
claimed for them. A continuous sheet of gravel covered by 
another of clay, spread over the slopes of a valley, is not 
what we should have expected. The river, in wandering 
from side to side of the wide valley it was wearing down, 
should often have cut into and truncated the deposits it had 
left at a higher level, and could not have exactly joined on 
