33 
and in the valley of the Thames from Oxford downward to its termination 
below London.' "* 
The date therefore of their introduction into the valley must be placed 
somewhere in the long interval that elapsed between the deposition of the 
upper and middle gravels, and the means may perhaps be found in the 
river Cherwell which rises in the district referred to by Dr. Buckland and 
joins the Thames at Oxford. Beds intermediate in age would most likely 
"be met with on further search which would show that these pebbles 
occur in increasing numbers and determine more exactly the date of their 
first appearance The presence in the lower gravel of fossils from the 
Oxford clay, coral rag, and the lower chalk, and probably also from the 
lias must be accounted for. Without attempting to offer a decided opinion 
the writer thinks that the following suggestion may not be quite beside the 
mark. Assuming that the chalk and green sand were once much more 
extensive than at present and that they overlapped the formations lying to 
the west, the beds of the Thames and its tributaries would, when at a 
higher level, be entirely in that formation. + Consequently only chalk 
fossils would be met with and most of these being brittle would be 
destroyed before they had travelled far down the stream. The same would 
happen to any derived from other systems over which the river passed 
in its earlier stages. But when, in its eroding process, the river had cut 
down into the Coral Eag and Oxford clay near Abingdon, the organic 
remains of those formations would be washed into the stream and might 
well be found, much worn but not destroyed, at the distance of a few miles 
only. And in the same way liassic fossils if present may have been 
brought in by the Evenlode, which has worn its way down to that system 
along almost its entire course. If this be the true cause of the presence of 
these shells in the lower gravel, intermediate beds may be looked for by 
which the date of their introduction may be more exactly determined. 
In fine, the alluvial loam without shell may be a result of the 
slackened current which must at length ensue on the continued erosion of 
the stream, or, as seems equally if not more probable, it may be the 
product of the almost annual floods to which this river, like most others, 
is subject in the winter. At all events there can be no doubt that the 
carrying power of the Thames for anything larger than silt has long ago 
dwindled down almost to nothing. 
* " All round the "Wrekin." 
t There are many facts that tend to prove this : the presence of chalk- 
flints where now no chalk is found; the projection of these strata to the 
west in Buckinghamshire ; and particularly the outlier of green-sand and 
flints at Orleigh Court, near Bidef ord, 43 miles from the nearest correspond- 
ing beds at Blackdown, 
