; in the Drift of the Valley of the Avon. 225 
found, although diligent search has been made at every fresh 
opening. 
When we consider that nearly all the drift gravel has been 
derived from the chalk, and remark the comparatively few bands 
of flint nodules which it contains, we feel it is impossible to 
form an adequate notion of the immense bulk of chalk which 
must have been denuded and disintegrated, to produce these large 
accumulations of flint gravel. 
One is often asked how many years old these gravels are? A 
question that can at present only be answered relatively. We can 
with tolerable certainty say one set of gravels is older than another, 
but no exact data exist by which any given thickness of deposit 
can at once be reduced to an equivalent in years. Many of the 
forces in operation would alter materially in the different periods, 
aud what might be true for one would be false for another. For 
instance, it is provable that during these periods considerable 
alteration in the relative level of the surrounding country occurred, 
yet any such change must have been very gradual, as the 
regular stratification of the chalk hills is undisturbed, and shows 
no trace of any sudden upheaval or depression. Again, if we 
examine the composition of the deposits, it is evident that the 
physical conditions under which the higher level gravels were 
deposited, differed considerably from: those of the subsequent beds. 
The large angular flints and the blocks of saccharoid sandstone were 
probably attached to masses of ice, and thus lightened would easily 
float into their present position ; whilst the absence of anything like 
stratification and the very mixed character of the deposit, proves 
that these various materials were roughly jumbled together. 
Hence we conclude the high level gravels are the result of tor- 
rential action during a period of great cold. This turbulent period 
was succeeded by one of comparative tranquillity, when the 
great mass of the brick-earth at Fisherton was brought down the 
valley in the shape of mud and sand: even then the temperature 
__was sufficiently cold to float the few large blocks of sandstone that 
_ are found irregularly scattered through the clay. The highest. 
level gravels are almost destitute of organic remains ; Paleontology 

