194 
THE GEOLOGIST. 
feet. The rate of accumulation, though it may often be slow, is very va- 
riable. A Eoman road in the valley of the Lea was found covered by 
two feet only of alluvium. Another such road in Cambridgeshire was 
covered with five feet of alluvium. The entire depth of the alluvium was 
not, however, ascertained in either case. M. Kozet gives another instance 
of a Eoman road, which he considers to have been kept in repair until 
about the eighth century, traversing the valley of the Dlieune. Its paved 
and even surface is now covered by twelve inches and a half of alluvial 
soil. A little lower down the valley this alluvium, which is very uniform, 
has been ascertained to be about thirteen feet thick. This he estimates 
would have required for its accumulation about 10,000 years. The allu- 
vial soil reposes there immediately on the so-called diluvium. The rapi- 
dity with which the alluvial soil will accumulate under favourable condi- 
tions is often very much greater. In places, thick beds of alluvium and 
of peat have been formed since the lloman occupation. Looking at these 
facts, and at the general fact, that as a rule, in the valleys of the Somme 
and of the Thames, for example, the lioman, British, or Gaulish remains 
are found at a depth from the surface bearing a considerable proportion 
to the entire thickness of the alluvium, the probability is, that the com- 
mencement of the alluvial deposits is not to be carried back indefinitely. 
One reason for believing the accumulation of the silty alluvium of our 
valleys to have been more rapid at one time than now is, that these val- 
leys, left rude and rugged at the end of the Quaternary period, would be 
subject to more frequent floods until their inequalities were filled up and 
levelled. Mr. Prestwich concluded by observing that for these and various 
other reasons he was confirmed in the opinion he expressed in 1859, that 
" the evidence, as it stood, seemed to me as much to necessitate the bringing 
forward of the extinct animals towards our own time as the carrying back 
of man in geological time." In making that observation, he had chiefly 
in view the distance of time at which the last of the great extinct mam- 
malia disappeared. If there should have been, between the modern 
valley alluvia and the latest Quaternary beds, some intervening period of 
time of which we are ignorant, that distance may be materially prolonged. 
If, on the contrary, they followed in immediate succession, — and he 
thought we have evidence that such was the case, for there seems rea- 
son to believe that some of the large pachyderms still existed at the 
commencenient of the alluvial period, whilst we know that manv of the 
ruminants lived on uninterruptedly from one period to the other,— he did 
not, for his part, see any geological reasons why the great extinct mam- 
malia should not have lived down to comparatively recent times, possibly 
not further back than 8000 to 10,000 years. 
" But this only brings us to the threshold of that dim and mysterious 
antiquity in which first appear those rudely-wrought flints— those evident 
works of design— those palpable shado\^ ings of man. Here our chrono- 
logy fails us altogether. If we look at our broad and long vaUeys, and 
then at the comparatively small streams now winding through them, and 
suppose these streams to have been the same in past times as they now 
are, we could hardly avoid the conclusion that the time required to pro- 
duce such excavations with such means must be almost incalculable. But 
It the view here proposed be correct, it would follow that with rivers so 
large in proportion to those now occupying the same valleys, with floods 
ot a force now unknown in the same districts, with a cold so severe as to 
shatter the rocks and to hasten the removal of their debris, we should 
Lave, 1 contend, agencies in operation so far exceeding in power any now 
