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beds below, to find its way out in springs at lower levels, or, 
possibly, beneath the sea, and so all denudation by the streams 
be stopped. No observations have, as far as I can tell, been 
made in any of the river basins with which we are now con- 
cerned upon the rate of retrocession of the rapids or falls, 
such as would enable us to form a numerical estimate of the 
number of years that must have elapsed since the implement- 
bearing terrace gravels were left where they now lie. 
But there are circumstances that give the impi*ession which, 
in most of those who have seen many similar examples, 
amounts to a conviction, that the time must have been in most 
cases enormously long. 
At the Reculvers, on the Thames estuary, a bed of gravel 
caps the cliff quite 50 feet above the sea. This has flint 
weapons in it. When the Thames ran at that level 
down by its mouth, it cannot have run at a lower level 
by London ; yet, as far as we know from old remains, London 
was as now 2,000 years ago. Teddington, to which they say 
the tide came up when first it got its name, was then no 
higher, and so we trace tbe valley far up into the oolitic hills, 
so far I doubt whether now we could identify the corresponding 
levels. How long did it take to cut back such a valley and so 
far, seeing that within the time of history we know of no 
great difference in its channel ? 
So for the Somme. The Romans left what they lost down 
in the peat quite 80 feet below the terrace on which tho 
city of Amiens stands. This terrace we can trace much 
further both up and down the valley. Beds of the same age, 
too, are found at Menchecourt at a lower level. They may bo 
synchronous with those of Amiens, if the rapids then came 
between. The rapids had passed Amiens before the Roman 
times. Where are they now? Par back towards central 
France. How long it took to cut the valley back so far I 
will not try to speculate, having no data, but I feel that 
it must be something very great, seeing that the historic 
period of 2,000 years has done so little. 
Another line of inquiry I will mention to conclude with. 
In the long periods of geologic time races appear and last 
awhile, and then are not, and a new group of living things 
represents them in the next succeeding age. How they went 
out we cannot tell. It was not by cataclysms, for they go 
one by one, and the deposits tell of slow accumulation ; but 
more as if some gradual changes over various regions of 
the earth made each successive place in time unsuitable for 
all tho life that once was there. First, those which were 
most susceptible and able to migrate went off. So nature 
has arranged for a constant succession upon earth’s surface ; 
