56 
circumstance “ Eon-somme,” only reached 308 feet in height above the level 
of the sea, and the drainage of this alone supplied the upper tributary. 
Three miles to the south-east was a hill reaching 400 feet ; but it appeared 
to add little, if anything, to the supply. They would have to go several 
miles to obtain a higher level, and directly they reached it they found that it 
no longer supplied the Somme ; but the Aisne and the Oise, which . were 
tributaries of the Seine, and belonged to a distinct system. If they continued 
their search for still loftier elevations, they would, still proceeding m a south- 
easterly direction, find hills rising to 800 and 900 feet ; but they gave oft 
their streams to tributaries of the Meuse, and they would be obliged to follow 
their waters through Holland before they were discharged into the sea. In a 
word, the whole system depended upon the water-sheds of the hills ns l n £ 
only to 300 and 400 feet above the level of the sea. . The. Somme depended 
mainly for its water upon the combined supplies of its chief tributaries, the 
Avre, the Noye, and the Celle ; but all along its course it was assisted by 
numerous smaller streams gathering the rain-water which fell upon the slopes 
of the numerous ravines descending into the main valley. 
But connected with the Somme system, it was pointed out that there were 
several parallel rivers following the same course as the Somme, ix., descend- 
ing from the south-eastern ridge in a north-westerly direction. To the north 
the Authie, and to the south the Bresle, the Yeres, the Eaulne, and the 
Bethune. They depended also upon the same sources of water, and were m 
every respect similar in their circumstances, and could scarcely have been 
different in their origin. If it were objected that springs now no longer in 
existence might have originally supplied a much larger body and a much 
greater force of water than now, it must be remembered that the district was 
a chalk district. Each ravine was as a rule dependent upon the rainfall of 
its own slope. All that could be done, therefore, was to increase the rainfall, 
and add, what perhaps there might be independent reason for adding, heavy 
snowfalls, and of long duration, by which the April suns provided an amount 
of water far in excess of what was thrown down the ravine now. And. yet 
that would affect the argument but little, because the sloping ravines 
converging to the great general valley must have been already there before 
the excessive rainfall or snowfall could be of any value. The sudden melting 
of snows on large flat expanses produced no material results ; it was the 
valley, the ravine, and the gully which gave the force to the water, and 
without them the water but evaporated into the atmosphere or soaked away 
as best it might. „ 
It was not a part of his task then to explain the phenomena of the Somme 
valley ; but with that map before him he felt called on. to say a few words as 
to the operations which he thought it suggested. He might add that the view 
he took was based not only on the data then before them, but upon the study 
of the levels of the Ordnance Survey in a much more minute degree than was 
represented by the figures on his diagram, and beyond this by many a tramp 
over the hills in question, sometimes in geological excursions, more often archaeo- 
logical. The great parallel lines of rivers, the furrows as it were stretching ; in a 
direction similar to that of the sloping chalk, suggested that the river valleys 
belonged to the operations consequent on the upheaval of the great mass of 
chalk from its ocean bed. He compared the result with what any one might 
see on any argillaceous shore, where the base was impervious and yet soft. 
The descending tide left channels and furrows, by which the surface was 
drained, but afterwards modified in character by evaporation and exposure 
to atmospheric influence. The great chalk expanse of a hundred miles Was 
enormous in comparison to the few yards of a tidal shore, and so were the 
valleys of 100 and 200 feet depth to the little drifts of two or three inches. 
But this was not all. If it were argued that the effect was not proportionately 
sufficient, it might also be reasonably replied that the emergence of this vast 
