84 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



one of the drift periods. A continuous bed of gravel is supposed to 

 occupy a plain situated a few hundred feet above the sea-level. The 

 constant infiltration of water from this gravel is presumed to have 

 eroded the surface of the chalk, until ultimately the passage of the 

 water has centred in numerous channels or pipes, into which the 

 gravel sank as the cavities enlarged ; and these vertical channels, 

 now that the country has been drained and opened out by another 

 and deeper system of valleys, exist as dry gravel-pipes. 



Diagram B. — In this case we have a continuous tertiary sand bed 

 occupying a level ranging from the sea-level to 500 or 600 feet above it, 

 and reposing upon a mass of chalk which, beyond the range of these 

 Tertiary Sands, is traversed by a deep river- valley. This valley 

 consequently gives vent to the water in the chalk and establishes a 

 low water-level, and therefore the water in the sands S, not being 

 permanently held up by the slight argillaceous bed at their base^ 

 passes into the chalk and escapes to the lower water-level in that 

 deposit. The indentations on the surface of the chalk represent 

 the cavities worn by the water in thus effecting its escape. As in 

 Diagram A, a fresh system of drainage has since dried up these old 

 water-channels and left them as disused sand-pipes. 



