PERCOLATION', AND EVArORATION. 53 



river flowing orcr it, if no impermeable bed intervene, would sink 

 through it and disappear from the surface if the plane of saturation 

 were lower than the river-bed. 



The Chalk is well known to be more impervious under the 

 London Clay than where it comes to the surface, this being most 

 probably due in part to the pressure of the London Clay and the 

 Woolwich and Reading beds on which it rests, and in part to 

 the thick bed of clay preventing the percolation of water into it, 

 and consequently preventing it from becoming more porous by the 

 dissolving action of water and carbonic acid. "Water, therefoi'e, 

 flowing in the Chalk from N.W. to S.E., meets with a partial 

 obstruction when it gets to this denser chalk under the London 

 Clay, and this must have the effect of retarding its rate of flow 

 towards the Thames. 



The preservation of this partial barrier to the comparatively free 

 passage of the water is necessary to prevent it from running too 

 freely away from the higher ground to the wells under London, as 

 it might do but for this, notwithstanding the impediment to its 

 flow by friction in its passage laterally through the Chalk. 



The abstraction of a large quantity of water by pumping from 

 a considerable depth anywhere in the Chalk, but more especially 

 along or near the line of its outcrop from under the Tertiary beds, 

 which line extends in Herts from near Bishop's Stortford in the 

 N.E. to Rickmansworth in the S.W., would be equivalent to the 

 destruction of this partial barrier, as the water would be removed 

 much more rapidly than it could percolate through the Chalk 

 under the London Clay. 



Such abstraction of water by deep pumping anywhere along this 

 line, and the removal to a distance of the water raised, would 

 unduly lower the surface of the water in the Chalk at the points 

 of abstraction, and this lowering would gradually extend in diy 

 seasons over the whole of the Chalk area to the N.W. The 

 springs which now find their way into our rivers would be drawn 

 upon, and would not only cease to rise, but in extreme cases might 

 be converted into " swallow-holes," the water in the rivers sinking 

 into the channels which have been formed by the springs. 



This conclusion is not merely formed from theoretical con- 

 siderations, but also from a change which must have taken place 

 in a district well known to me, and which may have taken 

 place in others. This district is the upper portion of the Valley 

 of the Colne. 



All the water which flows down the Colne from the London 

 Clay area on the S. towards North Mimms is now usually absorbed 

 by " swallow-holes " at Potterells and in the bed of the Colne 

 between Potterells and the London Clay area. I have seen this 

 part of the River Colne under almost every variety of condition ; 

 when the water did not reach so far as the "swallow-holes" 

 at Potterells, sinking into the bed of the river in other (smaller) 

 "swallow-holes" above this point; when the water was rushing 

 down them with great force (one at least is so large that a man 



