8 W. ATHTTAKEE ADDRESS : 



the stream, are three small sink-holes, which were dry, and pre- 

 sumably only act when the marsh is flooded. 



In the next valley to the north, swallow-holes are marked on the 

 Ordnance Map (Herts, Sheet 40) in Gobions "Wood, north-westward 

 of Littleheath. These were all dry on June 9, 1893. The lowest 

 marked is a large hollow, and in the highest marked, where a foot- 

 path crosses the channel north of Bolton's Farm, I saw chalk at 

 the bottom. These swallow-holes show an error in the Geological 

 Survey Map (Sheet 7), presumably from an iiprise of the beds 

 which escaped note when the surveyor (the late Mr. Trench) was 

 working on the old and not too exact one-inch map. This uprise 

 may perhaps be connected with that which brings up the Chalk 

 inlier at Northaw, on the E.S.E. Such an uprise might have some 

 effect in hindering the passage of water underground in the Chalk 

 from the upper swallow -holes of the Mimms Hall Brook north- 

 eastward. 



We come lastly to the stream from Marsh Moor and "Welham 

 Green, which passes through the northern part of Potterell's Park, 

 receiving the sewage of the house some little way above the highest 

 swallow-hole marked on the Ordnance Map (Herts, Sheet 35). 

 A little further down, westward of the house, there was water 

 for a short way on June 9, 1893, and the second marked 

 swallow - hole was in action. There are others below, as the 

 stream debouches on to the great swallow-hole tract above noticed. 



TJndergeoukd "Water-levei. 



The long series of swallow-holes in the Mimms valley, and their 

 intermittent action, show that the level of the underground water 

 varies. When this rises to the surface everywhere along the 

 bottom of the valley there is a continuous stream ; when it is lower 

 the stream ends in successive swallow-holes, according to that 

 lowering, and also reappears at successive points northward. There 

 must therefore be an inclination of the underground water-level 

 down the valley. 



Such inclinations, or water-slopes, occur nearly everywhere in 

 the Chalk, though nature rarely gives us proof of them, only 

 indeed in the case of bournes, or intcnuittent streams, that is to 

 say, only along the bottoms of some valleys. But by means of 

 wells and borings we are able to get some notion of the water- 

 slopes away from the valley-bottoms. 



As a general rule in a Chalk district it is found that such water- 

 slopes show a rough parallelism with the contour of the ground 

 above, that is to say, that as the ground rises the underground 



