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into the permeable chalk, sometimes down a sort of funnel, some- 

 times in a wee pool. In places, however, they occur in series 

 along valleys in the Chalk, a little below the Tertiary beds, as 

 is the case in the neighbourhood of North and South Mimms. 

 In such a tract the stream sometimes disappears low down its 

 course : as the season gets drier the point of disappearance gradually 

 ascends, and the lower swallow-holes are left dry, the reverse of 

 course taking place as the water increases. 



This subject is well illustrated in our county, and is by no means 

 an unexplored one, for our swallow-holes have been noticed by 

 many observers. Mr. Hopkinson and the Eev. J. C. Clutterbuck 

 have specially discoursed to you on some of them, and I must plead 

 guilty to having noticed them on various occasions in the course of 

 my work, in Geological Survey Memoirs. 



When contributing a Memorandum on the subject to the Royal 

 Commission on Metropolitan Water Supply * I thought that I had 

 done with it, for some time at least; but in the summer of 1893, 

 when it was too late to incorporate fresh notes in my paper, I had 

 the advantage of making a pretty thorough examination of the 

 Mimms swallow-holes, with Mr. T. H. Martin, Engineer to the 

 Barnet District Water Company. As there had been a long spell 

 of dry weather the stream-course was dry for a long way, and the 

 swallow-holes could be seen and examined in a manner that is not 

 possible when they are in action. My notes, therefore, may be 

 acceptable, at all events as a low-water record. 



The swallow-holes on the Catherine Bourne, about two thirds of 

 a mile north of Bidge, one of which (close to the county-boundary) 

 is rather lower than the neighbouring stream, were dry. The 

 valley of the Mimmshall Brook contains many swallow -holes. 

 Down this stream we will now go. On May 2 there was water 

 along Watery Lane, that part of the channel which serves the 

 double purpose of a roadway and of a water-course, for about 

 a quarter of a mile above Bridgefoot ; but on June 9 this was dry. 

 At the latter date there was a little water in the lower pond at 

 Bridgefoot. Below this the stream-course was dry, except for 

 mere pools of water in places. 



About a third of a mile below Bridgefoot one could see, at the 

 foot of the bank, many flint-pebbles and some brown loam, these 

 being suggestive of the basement-bed of the London Clay ; whilst 

 a little south of west from Mimms Hall pebbles and green-coated 

 flints were seen, suggesting the junction of the Beading Beds with 

 the Chalk. 



* ' Appeudices to Minutes of Evidence,' pp. 430-438. Fol. Loudon, 1893. 



