HKRTl'OEDSUIKE NATURAL HISTORY SOCIETT. Ivii 



and geological history of this peculiar rock, which is formed by 

 the consolidation, by infiltration of silica, of a pebble-bed of the 

 "Woolwich and Heading Series.*" 



Contiuuing, from the Rectory grounds, to ascend the hill on the 

 south-west of Borkhanipstead, Haresfoot Park, the seat of Mr. F. 

 A. Dorrien Smith, was soon readied ; and after walking through 

 the park, a descent was made into the valley of the Bourne, permis- 

 sion having been obtained from Mr. Dorrien Smith for the party to 

 follow the course of the stream, wliich flows over his property. 

 The Bourne was encountered where it is crossed by Harratt's End 

 Lane, and where its source was found when it was visited by the 

 Society two years ago (7th May, 1881). Now, however, it was 

 flowing here in a considerable stream, the field above the lane was 

 in great part submerged, and to find the present source of the river 

 it was necessary to walk about half a mile up the valley by the 

 side of the lake thus temporarily formed, and through some muddy 

 fields. Here, in a field in Buckinghamshire,! on the right bank of 

 the stream, in view of its present source, and of its course for some 

 distance, Mr. Evans explained the geological and meteorological 

 conditions to which the existence of this and other intermittent 

 streams is due, his remarks being somewhat as follows : — 



Such intermittent streams as this, to see the source of which we 

 have come to these muddy fields, are governed by the same laws as 

 are the other rivers of the district, and all streams which issue from 

 and pass over a porous subsoil like the chalk. All streams are 

 dependent upon the rainfall, if on hard or impervious soils directly, 

 for the water in that case at once runs off on the surface ; but if on 

 absorbent soils or subsoils such as the chalk, the result is different, 

 for much of the water percolates downwards, until it meets with 

 some obstruction. It arrives at a "plane of permanent saturation," 

 a natural subterranean reservoir, by which it must not be inferred 

 that there is anything like a subterranean cavern full of water in 

 the chalk, but merely that at a certain dej)th the chalk becomes 

 thoroughly saturated with water. There is a vast amount of water 

 continually in the chalk, and even in the driest weather there is 

 always water in most chalk-streams, as they are not immediately 

 dependent upon rain. The amount of water flowing from one of 

 these areas represents merely a fraction of the amount of rain falling 

 upon it. The average yearly rainfall over a period of 50 years in 

 this neighbourhood is 25 or 26 inches. For the 12 years ending 

 1872, an average of 25^ ins. per annum fell, of which about half 

 fell during the summer months (April to September), and the other 

 half during the winter months (October to March). 



In order to ascertain the amount of water which finds its way 

 more than one or two feet below the surface, Messrs. Dickinson and 

 Co. have had a series of gauges constructed, consisting of cast-iron 

 cylinders, 18 inches in diameter and 3 to 6 feet in length, turned 



* See ' Trans. Herts Nat. Hist. Soc.,' Vol. I, p. 33. 



t The Bourne forms a boundary between the counties of Hertfordshire and 

 Buckiughamshire ior a considerable part of its course. 



