28 My GARDEN. 



of the river is left almost dry. Here with our porous chalk the water 

 is retained, and is given off during the whole year. 



Upon reference to the section of the strata of the London Basin 

 (plate 4), it will be observed that a layer of impervious clay exists 

 below the chalk, which comes to the surface to the south of the chalk 

 hills on the north of Redhill and Reigate. In consequence of this 

 clay, the water is not discharged into the valley of the Darenth, but 

 is compelled to drain away to the north of the chalk. But then, 

 between my garden and North Mimms in Hertfordshire, it is again 

 covered with a layer of sands and clays for 100 feet, and over these 

 again by an impermeable layer of blue clay through which no water 

 can rise. The water, therefore, is by compulsion restricted to flow 

 at the edge of these two sets of clay beds. Hence it rises about my 

 garden to form the River Wandle, and on the other side the London 

 Basin it also rises, at Watford to form the River Colne, and elsewhere 

 to form the River Lea. Between these two sets of springs, north 

 and south of the London Basin, water can be obtained by sinking 

 wells through the impermeable strata to the chalk. 



Wallington Bridge is 95 \ feet above mean tide level of the 

 Thames ; and as there are two or three mills above the garden, it follows 

 that the springs of the Wandle are more than 100 feet above the level 

 of the Thames. A reference to the contour map, kindly prepared 

 for me under the direction of Sir Henry James at the Ordnance 

 Survey Office, Southampton, will show their height and the relative 

 position of the surrounding country. 



Water flowing from the chalk strata invariably contains chalk or 

 carbonate of lime in solution. Chalk by itself is extremely insoluble, 

 and one part is only soluble in 10,000 parts of water. It is, however, 

 very soluble in carbonic acid, and the water from the springs has 

 always carbonic acid by which the chalk is dissolved. 



To remove the chalk from the water, Dr. Clark has invented an 

 ingenious process, thus getting rid of the excess of carbonic acid. This 

 he effects by adding a small quantity of lime-water sufficient to com- 

 bine with the excess of carbonic acid, and to form the two into 



