li.J SPRINGS. . 37 



the Great or Bath Oolite. Three million gallons of water are 

 every day pumped up by the engine from a depth of 35 

 feet into the summit-level of the Thames and Severn canal. 

 This artificial withdrawal of water to feed the canal has 

 lowered the level of the water-bed in the surrounding 

 country, so that the natural discharge at Thames Head is 

 now contracted and takes place lower down the valley. 



About five miles south-east of the engine at Thames 

 Head is the Boxwell Spring, which discharges into the 

 Churn more than a million gallons daily. This large body 

 of water is drawn off from the surface of the Fuller's earth ; 

 and many other springs, such as the Ewen Spring and the 

 Ampney Spring, have a like origin. 



Other streams tributary to the Thames take their rise in 

 the springs of the porous sandy beds called Upper Green- 

 sand, which rest upon a stiff clay known as Gault. In 

 the lower part of the basin, or nearer to the mouth of 

 the river, the Chalk becomes the great reservoir ; and it 

 often happens, that water which has drained through the 

 porous upper part of this formation, and made its way 

 through fissures, is held up by the lower part of the chalk, 

 which becomes stiff and retentive. The Kennet, which is 

 one of the main feeders of the Thames, receives its water 

 chiefly from the chalk downs near Hungerford and Marl- 

 borough ; whilst the Coin collects a large body of water 

 from the Hertfordshire chalk. A great part of the Nevv 

 River water too is drawn from the chalk. Finally, as a 

 source of water in the Thames basin, the Bagshot Sands, 

 already noticed, must not be neglected. Thus, the Loddon, 

 a tributary from the south, which falls into the Thames 

 about five miles below Reading, derives part of its water 

 from the Bagshot series. 



As the Thames pursues its course, it receives supplies of 



