36 PHYSIOGRAPHY. [chap. 



an egg). The lower part of this oolitic series, which rests 

 directly on the Lias, is termed, from its position, the Inferior 

 Oolite. It is from some of the beds of this formation that 

 the Seven Springs are fed. Rain falling on the rubbly lime- 

 stones., the sandstones, and loose sands of the Inferior 

 Oolite, percolates downwards, through numerous cracks and 

 crannies, until it reaches the impermeable clays of the Lias, 

 when it gushes forth wherever a channel offers itself About 

 one-third of all the rain that falls upon the inferior oolites is 

 returned in the form of springs. So porous and open- 

 jointed are some of the rocks of this series that, where they 

 form the channel of the river, a considerable quantity 

 of the water which flows over them is absorbed, and the 

 volume of the stream may thus be diminished to a serious 

 extent. 



The Syreford Spring at the head of the river Colne — 

 one of the streams in the upper basin of the Thames — 

 has an origin similar to that of the Seven Springs. The 

 Syreford source yields daily from three to four millions 

 of gallons of water. This water has been absorbed by the 

 Inferior Oolites and is thrown out by the underlying Lias 

 clays. 



Many of the springs in the head-waters of the Thames 

 take their origin in the water-bearing limestone beds known 

 as the Great Oolite — a set of rocks separated from the 

 inferior oolite by the so-called Fuller's earth. This Fuller's 

 earth forms a thick bed of clay, which retains the water that 

 reaches it, in enormous quantity, by percolation through the 

 porous limestones and sands of the great oolite series. 

 Thus, the famous well at " Thames Head," the position 

 of which is indicated on the map in Plate I., rises in this 

 way from the surface of the Fuller's earth, and yields a vast 

 volume of water which has been collected in the fissures of 



