II.] . SPRINGS. 35 



springs of the finest drinking-water as that of the Thames." 

 The basin of the Thames, as we have seen above (p. 15), 

 has an area of more than 6,000 square miles, and rather 

 more than half of this area is formed of porous soil, 

 resting upon water-bearing strata, whilst the remainder 

 consists of clayey soil. The pervious ground, which ab- 

 sorbs the rain, is, for the most part, under cultivation ; the 

 impervious ground, which throws off the rain, is mostly 

 permanent meadow-land and pasture. 



In describing the springs in the upper part of the Thames 

 basin, or those springs which rise above Oxford, we may 

 be guided by the careful observations which have been 

 made for many years by Mr. Bravender, of Cirencester.^ 

 The source of the river Churn, or the extreme western 

 tributary of the Thames, is known as " The Seven Springs," 

 and is situated about four miles from Cheltenham on the 

 road to Cirencester. It has been estimated that these 

 springs yield on an average about 150,000 gallons of water 

 daily. The Seven Springs are thrown out from clayey beds 

 which belong to the series of rocks known to geologists as 

 Lias ; ^ the water having been absorbed by the overlying 

 loose limestones which are technically called Inferior Oolite, 

 and which contain cavities that serve as excellent subter- 

 ranean reservoirs for storage of water. The term " ooHte," 

 which has just been used, is applied by geologists to a large 

 series of rocks, which occupy a definite position in the scale of 

 strata and include many limestones which are composed of 

 peculiarly rounded grains, that give the rock somewhat the 

 appearance of the roe of a fish, whence the word oolite (cior, 



^ See his evidence before the Royal Commissions on Water Supply 

 and Rivers' Pollution. 



^ Tlie geological structure of the Thames basin is shown in Plate 

 v., and will be explained in detail in Chapter XVII. 



D 2 



