ii8 PHYSIOGRAPHY. [chap. 



surface of the ground that the river and its lateral streams 

 derive their supply of soluble impurities, much more being 

 probably due to the springs by which the streams are 

 chiefly fed. Spring-water is, in fact, much richer than river- 

 water in these soluble constituents. Nor is it difficult to 

 see why. 



To form a spring, the rain-water must sink to a greater or 

 less depth in the ground. During its underground passage 

 it exerts its solvent action upon the surrounding rocks. In 

 some cases, die water sinks to great depths, traversing long 

 and tortuous passages ; and, in such cases, it is only to be 

 expected that, when it reappears at the surface, it will be 

 highly charged with soluble constituents. Under pressure, 

 at great depths, it may absorb large volumes of such gases 

 as carbonic acid and sulphuretted hydrogen ; or it may dis- 

 solve saline matters of various kinds, and thus acquire 

 peculiar properties which confer upon it medicinal value. 



Analysis of the water of the Thames Head Well, near 

 Cirencester, shows that it contains 27-44 parts of solid im- 

 purity dissolved in 100,000 parts of the water; in other 

 words, o'02744 per cent. The most notable of the mineral 

 constituents which affect the quality of the springs in the 

 Thames basin is carbonate of lime. 



A large part of the course of the river lies, indeed, 

 through limestone rocks. In the upper part of the basin, 

 it is the limestones of the Oolitic formadons that furnish 

 most of the springs ; whilst, in the lower part, it is chiefly 

 the chalk. All limestones, from the softest chalk to the 

 hardest marble, consist essentially of carbonate of lime ; 

 and as this compound is slightly soluble in water, the springs 

 and streams of limestone districts always hold it in solution. 

 It is true that the proportion of carbonate of lime dissolved 

 by />ure water is extremely small ; not more, it is said, than 



