46 ON THE GENERAL DISPOSITION OF THE 



only most frequent in the stratified rocks, but that 

 they are principally found in the secondary strata and 

 among the uppermost of these. It has been, in a 

 few instances, remarked, that they burst out in new 

 places, disappearing in former ones ; or that they 

 sometimes appear where none had existed before. 



It was already observed that the theory of springs 

 was very defective. I some admit of explanation, 

 others seem hitherto to defy all attempts towards it. 

 The rain which falls on the surface, penetrating to a 

 certain depth, according to the nature of the subsoil, 

 at length meets with an impenetrable stratum, as I 

 just remarked, and, gliding along its surface, breaks 

 out at some favourable opening, from the effects of 

 hydraulic pressure. In other cases, it may be con- 

 ducted in a similar manner by means of the fissures 

 in which some classes of rock abound. Thus also, 

 if a bed of gravel lies above clay, the water may be 

 collected on the surface of the latter; and hence again 

 the limestones, which so often abound, not only in fis- 

 sures but in wide rents and caverns, sometimes conduct 

 even subterranean rivers, as in Derbyshire. A peculiar 

 disposition of such fissures will thus, when combined 

 with hydrostatic force, sometimes account for the 

 phenomenon, not uncommon, where springs arise so 

 high up in hills that their waters could not have been 

 collected from above; or where, as in the Strophades 

 and in the small island of Chisamil in Barra, they 

 break out in spots surrounded by the sea. 



But whatever explanations these offer of some cases, 

 there are innumerable others to which they are inap- 

 plicable, and for which theories have been invented by 

 Des Cartes and other philosophers, which, resting on 

 no solid foundation, it is unnecessary to quote. The 

 perennial spring at Willowbrig in Staffordshire, is 



