of Overjlowmg Wells. 1G3 



According to the slope, the undulations, or the declivity which 

 are presented by the plane of superposition of the permeable 

 deposits in which the waters flow between impermeable strata, 

 these great sheets of water are met with at all depths ; but it is 

 impossible to lay down any constant rule with respect to them. 



In order that these waters be capable of ascending, it is neces- 

 sary that the formations among which they occur be entire, in 

 the state in which they were originally deposited, and that they 

 be not intersected by large valleys, or deep ravines, in which 

 the waters would find a free and easy exit. 



It would be in vain to search for springs in depo.sits which, 

 at no great distance from the place of boring, are intersected 

 by deep valleys, or when the formations are internally cracked, 

 filled with tortuous sepai'ations, and greatly disturbed, whether 

 by the contraction attending the desiccation of the mass, or by 

 internal shocks, swellings or earthquakes ; or, lastly, when these 

 neptunian formations, such as plastic clay, chalk, oolite and 

 shell- limestone, are raised up, and present precipices at the sur- 

 face, as is the case, for example, with the plastic clay at Issy, 

 Vauvres, Auteuil and Passy, and with the chalk at Meudon, 

 Sevres, Auteuil, Bongival, &c. 



In these different localities, we need not expect success in 

 boring for springs, unless by penetrating deeply into the mass 

 of the chalk, in search of the sheets of water in its lower part, 

 or even by traversing it entirely, in order to come upon those in 

 the clays, oolites, and shell-limestones ; or, lastly, unless by pe- 

 netrating deeply into the latter, when they happen to be raised 

 to the surface, and present cliffs, or are intersected by valleys 

 of greater or less depth. 



On this subject it is necessary to observe, 1st, That if in a 

 country composed of elevated plains, such as those of Cham- 

 pagne, Normandy, Picardy and Bcauce, or any other of the 

 same nature, or of similar formraion, in place of boring to the 

 necessary depths for reaching the different water-sheets which 

 are commonly the most abundant, and, at the same time, those 

 which ri.se highest, the boring is stopped at higher levels less- 

 distant from the surface, it is more than probable that, in that 

 case, the ascending waters would stop more or less beneath the 

 surface of the ground, according to the depth of the borings. 



