4i\6 M. Arago on Artesian Wells 



ing originally been vast plains, and by their having subsequently 

 experienced an elevation, in such a way as to circumscribe the 

 horizontal portion in the bosom of hills or of mountains. We 

 shall add that the rocks of the secondary formation are arrang- 

 ed in layers ; — that some of these layers, occasionally of great 

 thickness, are composed of loose and very permeable sand; — that 

 in rising towards the extremity of the basins, these permeable beds 

 crop out on the sides of the hills and mountains ;— that the 

 surface water may, by infiltration, form in them extended sheets 

 of water ; — that these reservoirs, when the beds dip at a high 

 angle, must empty themselves with velocity towards the lower 

 levels; — that in their progress, these running waters, by de- 

 grees removing the sand, and even portions of the surrounding 

 rocks, come to form subterranean rivers which displace parts pf 

 original massive strata and excavate great voids, where previously 

 there were none. 



There is one of these secondary strata, viz., chalk, which is 

 furrowed in every direction by thousands of fissures. And it 

 is evident that rain water may easily find its way through it, and 

 circulate in its mass to the greatest depths. 



Tertiary Formations. 



The tertiary formations are stratified, that is to say, are com- 

 posed of a greater or less number of overlying beds, which, like 

 the courses of a wall, are separated the one from the other by 

 distinct and well marked joinings. 



This formation, like the former, usually assumes the form of 

 a basin, but commonly of far less extent than the other. We 

 should not forget that this shape appears to be the result of some 

 alteration in the position of the beds. It is during this altera- 

 tion, too, that the constituent elements of the tertiary series 

 have formed the ridge of slopes and hills which surrounds 

 them. 



During this process of change in the total mass of these 

 strata,* commonly dl the beds were more or less torn, and bro- 



• The greater number of basins, whether belonging to the secondary or 

 tertiary series, really owe their origin to this alteration in the strata, which 

 had originally been deposited horizontally. But sometimes also the second- 

 ary or tertiary deposit is formed in a pre-emtting ha,sin, and bounded by a 



