6i4 THE POPULAR SCIENCE MONTHLY. 



sion of water-sbects or water-levels occupying distinct stages, and 

 extending, with uniform cliaracters, under wbole countries, like the 

 strata to which they are subordinated. It is proper to remark here 

 that by the term water-sheet is not meant a real bed of water, lodged 

 in a cavity, between solid masses that serve as walls to it, but water 

 fillinof the minute interstices or the cracks of a rock. Continuous and 

 regular in sand, these sheets are usually discontinuous and irregular in 

 limestones and sandstones, in which the water only occupies more or 

 less 8i)acious fissures. When natural issues are wanting, human 

 industry is able, by boring, to make openings down to the subterranean 

 waters, which it causes to jet up to the surface, and sometimes to 

 a considerable height above. The thought of undertaking such works 

 is a very ancient one. The Egyptians had recourse to them forty 

 centuries ago ; and they were executed in France, in 112G, at Artois, 

 whence the name of artesian wells has been given to them. 



The water-levels of the cretaceous strata, from which the French 

 artesian waters issue, are not always of advantage ; but in the north of 

 France and in Belgium they constitute the most formidable obstacle 

 which miners have to encounter in reaching the coal-beds. 



A striking confirmation of the theory of the source of supply of the 

 artesian waters has been observed at Tours, where the water, spouting 

 with great velocity from a well a hundred and ten metres in depth, 

 brings up, together with fine sand, fresh-water shells and seeds, in such a 

 state of preservation as to show that they could not have been more 

 than three or four months on their voyage. Some of the wells of the 

 wady Rir have also ejected fresh-water mollusks, fish, and crabs, still 

 living, which must, therefore, have made a still more rapid transit. 

 Caves, in limestone regions, play a part of the first order in the move- 

 ments of the interior waters. Their presence is manifested at the 

 surface by depressions of various shapes, such as are called " swallow- 

 holes " in the north of England, and " sink-holes " in the United 

 States. These cavities draw in the surface-waters and remove them 

 from sight, to reappear at some other place, oftentimes in exceptionally 

 voluminous foimtains. They can be pointed out by the hundred in 

 some parts of France, although only a small proportion of them are 

 revealed by a visible discbarge. The internal hollows are often aligned 

 with dislocations of the ground, with which they are connected as 

 effects of fractures, ultimately corroded and rounded off by water. 

 The caves of Baume in the chain of the Jura correspond with a series 

 of tunnel-holes and sinkings from the prolongation of which arises the 

 river Seille. The Jurassic limestone of La Cbarente is marked by pits 

 of various depths, with yawning mouths, into which the Tardou6re 

 and the Baudiat disappear near La Rochefoucauld, to gush out bub- 

 bling farther down and give rise to the Tourne. In the departments of 

 the Var and the IMaritinie Alps, numerous sink-holes {scialets) feed, 

 through secret channels, powerful springs that issue from the sea- 



