sion of water-sheets or water-levels occupying distinct stages, and extending, with uniform characters, under whole 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 filling 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 spacious 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 1126, 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 movements 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 fountains. 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 discharge. 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 Charente is marked by pits of various depths, with yawning mouths, into which the Tardouère and the Baudiat disappear near La Rochefoucauld, to gush out bubbling farther down and give rise to the Tourne. In the departments of the Var and the Maritime Alps, numerous sink-holes (scialets) feed, through secret channels, powerful springs that issue from the sea--
