native wells, give exit, from a mean depth of seventy metres, to a volume of water fully equal to that of the Seine, at Paris, in its lowest stage. Cultivated lands have been created, the native population has doubled, and the value of the oases has more than quintupled; a complete transformation of this part of the Sahara has been effected, by the agency of underground waters, within thirty years. Most of the manufacturing cities of the middle and north of England are situated upon the New Red Sandstone, where, besides excellent building-stone and proximity to the coal-fields, they enjoy the inestimable advantage of the presence of inexhaustible reservoirs of water purified by natural filtration, and easy of extraction. Belfast, in Ireland, is similarly situated. The water-bearing gravels are particularly worthy of attention from this point of view. With the inexhaustible and easily accessible provisions of water which they contain, they present to man an almost infinite expansion. This accounts for the existence upon these deposits, from most ancient times, of numerous important cities and capitals, like London, Paris, and Berlin. But in London the arenaceous and phreatic stratum has limitations which were opposed for several centuries to growth in particular directions. For a long time, according to Mr. Prestwich's observations, the population, by an instinct easy to understand, continued strictly concentrated on the principal water-sheet, and on a few isolated strips of gravel, as at Islington and Highbury. In the suburbs, likewise, the thick populations were collected on the larger gravel-beds rich in water, while in the same region, although the soil was everywhere cultivated and productive, the houses were very sparsely scattered. But the situation has greatly changed within the last seventy years, a supply of water having been brought from a distance, and the city has spread very rapidly over the clayey grounds.
Numerous populations still depend on wells for their drinking-water; Lombardy and Venice, with two million inhabitants; the extensive plains of Hungary; at least half of the German Empire; a part of the Russian Empire, seven times as large as France, and populated by about twelve million souls; and, according to the explorer Abbé David, the whole of the great northern plain of the Chinese Empire, containing more than a hundred million inhabitants. Besides these vast plains which represent more than a third of the continents, there are numberless valleys, with water-bearing subsoils, which have attracted to themselves aggregations of men. We can then affirm that a very important fraction of the human race depends for its principal drink wholly upon water which is furnished by the phreatic strata of ancient or modern alluviums. We never find such concentrations of inhabitants in countries where the soil is formed from granitic and schistose rocks, without being covered by disaggregated materials. These rocks permit water to descend to their interior only with the greatest difficulty. Springs are likewise weak among them, but very numerous; and the
