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THE GEOLOGIST. 



Cartes Geoloaique et Hydrologique de la Ville de Paris. By M. Delesse. 



Savy, 1861. 



Eather more than a year ago, M. Delesse presented to the Geological 

 Society of France two maps of the city of Paris, executed under the orders 

 of M. le Baron G. E. Haussmann, Prefect of the Seine, — one a geological 

 map of the subsoil of the city itself, as far as authorized actual excava- 

 tions ; the other hyclrological, giving the quality and modes of flowing of 

 the springs, especially of the subterranean watercourses. An ordinary 

 geological map indicates only the rock found at the surface, but in the 

 former of the maps the attempt is made to make known the nature as well 

 as the form of the rocks which composed the under-soil. Such a labour is 

 exceedingly difficult, and the more so that the rocks of Paris are remark- 

 ably varied ; but the numerous works undertaken there of late years have 

 offered unusual facilities for the comparison of sections. 



The city of Paris is traversed by four superficial sheets of water — the 

 Seine, the Bievre, the stream of Menilmontant, and the canal Saint-Martin. 

 Independent of these, there exist the subterranean watercourses which are 

 attained on the sinking of wells. These the hydrologic map shows in a 

 very complete manner. The positions of these subterranean sheets are 

 very interesting. There is pointed out, first, a subterranean sheet in im- 

 mediate communication with the Seine, and designated the "infiltration- 

 sheet" (nappe a" infiltration), which extends under Paris, and is that 

 which furnishes the water of nearly all the wells. Its horizontal curves 

 are undulating lines nearly parallel. They are disposed symmetrically on 

 each side of the river, and accord with the superficial sheet. The level of 

 the infiltration-sheet is generally above that of the Seine, and rises in pro- 

 portion as it recedes from the banks of the river. Its form depends 

 entirely on the river reproducing all its variations. Then the islands of 

 Saint-Louis and JSTotre Dame are shown to have a distinct subterranean 

 water-sheet, equally an " infiltration- sheet," with concentric horizontal 

 curves nearly parallel with their contours; the sheet slightly elevated 

 towards the central part, and inclining, on the other hand, towards the 

 shores of the islands. Near the ancient barrier Blanche, some wells are 

 fed by a subterranean sheet, of which the average is about 142 metres. 

 This sheet is above the lacustrine limestone, and is altogether distinct 

 from the infiltration-sheet of the Seine. Near the ancient barriers Boche 

 Chouart and Fontarabie, subterranean sheets are met with at an average 

 of 137 metres ; these are also above the infiltration-sheet. 



The map shows well the flowing of these subterranean courses. For 

 example, in the iniiltration-sheet of the Seine, which extends everywhere 

 under Paris, it is seen that the water is necessarily directed from a higher 

 to a lower point ; and consequently, contrary to the ideas generally enter- 

 tained, it inclines towards the shores of the stream. Thus, however para- 

 doxical it may seem at first sight, the Seine plays, in respect to the sub- 

 terranean sheet, the part of a draining canal; it determines the flowing of 

 the water, and effects the drainage of the city. 



