80 
THE GEOLOGIST. 
Cartes Geologique et Jlydrologique de la Yille de Paris. By M. Delesse. 
Savy, 1861. 
liatber 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. Uaussmaun, Prefect of the Seine, — one a geological 
map of the subsoil of the city itself, as far as authorized actual excava- 
tions ; the other hydrological, 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 ditEcult, 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 d' injiliratwn), 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 tlie superficial slieet. 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 jN^otre 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. Kear 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. !N^ear the ancient barriers Roche 
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 infiltration-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 ; il determines the flowing of 
the water, and effects the drainage of the city. 
