684 SEC. 15. GEOGRAPHY. 



The waterbed supported by the green clays is by far the most important in 

 the plateau of La Brie, -whilst in the valleys the wells are supplied by the beds 

 of infiltration. 



The hydrological map of the Seine and Marne 'enables a judgment to be 

 formed of the depth it would be necessary to attain in order to reach the 

 water deposits, and an opinion can also be formed of the geological nature of 

 the soil which would be met with in boring. A judgment can also thus be 

 formed of how the Avater sources and beds act in an absorbent soil like chalk ; 

 a question which is of considerable interest at the present time, when a tunnel 

 is proposed to be dug between France and England. 



3. Lithologic or Submarine Chart of the Seas of Europe. This chart, 

 compiled from hydrographical observation, serves to convey an idea of the 

 nature of the rocks which are covered by the seas of Europe. 



The bed of the seas is characterised by horizontal curves traced from the 

 depths ascertained by a great number of soundings. The beds distinguished 

 are the stony rocks of various kinds, the clays, the mud, the muddy sand, 

 the gravelly mud, the sand, the grit, and the pebbles. 



These are represented by conventional colours, as in geological maps, but 

 one colour indicates the same lithological character. The localities most rich 

 in shells are also determined and marked by shadings. 



Among the rocks which form the bed of the seas, some are anterior to the 

 present epoch, and are constantly worn or beaten, and are covered by 

 deposits. They may be stony, like the granites or paving stones and the 

 limestones, but they are occasionally tender and brittle like the clay, and some- 

 times movable like the sand and the pebbles. 



The rocks anterior to the present period are seen principally in the parts 

 which jut out along the coasts, in the straits, and in the localities where the 

 tides run strong. The rocks belonging to the deposits of the present epoch 

 are essentially of the movable character, they fill especially the places 

 having hollows or depressions ; they cover the plateaux, and they accumu - 

 late chiefly in depths where the tides or currents are not rapid. 



The geological study of the coasts occasionally enables us to trace beneath 

 the sea the prolongation of the rocks which have emerged, and to study 

 portions of the geological submarine chart. 



4. Ancient and present Seas of France. An endeavour has been made to 

 represent the ancient seas of France in the silurian period, the trias, the lias, 

 the eocene, and the pliocene. Starting with the facts furnished by geology, 

 there have been comprised in one blue tint all the points in which the 

 existence of soils deposited by these various seas have been established. 

 But these soils are not now what they were when originally formed ; they 

 have been partially destroyed by the atmosphere and by flowing waters ; 

 they have been sundered by faults ; they have been subjected to numerous 

 and complicated overthrows ; and it is chiefly their actual condition that it is 

 proposed to describe. With this view, in order to study the lands in the 

 parts where they are now visible, as well as in those where the} 7 are covered 

 up, they have been represented in relief by means of horizontal curves. The 

 curve having the side O is particularly interesting, as it shows the inter- 

 section of the actual level of the sea with the surface of the land under con- 

 sideration, and, if we admit that the sea level remains constant, all the 

 portions of land which are formed above this curve have necessarily been 

 elevated. 



The total of horizontal curves shows the orography of the superior surface 

 of land, which has resulted from the wasting and changes since its deposi- 

 tion. 



By the aid of these maps we can well see that while a basin has received 

 the superposed soil, and the original elevations and depressions are to a 



