and Silurian Si/stems in Belgium. 151 



often no longer possible to distinguish it from a modified 

 chalk. 



These modifications being observed along a line running 

 from N. W, to the S.E., and terminating at Borcette, we may 

 conclude with great probability that the changes which the 

 cretaceous rocks have undergone were the effect of some 

 plutonian agencies connected with the phaenomena of the 

 mineral and hot springs of this town. 



I shall conclude this report with some remarks on the mo- 

 dern deposit of Flanders. 



It consists for the most part of a clay more or less plastic, 

 grayish, calcariferous, sometimes sandy, and containing pro- 

 ducts of art, and some shells analogous to those which live 

 upon our coasts at the present period. This clay forms a 

 horizontal bed which in some localities is three metres thick. 



Under the clay in a great many places is found a layer of 

 turf, which sometimes attains the thickness of five metres, and 

 which is composed, according to the observations of M. Bel- 

 paire (Memoir on the changes which the coast from Antwerp 

 to Boulogne has undergone), of two distinct parts, namely, the 

 upper part of terrestrial, and the lower of aquatic plants. Be- 

 neath the turf clay is also sometimes found, but more gene- 

 rally there is nothing but sand, in which shells analogous to 

 those now found in the sea are met with. 



The modern deposit forms a band bounded on the sea side 

 by the Dunes, and on the land side by a sinuous line traced 

 on the map which accompanies the Report*. It is easy to 

 distinguish this deposit from the neighbouring formations by 

 its clayey nature, the fossils it contains, and its perfect hori- 

 zontality; it is besides covered with rich pastui'es through 

 nearly the whole of its extent, which also distinguishes it from 

 the arid sandy soil that surrounds it. The connexion which 

 exists between this deposit and the clay of the Polders, which 

 are still actually forming, and the presence of shells similar 

 to those which live on the coasts, sufficiently indicate its re- 

 cent origin, and that the sea, at a period not very remote, 

 advanced into the lands nearly as far as Dixmude, Ghistel, 

 Bruges, Assenede, &c. as may be seen upon the map. 



Our honourable colleague, M. Belpaire, from the investi- 

 gation of historical documents, comes to the same result. He 

 even fixes the origin of the clay formation at the period of the 



* This refers to a small map accompanying the Report, as given in the 

 Bulletin, showing the geographical extent of the modern deposits of Flan- 

 ders, and the maritime limits of the ancient Belgiutn : the former appears 

 as an edge or border, extending from the ancient to the present coast- 

 line, and in the direction from Furnes to Antwerp. 



