246 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [Jan. 24, 



Abundant illustration might be derived from our own East Ang- 

 lian area to controvert this view, and prove that a great break and 

 a long interval of time intervened between the Crag beds and the 

 Boulder series, during which the North Sea was a terrestial area. 

 The Belgian evidence bears most on the length of the interval or 

 the lapse of time. 



Over the whole of the broad area of the Scaldesien, Diestien, and 

 Bolderien systems of the map of Dumont the formations are seve- 

 rally at present represented only by isolated masses or patches: 

 away from Antwerp, and as the country rises southwards, these 

 occur as the cappings of hills, or ridges. These sandy strata, which 

 perhaps may have admitted of easy denudation, must, from their 

 composition, have extended from place to place ; and what now re- 

 mains is not a thousandth part of what once was. In the case of 

 the valley of the Dyle, by Louvain, the denudation has extended down 

 to the Bruxellian beds of the IS'ummulitic series to a depth of 1 00 

 feet ; and the completeness with which all the materials have been 

 removed is well seen in the slope of the hills, as also on the level, 

 along the railway cutting. The denudation about the Bolderberg is 

 of nearly like amount as at Louvain. 



In these cases, as in that of Schelle, the loss and Campine sands 

 were spread out after the denudation of the Crag-sea beds ; and as 

 these belong to the later portion of the glacial period, the denudation 

 of the country must be referred to some intermediate stage *. 



4. Variation in Depth. — The movements of elevation and depres- 

 sion of the water-level which the North- Sea area experienced during 

 the Crag period, are very simple, and so far are in harmony with the 

 apparent short duration of that period. 



The chalk flints, and the Septarian blocks derived from the sub- 

 jacent Rupellien beds, which also occur elsewhere at the base of the 

 Diestien system (Edeghem) are the remains of shallower water con- 

 ditions than such as followed, or are proofs of depression. 



Both on our own Suffolk area, as on that of Belgium, the change 

 indicated from the Bed Crag to the Bryozoan, and from the Scalde- 

 sien to the Diestien, was of diminished depth, or is evidence of 

 elevation. Its subsequent emergence was in the same direction. 



The earhest condition of the Crag-sea area here indicated is sup- 

 ported by a study of the list of shells given by Mr. Nyst, as obtained 

 from the gravel bed at the base of the Diestien system at Edeghem f. 



The Faluns of Touraine have experienced an amount of denudation 

 such as is presented by the Belgian Crag. M. d'Orbigny has made 

 this remark with respect to the first-named formation, that removal 

 to so great an extent should have happened to synchronous accumu- 



* Considered with reference to a line of section from Fort 2. towards the 

 North citadel, the whole of the Crag series has been planed off from N.E. to 

 S.W., so that lower beds come to the surface ; and this has happened twice : — 

 1. during the Crag period, when by diminution of depth the deeper sea-bed was 

 denuded, and covered up by the Scaldesien ; 2. when its general surface was 

 eroded and removed. 



t Bull, de I'Acad. Koy. de Sciences de Belg. s6r. 2, vol. xiii, p. 29. 



