PRESTWICH BRITISH AND FOREIGN TERTIARIES. 239 



for some time. A subsequent extension of the sea to a short distance 

 further south then led to the formation of the lower marine sands of 

 Champagne, the Aisne, and the Oise, and the marine beds of the 

 Woolwich series in East Kent ; whilst the continuance of some small 

 or slow changes in progress during this period caused, after a time, 

 the littoral zone of this sea to be fringed with river or with lagune 

 deposits, in which fresh- or brackish-water areas the lignites and their 

 associated shell-beds were accumulated. A slight further subsidence 

 again, however, led to the partial return, over these freshwater or 

 fluviatile deposits, of the same sea with part of that fauna that the 

 changes of level or silting up of bays had temporarily displaced. 

 This last period appears to have been rather suddenly succeeded by 

 an extensive rise of the Lower Tertiaries to the south, and a further 

 depression to the north, or, I apprehend, more exactly to the S.S.E. 

 and N.N.W.*, whereby in the former direction a large area was pro- 

 bably again converted into dry land, whilst, in the latter direction, 

 the sea only became deeper and somewhat more extensive, covering 

 the area now occupied by the London Clay. Daring this latter 

 important period, the sea stretched over the south-east of England, 

 some part of the north of Normandy, Flanders, and part of Belgium, 

 as far east probably as Brussels, and thence apparently north-eastward 

 in a course which yet requires tracing. That that sea was extensive 

 is evident from the width and depth of the delta of the London Clay, 

 which, with a maximum thickness of 480 feet, exhibits a transverse 

 section in a straight line of not less than 200 miles, — conditions 

 which also could hardly have obtained without a large river f, and 

 therefore a large tract of adjacent dry land, unless possibly by the 

 wear of a long line of coast. 



The wider spread of the seas over the two countries is resumed at 

 the period of the Lower Bagshot Sands. The change seems to have 

 been a gentle one. The waters recommenced their deposition over 

 the shingle and sands capping the Lignite and Plastic Clay series in 

 the Paris district, and over the London Clay in England, and this 

 change was apparently the result of some extensive subsidence to 

 the south. For not only have the strata of this period a greater range 

 southward, but a new fauna abounding in more southern forms is 

 now introduced, and with it appears in extreme profusion the Num- 

 mulites planulatusy followed soon after by the several other species 

 of this Foraminifer which so distinguish the middle portion of the 

 Eocene or Paris group of strata of this part of Europe. 



Many species of the shells which had passed from the Lower 

 marine sands into the London Clay, or had migrated to some adjacent 

 district, reappear in the " Lits Coquilliers " ; but few of them had 

 their existence prolonged to the period of the Calcaire grossier. 



* And not, therefore, in any way connected with the rise of the Wealden and 

 Pays de Bray, the final elevations of both of which tracts I believe to be subse- 

 quent to this period. 



t The large quantity of organic remains derived from land, and at the same 

 time the absence of freshwater shells, must surely indicate the proximity of a 

 considerable tidal river. For this and many other reasons the debris seems to me 

 to have been derived from such a source rather than from the wear of a coast. 



