PRESTWTCH — BRITISH AND FOREIGN TERTIARIES. 131 



quillity restored, then these pebbles of the older Tertiaries were 

 subjected to the attacks of boring molluscs, and small corals grew 

 amongst the pebbles. The sea-sediments now changed entirely : 

 quartzose sands, derived from other shores and different sources, and 

 formmg the mam and common mineral element in the two countries 

 are substituted for the calcareous deposits of the Calcaire grossier' 

 In England a change of nearly like value took place : the alternating 

 sandy beds of theBracklesham Sands are suddenly succeeded by a thick 

 mass of compact clays and quartzose sands, the change being marked 

 by a thm but contmuous bed of well-rounded chalk-flints strewed 

 over the bed of the sea in which the Barton Clay commenced its 

 formation. Some of these pebbles are as l.irge as cannon-balls • they 

 are evidently removed from the position in whicli they were formed 

 for they usually overlie loose sands— a floor on which the flints could 

 not have been worn down as we now find them. 



Accompanying this break in the sequence and this change in the 

 mineral character, a partially new fauna appears in both areas, but 

 still many of the old forms remain. The change in this respect in 

 the hnghsh and French areas is about equal in degree. 



A long period of rest now succeeded, during which the lower beds 

 of the Sables moyens, with their large and varied fauna, were formed. 

 In England the same relatively greater amount of subsidence which 

 we noticed at the former period was continued in this one : the Bar- 

 ton Clay is many times thicker than the corresponding beds of the 

 Sables moyens. The fossils in it are not so numerous, and are much 

 dispersed. Other changes then succeeded in the French area ; and 

 probably the sea again became more restricted*. In Belgium the 

 change appears less marked. 



The Barton-clay sea seems again to have been more connected 

 with water opemng to the northward than did that of the Bracklesham 

 Sands ; for several species of the London-clay sea, which, as I have 

 before shown, was probably connected with northern seas, that had 

 disappeared m the intermediate Bracklesham period, reappear in 

 the Barton series. In fact the fauna of this group, together with 

 that of the Sables moyens, has not so southern an aspect as that of 

 the Calcaire grossier and Bracklesham period. 



It is interesting to note the evident migration of species eoina: 

 on all through these periods. INIanv physical changes, some slow and 

 others rapid, took place from time to time ; many of them were only 

 local, whilst several extended over wider tracts and affected the 

 several areas, but in such cases we find that the effects were in some 

 places stronger than in others. It would seem that larger or smaller 

 portions of the fauna of each period or of each subdivision were in all 

 mstances preserved and transmitted or continued upwards. Some- 



* M. p'Archiac has suggested that the great accumulation of barren sands on 

 the north-eastern edge of the area of the Sal)lcs movens are probably the dunes 

 of an old land (D Archiac, Progr. de la Geol. vol. ii. p. 571). The large sliingle 

 beds mentioned by M. Graves might also suggest that land ^vas not far off-nos 

 sibly part of the Pays de Bray might at this i)eriod have been an island with 

 ranges of chalk chtfs. 



K 2 



