280 Mr. S. V . Wood on the Events which 'produced and terminated 



and reflux of the tide through the north-east opening during 

 the formation of the lower greensand in impeding the accumu- 

 lation of sediment ; for if my views are well founded, the sea, after 

 the close of the lower greensand deposit, found its way through 

 various parts of the barrier then changing into the condition of 

 a cluster of islands, so that the flux and reflux in the region of 

 the old strait became less and less, until a deep-water condition 

 was attained permitting the accumulation of this great thick- 

 ness of chalk. Again, according to the same authority, at a 

 distance of only twenty miles south of Norwich, at Diss, where, 

 if my delineation of it be tenable, the north- western shore of the 

 palaeozoic barrier was near, and the water consequently shallower, 

 the chalk has a thickness of only 500 feet. A much less thick- 

 ness than that at Norwich appears to prevail pretty generally 

 over the region of the barrier, except that at Harwich it rises 

 to 880 feet j while in the Isle of Wight, over the central part of 

 the old basin, and therefore in a deep part, the greatest thick- 

 ness of any is found*. The increased thickness on the borders 

 of Essex and Herts and in Sussex corresponds with the edges 

 of the barrier being there passed f. 



The presence of the upper greensand overlying gault in Nor- 

 folk and Essex as well as in Kent and Sussex, although very 

 attenuated in the first county, shows that the diminished thick- 

 ness of the chalk in the parts where such diminution is observed 

 is not due to those parts undergoing submersion at a later stage 

 of the chalk than others. Yet we may infer that the shallower 

 condition of the water over the barrier impeded the accumula- 

 tion of sediment there, a result to which perhaps some islands 

 still remaining above water during the deposit of the upper 

 greensand and of the earlier part of the chalk may have contri- 

 buted by inducing currents between them. 



It is worthy of remark, also, how the mineralogical character 

 of the cretaceous deposits around and over the palaeozoic barrier 

 mark the successive stages of the depression which prevailed 

 through that age, until the close of the middle chalk or chalk 

 with flints. While the basin was a quiescent inlet, the sediment 

 swept into it was of an arenaceous character, more or less inter- 



* About 1.300 feet. Fitton, Geol. Trans. N. S. vol.iv. p. 318. 



t Mr. Prestwich (Quart. Journ. Geol. Soc. vol. viii. p. 256) attributes the 

 varying thickness of the chalk to denudation anterior to the deposit of the 

 lower tertiaries, and shows that from this cause the chalk over the North 

 Downs has in places been reduced to the lower beds only, and to a thick- 

 ness of 400 feet; and this agrees with the views above, as, the region of 

 the Weald being beyond the barrier, the deeper-water conditions pre- 

 vailed, permitting a free accumulation of sediment that is now in places 

 extensively removed. The varying thickness of the chalk in many places 

 over the barrier cannot, however, be explained by denudation. 



