PRESTWICH — WOOLWICH AND READING SERIES. 135 



The prevailing set of the currents or tide is shown by the pre- 

 vailing dip of the layers of false stratification being northward, or 

 from oiF the presumed island, at angles varying from 10° to 35°. 



The irregularity of this river-accumulation is shown in the extreme 

 irregularity of the beds, which constantly exhibit the shiftings and 

 changes seen in the sand-banks of existing estuaries. Not only are 

 the several members of the "Lower London Tertiaries" divided by 

 irregular surfaces, but the Woolwich series itself often presents in its 

 central area instances of its several beds being deposited upon slightly 

 eroded surfaces one of another. 



Further, the occurrence of Lithoclomus in the eastern area, and of 

 Pholas in the central area, shows the near proximity of a coast-line. 

 The beautifully preserved plants of Reading also indicate neighbour- 

 ing dry land. 



Judging from all these phsenomena I infer that the period of the 

 Thanet Sands was brought to a close by a movement of elevation, 

 which threw off the sea from the shores of the island we have alluded 

 to, and swept down into the changed and shallower sea-bed the 

 coarser sand and rounded shingle existing ready-formed on the 

 coast-line. To such a movement I attribute the pebbly light green 

 sands forming the base of the Woolwich and Reading series ; the 

 ordinary currents of the sea having, after the first movement of 

 elevation, distributed the debris thus amassed and formed the beds 

 on which the Ostrea Bellovacina lived. In the mean time the size 

 of the island was necessarily increased by this movement of elevation, 

 and its drainage having thereby become larger, and its streams and 

 rivers more important, one of these rivers, still not a large one, must, 

 during a slightly subsequent period of quiet, have brought down 

 and accumulated the Woolwich shelly clays with their estuarine and 

 fluviatile shells. This settled state of things was, however, not long- 

 continued ; renewed, but slow movements, probably of subsidence, 

 must have taken place ; the Woolwich river action must have ceased, 

 or rather its direction changed ; for that its debouchure was slightly 

 altered, is indicated by the fact that the next fluviatile zone was not 

 accumulated over the older beds, but on one side of them ; instead of 

 being chiefly at Upnor, Woolwich, and London, it took place from 

 Bromley to Deptford, Wandsworth, and Guildford. 



In the mean time in the more open sea of East Kent these slight 

 changes were less felt, and the marine condition of the strata and 

 their character as they existed at the " Thanet Sands " period re- 

 mained for a time comparatively unaltered. To the westward, on the 

 contrary, some new and large river-action appears to have been opened 

 out from another land by the changes which led to the formation 

 of the Woolwich and Reading series. The debris forming; the mottled 



than mere chalk cliffs, and that source was prohably some of the subcretaceous 

 arenaceous strata, which present a hthological base siniiLir to that of the Lower 

 Tertiary sands, although the materials are somewhat differently sorted and some 

 few of the more soluble and finer portions are washed out. 



