342 ME. F. W. HARMER ON THE LENHAM BEDS [Aug. 1 898, 



Seams of reef-building polyzoa are present at various localities 

 in the upper part of the Crag (zone G of Prestwich), as, for example, 

 at Aldeburgh (pits 30 & 33), at Iken (pit 24, etc.), and at Sud- 

 bourne (pit 18) ; but this is also the case, as Sir Joseph reminds us, 

 at Sutton, in what he regards as zone E.^ 



At pit 22, where the ferruginous Crag (zone G) is composed 

 simply of comminuted material, specimens of Fascicularia are very 

 common, but they occur also at other localities in the unaltered 

 Crag, as at Broom Hill (No. 11), and at Sutton (Prestwich's zones 

 E & E). Generally the Crag along its eastern margin, as at 

 pits 15, 20, 22, & 23, is composed of comminuted material only, 

 the seams of large shells occurring principally in the western por- 

 tion ; this may be accidental, or it may be due to the action of 

 currents varying in strength. 



Thus no satisfactory division can be drawn between the shelly sands 

 and the ferruginous rock, ^^e have seen that at one locality (No. 3) 

 the indurated Crag comes down to within 10 feet of the base of the 

 formation, while at another, within a distance of 2 miles (No. 12), 

 the shelly sands have a thickness of 40 feet. It is clear that the 

 rock-bed has been affected by the percolation of acidulated water, as 

 it is in this way that the arragonite-shells have been removed. 

 The irregular line separating the two varieties of Crag may be due 

 therefore to the greater or less depth to which the infiltration has 

 penetrated. That this explanation is the correct one is, I think, 

 proved by the Iken section, where for a short distance only a small 

 portion of the shelly sands has been accidentally protected from its 

 operation. 



If, then, the separation between the two principal and apparently 

 self-evident divisions of the formation breaks down, it will add to 

 the difficulty of maintaining the distinction between the remaining 

 zones B, C, D, E, & E, into which Prestwich proposed to divide one 

 of them — divisions which cannot be worked out stratigraphically, 

 and which do not, I submit, represent any distinctive palseontological 

 horizons. 



I have discussed the zone-theory of Sir Joseph Prestwich at con- 

 siderable length, and with much detail, not only because it seems 

 due to so eminent a man to do so, but also because of the great 

 importance of the subject. It is hopeless to attempt to arrive 

 at any clear conception of the history of the older Pliocene period 

 in England, unless we can first ascertain whether this hypothesis, 

 with all its far-reaching consequences, should be accepted or not. 



Let us, however, enquire whether there is any evidence in favour 

 of his view that, commencing with a gradual invasion of the English 

 part of the area by the sea (indicated by the basement-bed A), the 

 formation of the Coralline Crag was attended, first by an important 

 subsidence, and then by a re-emergence of the Anglo-Belgian basin. 

 The former is said to have taken place during period B,^ beds C & 



^ Quart, Journ. Geol. Soc. vol. xxvii (1871) p. 119. 



- Ibid. p. 135. Only one exposure of zone B is recorded by Prestwich, 

 namely, a bed at Sutton 4 feet thick. 



