162 A. M. Davies — Base of the Oautt of E. England. 



It is clear, however, that from the first coming-in of Gault Clay 

 in Norfolk to its last fossiliferous exposure on the banks of the 

 Thames, wherever its basal zone can be recognized it is that of 

 Hoplites interruptus. 



If we now turn to the underground distribution of the Gault 

 in the area between the two outcrops we have been considering, 

 we find that the Gault, taken as a whole, is continuous under 

 the entire district so far as the evidence goes. It varies greatly 

 in thickness, but if we plot out all the borings on a map and 

 sketch out lines of equal thickness between them, we find that, 

 in spite of many irregularities, these lines have a general N.W.-S.E. 

 trend, and indicate a north-easterly thinning, which becomes 

 strikingly manifest at Weeley, Stutton, and Harwich. As this 

 north-easterly thinning is accompanied by disappearance of the 

 Lower Greensand, it is due probably to overlap, and not to 

 erosion of the Upper Gault like the northerly thinning along 

 the outcrop. There is, therefore, an a friori probability that the 

 lower zones of the Gault as we pass to the north-east first take 

 on a shallow - water facies (which would be called ' Lower 

 Greensand ') and then are overlapped altogether. But so many 

 different circumstances combine in affecting the thickness of 

 a deposit that it is not safe to base any definite conclusions on 

 it alone. From only one of the borings in this area have fossils 

 been recorded which could help in the determination of the basal 

 zone of the Gault, and that is the Eichmond boring. Dr. Gregory 

 has discussed' the evidence from this boring, and concludes that 

 it is in favour of the absence of the H. interruptus zone. 



After pointing out that H. splendens occurred both at 1,100 and 

 at 1,128 feet depth, he says: "This species ranges from the 4th 

 to the 7th zone of the Gault. If we make the somewhat improbable 

 assumption that this [the lower] specimen came from the very 

 lowest possible point at which it could have occurred, then zones 1-3 

 are reduced to a thickness of only 13 feet. Thus, either the three 

 basal zones are proportionally much thinner at Richmond than 

 they are at Folkestone, or else one or more of them is there absent. 

 The latter view appears the more probable." 



I am afraid this argument will not stand a close examination. It 

 is true that in the Weald Memoir the range of H. splendens is given 

 as zones 4-9 (not 7), but Price records it from his zones 2-8,^ 

 answering to De Eance's 3-8. Combining the two, we find the 

 maximum range to be through about 27 feet, starting at 10 feet 

 from the bottom. At Richmond the two specimens were 28 feet 

 apart, and the lower one 13 feet from the bottom — a curiously close 

 approximation to the Folkestone range. Moreover, H. splendens is 

 not 80 restricted everywhere as at Folkestone. I have found it 

 along with H. interruptus at St. John's, near Sevenoaks, aud 

 Mr. R. B. Newton has recorded it from the interruptus-zone of 



1 Loc. cit., p. 101. 



2 Quart. Journ. Geol. Soc, vol. xxx, p. 342. 



