part 4] JURASSIC chronology. 411 



These are the facts which have to be accounted for, and the 

 following theories may be put forward : — 



(1) The lithographic or white-stone matrix began to be de- 

 posited in ^YG-'^'alciferum time, and its deposition continued until 

 the time of early Diimortieria^ although there were vai-ious breaks 

 in the record due to penecontemporaneous erosions. 



(2) The whole of the Watton Bed was laid down at about one 

 date, Hammatoceras (that is, the base of the Watton Bed corre- 

 sponds in date with the upper part of Walker's Bothenhampton 

 section — it is ^o&t-striatulum wholly) : therefore, it is altogether 

 later in date than an}'- part of the Junction-Bed of the Western 

 Cliffs, and than all but the last layer of the Bothenhampton 

 section. So far as agreement in lithic character is concerned, 

 this would appear to be correct. So far as faunal contents are 

 concerned, nothing that has been found lower than layer 4 is in 

 favour of it. 



(3) The Watton Bed consists of two similar deposits of rather 

 widel}^ different dates ; there was a deposit of, sa}'', ^^vQ-falciferum 

 date which had the characters of white lithographic stone, and 

 made up the lowest part of the bed, then there was a break, 

 possibly with denudation, yAxA^ faJciferum and ^o^i-falciferiim 

 deposits were being laid down in surrounding areas, as for instance 

 in that of Thorncombe ; then followed a time of deposition at 

 Watton Cliff, for which materials were obtained ixovix falciferum 

 and later deposits, from the ^vQ-falciferum white-bed deposit, 

 and even from the thorncomhiensis rock. Afterwards came 

 another period of definite lithographic-stone deposition beginning 

 in Hammatoceras hemera (laid down not only at Watton Cliff, 

 but at Bothenhampton), and continuing to the earliest part of 

 the Dumortieria hemera. In other words, two deposits of similar 

 character, but of widely different dates, have coalesced. 



Let us consider these theories : the first theory carries various 

 difficulties. That a homogeneous deposit existed for many nemerse, 

 and at the same time shows various non-sequences, is not a diffi- 

 culty ; for such a deposit, lasting through more than thirty 

 hemerse with various non-sequences, is illustrated in my last paper.'^ 

 But the difficulty is to suppose that during all the long time of 

 about eighteen hemerse which the fauna of the Watton Bed 

 would require for its deposition (that is, from ^Ye-faJciferum 

 to early Dumortieria^, the white-stone conditions had so remark- 

 ably restricted a geographical range — not extending to the cliffs 

 west of Ej^pesmouth, about 7 furlongs westwards, where different 

 conditions of deposit obtained, nor to Bothenhampton, about 

 2 miles away to the north-east, until the time of Hammatoceras 

 hemera, when white-stone conditions did come in at that locality. 



jSText there is evidence that white-stone conditions were not 

 really continuous. Some small Dactyloids are in a brownish 



1 I, 9, p. 100. 

 Q. J. G. S. No. 312. 2 r 



