512 R. M. Brydone — Zone of 0. inlula, S. Englisli Chalk. 



South England dependent on the range of A. gramdattis ; and it is 

 impossible that such a boundary should ever be practicable in the fiekl. 



For Yorkshire Mr. Jukes-lirowne's test is the presence of A. granu- 

 latus. There is nothing to my knowledge which justifies an 

 assumption of correspondence between my zone of 0. pilula in South 

 England and all Yorkshire Chalk (above the zone of Marsupites) 

 which contains A. granulatus. There is not even yet any evidence 

 that A . granulaius has in Yorkshire the definite and constant range 

 which it should have if its range is to determine a zonal boundary, 

 much less that, if it has such a range, that range corresponds with a 

 zone. Assuming that it has in Yorkshire a definite upward range, it is 

 equally possible to argue that the Yorkshire range of A. granulatus is 

 larger than my zone of O.pilula because the South England range 

 of A. granulatus is larger than my zone of 0. pilula, and to argue 

 that the Yorkshire range of A. gramdaius is less than my zone of 

 0. pilula because in South England an interruption of the range of 

 A. granulatus and apparently a passage of A. granulatus into 

 A. quadratus take place in the middle of my zone of 0. pilula. 



The net result is that Mr. Jukes-Browne's zone of 0. pilula is not 

 identical with mine; and if his stands mine must fall. 



Study of the zone of 0. pilida inevitably draws attention to the 

 remarkable differences between districts in the matter of marl in 

 the Upper Chalk. The Hampshire student would leave that county 

 convinced that above the zone of M. cor-testudinarium marl is an 

 anomaly except for the subzone of abundant 0. pilula and the zone 

 of B. mucronata. Even the subzone of abundant 0. pilida seems to 

 be free of marl in the cinctus belt in Hants. 



He would be surprised on reaching the Sussex coast to find that 

 while the zones of M. cor-anguinum (apart from the Vintacrinus band) 

 and A. quadratus were still practically devoid of marl, marl seams set 

 in at the base of the Uintacrinus band and occurred at frequent 

 intervals throughout that band and the zones of Marsupites and 

 0. pilula, and he would find the same state of things in Dorset, though 

 Avith a marked reduction in the extreme west at Middle Bottom in 

 the degree of marliness. (It is curious that Dr. Rowe should have 

 described the zone of 0. pilida at Cockpit Head, which is full of marl 

 seams, as "free from marly veins and bands".) In the Isle of Wight he 

 would at last fi.iid the zone of A. quadratus beginning to give way to 

 marliness at Culver Cliff and substantially afleeted by it at Scratch ells 

 Bay, though even here the zone of M. cor-anguinum still retains its 

 character for purity. The obviousexplanation,whichIhave heard given, 

 is that the marl was introduced by currents. This explanation is quite 

 inadmissible. No muddy current breaking into a calm sea could possibly 

 spread itself so uniformly over such great distances as to deposit these 

 exceedingly thin layers for the miles and miles for which they can be 

 traced with certainty, and the far greater distances for which some of 

 them, such as the marl seam which closes the zone of O.pilula, can be 

 inferred to run without a break. To distribute itself in this way the 

 current would require a speed and volume which would have swept the 

 sea bottom into chaos, audit must have been shut out whenever a marl 

 seam gives place to chalk, and reintroduced whenever chalk is succeeded 



