R. M. Brydone — Zone of 0. inlula, S. English Clialh. 449 



V. — The Zone of Offaster pilula in the South English Chalk. 



By E. M. Beydone, B.A., F.G.S. 



(Continued from p. 411.) 



III. Dorset. 



rpHERE are three exposures of the horizon of the zone of 0. pilula 

 1 on the Dorset coast — at Ballard Head, Arisli Mell, and Middle 

 Bottom. Dr. Rowe's description of the Ballard Head section is so 

 unattractive, and the section itself so inconvenient for access and 

 study, that I put it aside to be attempted only if I had time to spare 

 after working out the others, which has not yet happened. Of the 

 other two, that at (or rather under) Middle Bottom is so much the 

 better that I take it first. 



A. Middle Bottom (hettveen White Nothe and LxdicortK). 



As is well known from Dr. Rowe's works,* the bay at the head of 

 which is Middle Bottom provides a section in chalk lying above the 

 zone of Marsupites and having an accessible thickness of about 

 200 feet, the point at which the highest beds are accessible being 

 under Middle Bottom itself. At this point the beds are apparently 

 flat in all directions, and I did not notice anything about them which 

 I could reconcile with Dr. Uowe's description of them as "riven and 

 shattered in every direction". For some distance westwards from 

 the recess under Middle Bottom the beds seem to continue flat, but an 

 easterly dip (apparently only a component of a stronger S.S.E. dip) 

 then sets in and brings up the beds below in regular succession. The 

 sequence measured was as follows : — 



Highest Flint Seam in Keach in the Eecess under 

 Middle Bottom. 



' Coast Sections, II, pp. 11-13. 



decade VI. — VOL. I. — NO. X. 



29 



