246 R. M. Brydone — Chalk Zone of Holaster planus. 



The " Bicavea bed" is universally accepted as part of the zone of 

 Holaster planus. The smooth white chalk in massive courses is the 

 typical chalk of the zone of Terebratulina lata. Where between 

 them is the boundary between the two zones to be drawn ? 



[It will be convenient to have short names for the two beds 

 numbered 2 and 3 above. Membranipora Vectensis, Bryd., is a 

 striking form very characteristic of Bed 2, and it will be called the 

 "Vectensis bed". Bed 3 is described in all published sections as 

 containing a layer of green-coated nodules, and I have therefore used 

 these words ; but in the Compton Bay section by far the most prominent 

 layer is a continuous greenish-yellow stony layer, forming a sort 

 of culminating point to the progressive hardening which has been 

 going on both from above and below. The "layer of green-coated 

 nodules " has been named by Bowe the " spurious Chalk Bock", and 

 the bed will be called the " spurious Chalk Bock bed".] 



For a long time before 1903 the "spurious Chalk Bock", or 

 perhaps more accurately the "spurious Chalk Bock bed", was 

 regarded as the direct equivalent of the Chalk Bock. Up to 1889 

 it figured as the top bed of the zone of T. lata, but in that year 

 Strahan 1 included it in his " Upper Chalk ", and so impliedly, if not 

 directly, in the zone of H. planus. 



In 1903 Jukes-Browne 2 drew the boundary between these zones 

 at the "Grey Marl" on the grounds that Holaster planus and 

 Micraster Leslcei do not become abundant until we get above it, and 

 that it was uncertain whether any specimen or fragment of Micraster 

 had up to then been found below it. In 1908 these arguments were 

 almost annihilated by Bowe, who recorded that Holaster planus was 

 as common below the "Grey Marl" as above it, and that Micraster 

 in the shape of M. cor-bovis was by no means rare below it — observa- 

 tions which entirely accord with my own. He transformed the 

 observation about Micraster Leslcei by the statement that it did not 

 occur at all until above the "Bicavea bed", but he, too, adopted 

 the conclusion that the base of the zone of Holaster planus was the 

 "Grey Marl" on the grounds that Echinocorys scutatus, Micraster 

 Leslcei, and Micraster precursor do not occur below it. 3 I have no 

 reason to dispute these statements, but their force as arguments in the 

 above connexion is quite another matter. On p. 221 we find that 

 neither did M. Leslcei or M. precursor occur in the first bed above the 

 "Grey Marl" (i.e. the " Bicavea bed "), and only one specimen of 

 E. scutatus was found there. Now the "Bicavea bed" is unique 

 in its quality of breaking up into blocks, which, owing to the 

 presence of a marl seam both above it and below it, contain no element 

 of any other bed, and whose horizon can be exactly determined 

 owing to the ubiquity in it of Bicavea rotula. The result is that the 

 surface of this bed which is available for study under the most 

 favourable conditions is only limited by the number of blocks of it 

 lying above wavewash on the falls of Culver Cliff and in Compton 

 Bay, and its total area must be estimated in thousands of square 



1 The Geology of the Isle of Wight (Mem. Geol. Surv.), 1889. 



2 The Cretaceous Bocks of Britain (Mem. Geol. Surv.), pt. iii, 1904. 



3 Op. cit., p. 220. 



