96 
THE CRETACEOUS ROCKS OF BRITAIN. 
of the massive whitish chalk which constitutes the zone in that 
island, we have here alternating beds of chalk and marl, and the 
greater thickness may be due to the local increase of argillaceous 
matter in the Isle of Purbeck. Possils are scarce in these beds, 
but Am. ( Acanth .) navicularis occurs to within 20 feet of the top. 
The Belemnite Marl, which is here only five feet thick, increases 
to 12 feet near Corfe, where it has been extensively quarried and 
mined for cement-making, and where an unusually large number 
of fossils reward the labours of Mr. Rhodes (see list at the end of the 
chapter). The presence of Scaphites cequalis and Plicatula injiata 
among them justifies the retention of this zone in the Lower Chalk 
or Cenomanian stage. 
The outcrop of the Lower Chalk reaches the coast again at 
Flower’s Barrow by Wor barrow Bay, where a good clear section is 
exposed in the cliff. This was measured by Mr. Strahan* and 
subsequently by Mr. Rhodes when collecting fossils there; they 
agree closely in total thickness, but as Mr. Rhodes’ account gives 
rather more detail, it is here reproduced : — 
ft. 
Buff marl (Belemnite band) - 6 
Buff blocky chalk with marly partings - - about 55 
Massive grey chalk with siliceous nodules - - - 21 
Massive grey chalk, glauconitic at the base 6 
Sandy glauconitic chalk with scattered phosphatic 
nodules (“ Chloritic Marl ”) .4 
92 
It will be noticed that the zone of Ammonites varians , or Group 
A, has here put on quite a different aspect; it no longer contains 
layers of chalk marl, but consists entirely of massive chalk, and it 
contains lumps of grey siliceous stone with cores of black flint, 
lumps which do not come away freely as flints usually do, but adhere 
to the surrounding chalk, so that there seems to be a kind of pas¬ 
sage from chalk to flint. Such flints occur in this zone throughout 
the west of Dorset and the adjoining part of Devon. The only 
fossils found in it at Wor barrow were Am. [Acanth .] rotomagensis, 
Pleurotomaria perspectives and Ostrea vesicularis. 
The Group B is similar to that in Ballard cliff', except that it is 
only half as thick, and the total thickness has diminished from 143 
to 92 feet. 
In Mupe Bay (formerly spelt Mewps) the Lower Chalk occupies a 
very narrow space, owing to the high dip, which is from 70° to 80°. 
Mr. Hill examined this section in 1893, and again in 1900, and 
felt convinced that what had been regarded as “ Upper Green¬ 
sand ” was in reality a downward continuation of the sandy 
basement bed of the Chalk, and on the second visit he obtained 
fossils which confirmed this opinion. In April, 1901, Mr. Strahan 
revisited the section, in company with Mr. Hill, and was able to 
* Geology of the Isle of Purbeck, etc., 1898, p. 176. 
