MIDDLE CHALK—SOUTH DORSET. 
415 
CHAPTER XXXII. 
THE MIDDLE CHALK IN SOUTH DORSET. 
General Description. 
The Middle Chalk of South Dorset resembles that of the Isle of 
Wight, but its thickness appeal’s to be rather less where the most 
satisfactory measurements can be made, being only about 128 feet 
in Ballard cliff and about 134 at White Nothe. 
A peculiarity in the Middle Chalk of Dorset is the setting 
in of a band of hard smooth chalk at the base. This band is 
six feet thick in Ballard cliff and thickens westward till it is 
more than 20 feet in Mupe Bay and Lulworth Cove ; west of that 
we have no measurement of its thickness, but it probably 
becomes less. As in other parts of England the Belemnite Marls 
are succeeded directly by the Melbourn Rock, we have had some 
doubt whether this intermediate band should be regarded as be¬ 
longing to the Lower or to the Middle Chalk. It contains very 
few fossils, but such as do occur appear to be of Middle Chalk 
species; further, in some localities there is no very clear demar¬ 
cation between it and the representative of the Melbourn Rock. 
Consequently we have grouped these beds as Middle Chalk and 
Dr. Rowe informs us that he takes the same view. 
The first description of the zones composing the Middle Chalk 
in the coast sections was that given by Professor Barrois in 
1876 (“ Recherches ” pp. 78 to 102). The district was re-sur¬ 
veyed by Mr. Strahan in 1888-90, and is described in his 
memoir on the “ Geology of the Isle of Purbeck and Weymouth " 
(1898). In this he recognises the Melbourn Rock at the base of 
the stage, and for its upper limit he takes a layer of green coated 
nodules, which he regards as the equivalent of the Chalk Rock 
and places at the base of the Upper Chalk. This layer is no doubt 
the equivalent of the layer which has been called Chalk Rock in 
the Isle of Wight, and which we have discussed in the last chapter. 
We feel sure, however, that the true base of the Upper Chalk is 
to be found rather higher up, and that it must be taken, as in the 
Isle of Wight, where Micrasters become abundant and are assoc¬ 
iated with Echinocorys scutatus as well as Holaster planus. For¬ 
tunately these coast sections have been studied during the presen- 
summer (1899) by Dr. A. W. Rowe and Mr. C. D. Sherborn, and 
their careful collection of the fossils and knowledge of the zonal 
faunas has put the matter beyond doubt. They permit me 
to say that, so far as the Mupe Bay section is concerned, the 
