408 
THH CRETACEOUS ROCKS OF BRITAIN. 
CHAPTER XXXI. 
THE MIDDLE CHALK IN THE ISLE OF WIGHT. 
General Description. 
Starting from the southern side of Culver Point the outcrop of 
the Middle Chalk forms a narrow band along the Downs as far as 
Carisbrook Castle in consequence of the high dip. West of this 
the space occupied by this division widens out and extends south¬ 
ward owing to the flattening out of the beds between the two anti¬ 
clinal folds described by Mr. Strahan as the Sandown and Brixton 
anticlines.* Neither of these flexures stretches completely across 
the island, and where one meets the other the beds flatten out 
and appear even to be deflexed into a short and shallow syncline 
which crosses the valley of the Medina between Rookley and 
Gatcombe Down (see Fig. 73). 
North-west of Shorwell the dips become greater again as the 
influence of the Brixton anticline makes itself felt, and the basset 
surface of the Middle Chalk is again reduced to a narrow band, 
which runs into the sea at' Compton Bay. 
The first adequate description of the beds which form the Middle 
Chalk or Turonian of the Isle of Wight was that published by Pro¬ 
fessor Barrois in 1875.f In this memoir he clearly distinguished 
the zones of Inoceramus labiatus (—zone of Rhynchonella Cuvieri ), 
of Terebratulina gracilis and of Holaster planus , and though in 
1876 (Recherches, p. 107) he altered his opinion with regard to 
the last zone, we believe his original identifications to be correct. 
In 1865 Mr. Whitaker gave some account of the Isle of Wight 
Chalk, and indicated a thin bed of hard chalk with a layer of green- 
coated nodules which he regarded as the equivalent of the Chalk 
Rock of Berkshire and Wiltshire, which lie had described a few years 
earlier. As the lied is certainly not far from the horizon of the Chalk 
Rock, and as in those days zones and zonal fossils were little studied, 
it was a very probable correlation, and it has been accepted by all 
subsequent writers on the Isle of Wight. We know now that the 
true Chalk Rock lies in the zone where Holaster planus is asso¬ 
ciated with many Micrasters, and, as Professor Barrois pointed out 
in 1875, the fauna of this zone does not begin to come in till we get 
some feet higher than the layer of green-coated nodules, and above 
a seam of black marly clay. He then placed the nodule-bed in the 
Terebratulina zone, but, afterwards, assuming that this bed was 
* Geology of the Isle of Wight, Mem. Geol. Survey, Second Edition, 
p. 240, 1889. 
t Bibliotheque de l’Ecole des Hautes Etudes, Sc. Nat., Tom, xiii, 
