410 
THE CRETACEOUS ROCKS OF BRITAIN. 
the equivalent of the Chalk-rock, he referred it to the zone of Hoi. 
planus , and the overlying nodular chalk to the zone of Micraster 
co rtestudinarium . 
We think that Professor Barrois’s first view was correct, and that 
the real mistake lay in the correlation of the layer of green nodules 
with the Chalk Rock. There is no doubt that beds resembling 
Chalk Rock occur at several horizons, and in the north of Hamp¬ 
shire there is one in the Terebratulina zone below true “ Chalk 
Rock.” The mere lithological resemblance carries little weight 
unless it is confirmed by the evidence of the fossils, and this is not 
the case in the Isle of Wight. 
The characteristic fossils, Holaster planus and Micraster Leskei 
do not indeed become abundant till we get above another seam 
of marl which lies 8 or 9 feet higher than the seam of black marl 
or clay, and it is at this light grey marl that we propose to terminate 
the Terebratulina zone and with it the Middle Chalk. 
We consider that the Chalk Rock of Wiltshire, Berkshire, and 
more northern counties is represented in the Isle of Wight by the 
lower part of the rough nodular chalk which overlies the upper 
seam of marl. These beds will be described in another volume 
under the head of Upper Chalk* and it is only necessary to note 
here that below them Micrasters are so rare that it is uncertain 
whether any specimen or fragment of one has yet been found in 
what we regard as Middle Chalk. 
In passing from the eastern to the western extremity of the Isle 
of Wight the Middle Chalk undergoes a certain diminution of thick¬ 
ness. Mr. Strahan’s measurements at Culver in 1887* gave a 
thickness of 183 feet 5 inches for the beds between the base of the 
Melbournrock and the black clay-band above mentioned. In 1897 
Mr. Hill succeeded in measuring the same beds in Compton Bay, 
and found the thickness only 141 feet 7 inches, showing a difference 
of about 42 feet (see Fig. 74). If we take the measurements up 
to the seam of grey marl they become 194 and 150 feet respectively. 
Stratigraphical Details. 
Complete sections of the Middle Chalk are exposed in the cliffs 
near Culver Point and in Compton Bay, and portions of this division 
are to be seen in some of the inland quarries. We shall take the 
exposures in order from east to west. 
The greater part of the Culver Cliff section is difficult of access, 
but the highest and lowest beds can be easily reached, and Mr. 
Strah an was able to estimate the total thickness by measuring from 
point to point. The following account is based on that published 
by him in the Memoir on the Isle of Wight (1889), supplemented 
qv notes taken by Mr. Hill on a more recent visit in 1897 : — 
*See Geology of the Isle of Wight, Mem. Geol. Survey, Second Edition 
(1889) p. 89. 
