458 
THE CRETACEOUS ROCKS OF BRITAIN. 
exposes hard yellowish lumpy chalk, which is probably not far 
above the Melbourn Eock, but is much weathered and disinte¬ 
grated. No fossils were found. 
A quarry just north of Streatley shows about 30 feet of the 
Terebratulina zone, the chalk being firm and white with a few 
scattered flints in the highest part; from this pit Prof. Barrois 
obtained Ostrea vesicularis, Spondylus spinosus, and fragments of 
Inoceramus. 
2. Oxford and Bucks. 
It will be convenient to describe the Middle Chalk of these two 
counties together, because this division crosses their narrowest 
parts where they have a combined width of only about 26 miles, 
and because certain inlying tracts include parts of both counties. 
The general succession and aspect of the beds is similar to those 
of the Middle Chalk in Berkshire. The Melbourn Eock rises 
from the alluvium of the Thames near Cleeve Mill, north of Coring. 
Thence it passes north-eastward and farms a conspicuous feature 
or ridge in advance of the main escarpment as far as Wat- 
lington and Lewknor. Beyond that place its outcrop retreats 
to the foot of the main escarpment. The Chalk Eock and its 
associated beds rise from the Thames near Whitchurch and mount 
to higher and higher levels as they pass north-eastward till they 
reach a height of between 700 and 800 feet. 
The chalk which lies between these two bands of hard rockv 
chalk occupies an area of some width between Ipsden and Wat- 
lington, but the tract then becomes much narrower until it reaches 
the valley of the Loudwater. 
In Buckinghamshire the escarpment of the Chalk is breached 
by the heads of several valleys that drain south-eastward into the 
Thames, and along these valleys the Middle Chalk is exposed 
for long distances. Thus, in the valley of the Loudwater, Middle 
Chalk is exposed along its whole length from near Eisborough 
to its junction with the valley of the Thames. 
Flints occur throughout the Terebratulina zone, chiefly 
along a few definite places which are some distance apart 
vertically, and they are only sparsely distributed along these planes. 
It is rather remarkable that in passing across the valley of the 
Thames the Middle Chalk shows a sudden and considerable in¬ 
crease of thickness. In Berkshire, west of Moulsford, the thick¬ 
ness cannot be more than 160 feet, while on the Oxford side, near 
Ipsden, it is at least 200 feet; of this about 60 feet may be assigned 
to the zone of Rh. Cuvieri and about 140 feet to the 
Terebratulina chalk. 
In Buckinghamshire its total thickness appears to be from 
200 to 220 feet, and it is apparently the upper zone which thus 
increases in thickness. As stated on p. 362, we now include in 
this zone those beds below the Chalk Eock which were referred 
to the zone of Holaster planus in the Memoir on the Geology of 
