166 
THE CRETACEOUS ROCKS OF BRITAIN. 
In Berkshire the zone of Holaster subglobosus loses the abnormal 
characters which it exhibits in Wiltshire, and assumes those 
which prevail both to the northward and to the eastward. The 
Wiltshire type is probably continued for some distance into Berk¬ 
shire, and there are so few exposures in this western area that we 
cannot say precisely where the change takes place. Possibly it is 
coincident with the incoming of the Totternlioe Stone. At any 
rate, to the north of Wantage this zone consists of greyish chalk 
for a certain distance above the Totternhoe Stone, probably 30 
or 40 feet, but this passes up into whitish blocky chalk without 
definite bedding, and of nearly the same character throughout. 
The subzone of Actmocamax plenus is very thin in Berkshire, no 
exposure showing a greater thickness than 2J feet, and, instead 
of consisting wholly or mainly of marl as in most of the southern 
and eastern districts, it here includes a bed of hard, white, compact 
chalk about a foot thick; lying generally between two layers of 
grey marl, this white bed is very conspicuous in section. 
All the exposures mentioned in the following pages occur along 
the main outcrop, which runs from Ashbury in the west to Cholsey 
and Moulsford in the valley of the Thames. There are, however, 
two small inkers of Lower Chalk on the south side of the great 
escarpment; these occur in two of the deep valleys which traverse 
the downs north of Lambourn. 
Stratigraphical Details. 
Zone of Ammonites Varians. 
Chloritic Mori. —The junction of the chalk with the 
underlying Greenland is not often exposed in Berkshire, but, 
wherever it has been seen, there is a complete passage upward from 
the glauconitic sand of the Selbornian through sandy glauconitic 
marl with fossils and pliospliatic nodules into a marly chalk with 
scattered green grains. The passage is often so complete and 
gradual that it is difficult to say where the one formation ends 
and the other begins. 
One of the best exposures is in a small sand pit by the side of the 
main road near Westcott, four miles west of Wantage. The section 
seen here in 1887 was — 
ft. 
^ jChalk marl with minute green grains- - - about 3 
(Harder glauconitic marl with larger green grains - ,, 2 
2. Green sandy glauconitic marl, with some phosphatic 
nodules and fossils 2J 
1. Dark green sand ----- seen for ,, 2j 
Among the phosphates were Am. [Schloenb.] varians , Nautilus 
sub-l(Evigatus ? Plicatula inflata, and a Pleurotomaria , 
