94 
THE CRETACEOUS ROCKS OF BRITAIN. 
in the Dorset nodule bed, whereas in the Isle of Wight this species 
is so rare in the Chloritic Marl that only one or two specimens have 
been found, though it is common enough in the chalk between 
10 and 20 feet above that horizon. 
These facts, taken in conjunction with the visible; signs of erosion, 
make it probable that the so-called Chloritic Marl of Dorset is merely 
the local base of the Chalk, and corresponds to a slightly higher 
horizon than that characterised by Stauronema Carteri. There 
is a gap, and some beds are missing, either because deposition ceased 
for a time or because the influx of a strong current destroyed some 
of the previously-formed deposit. Probably some beds were formed 
and broken up again, because many of the phosphatic casts in the 
nodule bed are broken and rolled ; others, however, are sharp, and 
cannot have been transported from a distance. On the whole, it 
seems improbable that any deposition was in progress during the 
epoch of the Stauronema beds, for, if such had been the case, phos- 
phatised remains of that calcareous sponge would probably have 
occurred as derived fossils in the existing nodule-bed. 
With regard to the upper portion of the stage, which we have 
hitherto called the zone of Holaster subglobosus, we are in Dorset 
confronted with a zonal difficulty of some importance ; for through¬ 
out Dorset this species of Holaster is common in the zone of Am. 
(, Schloenbachia ) varians, and is very rarely found in the higher beds, 
a distribution the exact reverse of that which prevails in the mid¬ 
land and south-eastern counties. 
Thus the species, which forms a good index of the zone elsewhere, 
would completely mislead a collector in Dorset; but the zone, re¬ 
garded as so much chalk, remains, and if another local index of it 
could be found the difficulty would be slight, for we should merely 
have to say that the zone of “ so and so ” in Dorset is the equivalent 
of the zone of Holaster subglobosus, in Kent and elsewhere. Unfor¬ 
tunately in Dorset these beds have yielded but a small number of 
fossils, and none of them have a restricted range. Thus the zone 
here is marked oh rather by negative characters than by the pos¬ 
session of any special fauna. It is not merged into the zone of 
Schloenbachia varians, for the range of that species and of others 
associated with it is strictly limited, and it may be urged that if 
the beds in question were more fossiliferous the usual restriction 
of some of the species would be observable. Still, it is useless to 
deny that in this case the zonal method fails us ; for, in spite of the 
fact that the beds themselves are well developed and have in the 
Isle of Pur beck a thickness of from 50 to 104 feet, it has not yet 
been proved that there is such restriction of species within this 
thickness as the zonal method teaches us to expect. 
Another peculiarity in the Chalk of Dorset is the setting in 
of a band of hard smooth chalk above the Belemnite Marl. This 
band is six feet thick in Ballard cliff and thickens westward till it 
is more than 20 feet in Lulworth Cove. As in other parts of 
England the Belemnite Marls are succeeded directly by the 
