202 
ELEMENTS OF GEOLOGY. 
To begin with the uppermost or Chillesford beds, it will 
be seen that about 9 per cent, only are extinct, or not 
known as living, whereas in the Norwich, which succeeds in 
the descending order, seventeen in a hundred are extinct. 
Formerly, when the Norwich or Fluvio-marine Crag was 
spoken of, both these formations were included under the 
same head, for both at Bramerton and Thorpe, the chief lo¬ 
calities where the Norwich Crag was studied, an overlying 
deposit occurs referable to the Chillesford age. If now the 
two were fused together as of old, their shells would, ac¬ 
cording to Mr. Wood, yield a percentage of fifteen in a hun¬ 
dred of species extinct or not known as li\ ing. 
NUMBER OF KNOWN SPECIES OF MARINE TESTACEA IN 
THE CRAG. 
CHILLESrOKD AND ALDEBY BEDS. 
Total Number. 
Bivalves . 
Univalves 
Brachiopods . 
Not known as 
living. 
Percentage of 
Shells not known 
as living. 
9*5 
NORWICH OR FLUVIO-MARINE CRAG. 
Bivalves . 
. 61 
10 
) 
Univalves 
. 64 
12 
}■ 
17-5 
Brachiopods . 
1 
0 
RED 
CRAG. 
(^Exclusive of many derivative shells.') 
Bivalves . 
. 128 
31 
) 
Univalves 
. 127 
33 
L 
25-0 
Brachiopods . 
1 
1 
coralline crag. 
Bivalves . 
. 161 
47 
> 
Univalves 
. 184 
60 
31-5 
Brachiopods . 
. . 5 
3 
) 
To come next to the Red Crag, the reader will observe 
that a percentage of 25 is given of shells unknown as living, 
and this increases to 31 in the antecedent Coralline Crag. 
But the gap between these two stages of our Pliocene de¬ 
posits is really wider than these numbers would indicate, for 
several reasons. In the first place, the Coralline Crag is 
more strictly the product of a single period, the Red Crag, 
as w^e have seen, consisting of separate and independent 
patches, slightly varying in age, of which the newest is 
probably not much anterior to the Norwich Crag. Second¬ 
ly, there was a great change of conditions, both as to the 
