196 
THE CRETACEOUS ROCKS OF BRITAIN. 
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the same assemblage of common fossils as those of the Cambridge 
Greensand. Hence both the stratigraphical and the palaeonto¬ 
logical evidence lead unmistakably to the conclusion that the 
nodules of the Cambridge bed have been derived from the Gault, 
and principally from beds of Upper Gault like the upper marls of 
Buckinghamshire, which we may reasonably suppose originally 
extended northwards and eastwards over the whole area that is 
now occupied by the Cambridge Greensand. 
Besides the pliospliatic nodules other foreign bodies occur here 
and there in the shape of detached rock-fragments, which are some¬ 
times of considerable size. They include pieces of red and purple 
sandstone resembling those of Devonian and Old Bed Sandstone 
age, others of grey sandstone like Carboniferous sandstones, of soft 
schist and hard grey slate, of quartzite and hornstone, with many 
varieties of igneous rocks—granite, quartz-porphyry, basalt, 
hyperite, and obsidian. 
Some of the softer rocks are waterworn, but the great majority 
are angular, and some are remarkably so, resembling the 
angular blocks of a glacier moraine. They vary in size from a 
mere pebble up to a block measuring 14 x 1'2 x Cinches. Oysters, 
Plicatuhe , and pliospliatic nodules are attached to their surfaces, 
and some at any rate seem to have been dropped in the Gault mud 
when the phosphates were being formed.* The source from which 
they were derived will be discussed in a later chapter. 
Besides the derived fossils, there are a certain number of 
contemporaneous species in an ordinary state of preservation. 
A list of these is given at the end of the Chapter. 
The main outcrop of the Cambridge Greensand follows very 
closely the line of the valleys of the Bhee and Cam to a point nearly 
opposite Water beach, where it passes beneath the Fen land. It has, 
however, been worked out beneath the alluvial deposits of Swaft- 
ham, Burwell, and Soliam Fens, the most easterly pits observed 
when the area was surveyed being about two miles north-east of 
Soliam. There can be little doubt that it continues beneath Milden- 
hall and Lakenheatli Fens, but how far the nodule-bed extends 
is at present unknown. 
The Chalk Marl. 
The lower part of the Chalk Marl or zone of Ammonites varians 
is generally a lumpy bluish-grev marl, somewhat argillaceous, 
but containing from 60 to 70 per cent, of carbonate of lime. (See 
analyses in Chapter xxii.) The higher part is a blockv and marly 
chalk, probably containing over 80 per cent, of carbonate of lime. 
The lower beds have been exposed from time to time in the 
* Mr. H. Woods has recorded (Gccl. Mag., Dec. 4, Vol. ii. p. 377) the 
occurrence of two angular boulders of coarse-grained quartzite in the 
nodule-bed at the base of the Upper Gault at Stanbridge, in Bedfordshire, 
and smaller stones have been found in the Gault at Barnwell, near 
Cambridge. \ >• 
