194 
THE CRETACEOUS ROCKS OF BRITAIN. 
As ail instance of the irregularity of the surface on which the 
base of the Chalk rests near Cambridge, Fig. 45 is reproduced from 
p. 37 of the memoir above quoted. The broken line shows the 
Fig. 45. — Section across the fields N.E. of Haslingfield. 
Scale horizontal and vertical 300 feet to an inch. 
a. Chalk-marl and coprolite-bed. b. Gault. 
The broken line shows the former continuation of the coprolite-bed. 
former continuation of the basement bed before the general detrition 
of the country had been carried to the present stage. The summits of 
the Gault ridges are, in fact, truncated by the present surface 
of the ground, and when the district was worked for “ coprolites ” 
these small inkers of Gault land formed what the workmen termed 
“ patches of dead ground ” within the productive area. 
Smaller rolls could often be seen in the trenches of the coprolite 
workings, forming ridges which sometimes rose to a height of 5 or 
6 feet above the intervening hollows (see Fig. 46). 
Fig 46. —View of a coprolite pit near Horningsea. (now closed) 
a. Gault. h. Cambridge Greensand, c. Chalk Marl. 
The matrix of the Cambridge Greensand is a green sandy marl, 
the sand consisting principally of glauconite grains with a small 
number of minute quartz grains. The proportion of sandy matter 
is greatest at the base, the bottom 5 or 6 inches of the deposit often 
having more glauconite than marl; upwards the green grains 
rapidly decrease in number, and generally disappear at a height of 
from 2 to 3 feet above the base. 
