464 
THE CRETACEOUS ROCKS OF BRITAIN. 
of Porton, about 7 feet of hard nodular chalk being seen with 
a marked plane of division at the top, above which are several 
feet of shelly chalk with fewer nodules. 
The best exposure of the Rhynckonella Guvieri zone in this dis¬ 
trict is in the cutting and quarries by the side of the line south of 
Hitchin Station. The quarries are carried down below the level 
of the line, and are sometimes worked down to the bottom of the 
Melbourn Rock. The succession is as follows : — 
feet. 
Soil and chalk rubble -.4 
Firm white chalk with Galerites subrotundus, and a layer of 
flints at base --------- 6 
Firm whitish chalk with much Inoceramus shell, Galerites 
subrotundus and Rhynch. Cuvieri - - - - - 14 
Harder white chalk with Rhynch. Cuvieri, Inoceramus myti- 
loides, and Terebratida semiglobosa - - - - - 10 
Hard white nodular chalk with the same fossils, and also 
Discoidea minima and Cardiaster pygmceus - - 10 
Very hard nodular yellowish rock with Rhynchonella Cuvieri , 
top of Melbourn Rock.-4 
Below this there is said to be hard, nodular, greenish-grey rock 
about 6 feet thick, making the whole about 50 feet thick, but as 
the highest bed is rather soft homogeneous chalk, and neither 
Rhynchonellce nor Inocerami are common in it, the layer of flints 
may be regarded as the top of this zone. 
The lower part of the zone is again seen in the large quarry at 
Ash well, and the beds exposed are : — 
Middle 
Chalk. 
feet. 
Hard but brittle yellowish chalk, with many Rhynch. 
Cuvieri and Inoc. mytiloides - 
Brittle homogeneous chalk (4 in.) breaking vertically 
into oblong blocks, resting on soft shaly chalk 
Hard chalk with nodules, weathering into layers 
Softer yellowish shaly chalk, breaking into lenticular 
lumps, some nodules ------ 
Hard rocky chalk, weathering yellowish, with 
s a rough nodular surface -. 
/ Soft greenish-grey chalk ------ 
Hard white chalk -------- 
Greenish-grey shaly chalk ------ 
^Blocky whitish chalk - - - 
The three lowest of these beds represent the Melbourn Rock, 
but the shaly chalk hi the middle is very unusual at this horizon. 
Lower 
Chalk. 
10 
U 
1 
1 
30 
Terebratulina Zone. 
The highest beds of this zone are exposed hi the quarry by the 
Chiltern Waterworks, two miles south-west of Tring, the section 
as seen in 1884 being : — 
feet. 
Chalk rock (seen in one corner) - - - 8 
Hard white nodular chalk. 8 
Thin layer of marl. 
Firm even white chalk - - - - - - - - 11 
Thin layer of marl. 
Firm white chalk, massive - - ■=• - - 8 
