504 STRATIGRAPHICAL GEOLOGY 



Feet. 

 5. Soft grey shaly marl enclosing a band of hard white 



chalk, Actinocamax plenus . . . . . . 3 to 5 



4. Tough blocky chalk without distinct bedding, Holaster 



subglobosus, Acanth. rotomagensis . . . . . 60 to 80 

 3. Totternhoe stone, hard grey sand chalk, RJiynch. Man- 



telliana, Lima echinata, Kingena lima, Pecten Jissicosta 2 to 20 

 2. Soft marls with occasional layers of hard marly rock in 



Oxford and Bucks, Schlcenb. varians, Ehynch. Martini, 



Inoceramus latus . . . . . . . . 60 to 80 



1. Marl with green grains, passing northward into the nodule 



bed known as Cambridge Greensand . . . . 1 to 2 



The Cambridge Greensand (see Fig. 162) commences near 

 Barton-le-Clay (north of Luton), to which locality the nodule bed 

 in the Gault has been traced, and there can be little doubt that all 

 the nodules and the phosphatised fossils which occur in the 

 Cambridge Greensand have been derived from the erosion of the 

 Upper Gault. This derived fauna comprises some 210 species 

 of Invertebrata, and about 80 of these have not yet been recog- 

 nised elsewhere in England ; out of the remaining 1 30 no less 

 than 114 occur in the Gault of Folkestone, nearly all the 

 commoner fossils occurring in the Upper Gault. 17 The commonest 

 species are Sch. rostrata, Hoplites auritus, H. raulinianus, Avicula 

 gryphceoides, Terebratula biplicata, Plicatula gurgitis, and Rhyn- 

 chonella sulcata, but it is noteworthy that the last is a very rare 

 fossil in the Gault elsewhere. 



The Vertebrate fauna of the Cambridge Greensand is very 

 remarkable, the remains of many species of Ichthyosaurs, Plesio- 

 saurs, Dinosaurs, Chelonians, and Pterodactyles having been found, 

 together with bones belonging to a bird (Enaliornis). Most of 

 these bones have doubtless been washed out of the Gault, but 

 some may be Chalk Marl forms. 



The indigenous fauna of the Cambridge Greensand is found in 

 the marly matrix of the bed, and some of the species are Vermi- 

 cularia umbonata, Discoidea subucula, Micrabacia coronula, Rhyn- 

 chonella lineolata, Kingena lima, and Terebratulina triangularis, 

 most of which occur also in the Chalk Marl above. 



Another speciality of this district is the Totternhoe stone, 

 so named from a place near Dunstable, where it is 20 feet thick, 

 and is quarried for building purposes. It varies greatly, however, 

 in thickness, and is sometimes represented by two beds of stone, 

 with Chalk Marl between them. At its base there is usually a 

 layer of green-coated phosphatic nodules, and small fragments of 

 phosphate are common in the stone above. 



In Norfolk the Lower Chalk becomes greatly reduced in 

 thickness and at the same time harder and more purely calcareous, 

 both these changes being evidently due to a diminution in the 



