14 LONDON DISTRICT. 



(J. Sowerby), a still more common brachiopod, has a smooth 

 inflated shell and gently -folded margin; its beak is pierced by 

 a tiny foramen. Various species of oysters have wide ranges, 1 

 as have also the scallop-shells Pecten (Chlamys) cretosus Defrance, 

 with its numerous, narrow, slightly spiny ribs and P. (Neithea) 

 quinquecostata J. Sowerby, distinguished by its prominent main 

 ribs. Spondylus spinosus (J. Sowerby), marked by regular, 

 strong, rounded ribs, often found with thick spines still attached, 

 is among the commonest bivalves of the Chalk. Inoceramus 

 lamarcki Parkinson, a large inflated bivalve with prismatic 

 structure and a surface marked by strong concentric folds, and 

 its larger and flatter variety cuvieri J. Sowerby, have a wide 

 range in the Middle and Upper Chalk. Vertebrates are repre- 

 sented chiefly by fishes, of which the sharp teeth of the shark 

 Lamna and the crushing palatal teeth of Ptychodus, with their 

 shiny rugose surface, are typical. Scales and fin-rays of the 

 bony fishes, sometimes representing the complete skeleton, 

 such as Hoplopteryx, are also preserved. 



Of the various bands recognized by their lithological 

 characters, the Chalk Rock is known over a great part of the 

 country ; in the south-eastern part of our area it is not always 

 to be found, though the same horizon, as determined by fossils, 

 can be traced, as stated above. The Melbourn Rock is also 

 nodular. On the north-west side of London a local development 

 of grey somewhat gritty chalk immediately above the Chalk 

 Marl is known as ' Totternhoe Stone ' ; it likewise "contains 

 phosphatic nodules. As their names indicate, these three bands 

 are all harder than the bulk of the Chalk. The ' Glauconitic ' 

 or ' Chloritic Marl ' at the base of the formation forms a kind 

 of passage from the Upper Greensand. 



The total thickness of the Chalk in the boring at Ottershaw 

 suggests that the Marsupites zone underlies the Tertiary strata 

 there, as also at Richmond and Southall. The Quadratus Zone 

 may be present at Egham, where the boring already mentioned 

 shows 346 feet of chalk above the Chalk Rock. The still higher 

 Zones of Belemnitella mucronata and Ostrea lunata do not occur 

 anywhere in the London District. 



On comparing Sheet 4 of the London Map with the newer 

 Dartford Sheet (271) it will be seen that the Chalk outcrop 

 between Plumstead and Erith is continuous on the latter, the 

 faulted mass of Blackheath Beds shown on the former being 

 omitted. This discrepancy is explained below (p. 37). 



CONDITIONS OF FORMATION. 



That the Chalk was deposited in moderately deep and clear 

 water is inferred from the nature of the materials forming the 

 mass of the rock, rather than from the more conspicuous organic 



1 For figures and descriptions of these and other Chalk bivalves see Woods, H., 

 " Cretaceous Lamellibranchia of England," Pal. Soc., 1899-1912. 



