10 



LONDON DISTRICT. 



points of view, inasmuch as flints characterize the higher portions 

 of the Chalk. It is now further divided into zones, which are 

 bands of rock each characterized by the occurrence of certain 

 species and assemblages of fossils. These zones indicate succes- 

 sive stages in the animal population of the sea on the bed of 

 which the Chalk was originally deposited ; and a zone is denoted 

 by the name of one distinctive species, the range of which usually 

 corresponds with the upper and lower limits of the zone. Lists 

 of fossils from each zone will be found in several of the papers 

 referred to below. The following table summarizes the present 

 zonal divisions of the Chalk recognized in this area : 



Upper Chalk; with many 

 flints. 



' Chalk Rock ' at base 



(9) Zone of Actinocamax quadratus. 



(8) ,, Marsupites testudinarius : about 

 60 ft., with sub-zone of 

 Uintacrinus at base. 



(7) ,, ,, Micraster coranguinum : up to 

 200 ft. 



(6) ,, Micraster cortestudinarium : 

 40-50 ft. 



(5) Holaster planus : 40-60 ft., 

 with sub-zone of Hetero- 

 ceras reussianum at base. 



Middle Chalk j*J - 

 Mel bourn Rock at base 



(4) Zone of Terebratulina : up to 160 ft. 

 (3) ,, Rhynchonella cuvieri : up to 

 70ft. 



Belemnite Marl at top 

 Lower Chalk - 



Chalk Marl with Chloritic 

 Marl at base. 



(2) Zone of Holaster subglobosus : up to 

 100 ft., with sub-zone of 

 Actinocamax plenus at top. 



(1) Schloenbachia varians : 80 to 

 120 ft. 



The divisions into Upper, Middle, and Lower Chalk are not 

 shown on the special London maps, but will be indicated by 

 colour on the new maps of the Beaconsfield and Dartford areas 

 (Sheets 255 and 271). It is only along the eastern part of the 

 southern margin of our district that the Lower Chalk crops out, 

 while the Middle Chalk comes to the surface there and in the 

 north-west corner. Of the former a small area occurs south of 

 Shoreham, the pale green Belemnite Marls being visible in the 

 railway cuttings, and beneath them soft white chalk of the 

 Subglobosus zone, wherein the small sparsely -ribbed brachiopod 

 Rhynchonella mantelliana (J. de C. Sowerby) is commonly found ; 

 these exposures are, of course, inaccessible to the public. 



The Middle Chalk borders this outcrop in the Darent Valley 

 and extends as far north as Lullingstone. It has also been traced 

 in two tributary valleys on the east, round Upper and Lower 

 Austin Lodge and again as far north as Maplescombe, the hard 

 Melbourn Rock at the base forming a noticeable feature ; to 



