ZONES OF THE CHALK. 11 



the west of the Darent the Middle Chalk enters our map from 

 the south in two valleys situated half a mile east of Downe and 

 of the Kent-Surrey boundary respectively. 1 



In the north-west the Middle Chalk occupies the valley of the 

 Chess above Chorley Wood. 



Examples of the zone-fossil of. the lower of the two divisions 

 of the Middle Chalk are commonly found. This small, inflated, 

 closely -ribbed brachiopod (Rhynchonella cuvieri d'Orbigny) is 

 often accompanied by the tongue-shaped shells of the bivalve 

 Inoceramus labiatus (Schlotheim), marked by close concentric 

 furrows. The higher zone is named after a very small brachiopod, 

 Terebratulina, a shell rarely more than J inch in length, and 

 ornamented with radiating ribs which often bifurcate and bear 

 tiny granules. At this horizon one finds occasional examples of 

 the heart-shaped sea-urchin Micraster, which occurs abundantly 

 in the higher zones of the Chalk. The characters of this urchin 

 as found in the Terebratulina zone may be regarded as primitive 

 in comparison with those from higher horizons. Thus we note 

 the absolute smoothness of the narrow tract between the double 

 rows of pores bounding the five sunken areas that mark the 

 upper surface of the test (or shell) . In higher zones the appearance 

 of tiny plates along this tract forms one of the features by which 

 the horizon of a Micraster can be determined. In the Terebra- 

 tulina zone Micraster corbovis Forbes is the species usually met 

 with. The thinness of the test is a notable feature. In either 

 of the zones of the Middle Chalk one finds tests of the sea-urchin 

 Conulus castaneus (Brongniart), sub -pentagonal in outline, and 

 G. subrotundus Mantell, a globular form, distinguished from 

 G. castaneus by its more circular outline and its dome -shaped 

 upper surface. There occur also certain fossils which are not so 

 restricted in range and may be found in various zones of the 

 Chalk ; these are referred to later. 



The whole of the remainder of the Chalk shown on the four 

 sheets of our map belongs to the upper division, and the distri- 

 bution of the zones has been studied by several workers. 2 The 

 Zone of Holaster planus surrounds the outcrops of Middle Chalk 

 just mentioned ; at its base is a band, usually about 5 feet in 

 thickness, recognizable by a peculiar assemblage of shell -remains, 

 mostly preserved as casts. Of these the commonest are uni- 

 valves, the top-like shells Trochus scJilueteri Woods, T. beroc- 

 scirense Woods and Solar iella gemmata (J. de C. Sowerby) being 

 typical representatives. Among the bivalves are the finely- 

 ribbed, mussel-like Septifer lineatus (J. de C. Sowerby) and a 

 form of Inoceramus peculiar to this horizon, the small /. costellatus 



1 Dewey, H., ' Summary of Progress for 1914 ' (Mem. Geol. Surv.}, 1915, 

 pp. 27, 28. 



2 See especially Young, G. W., ' Chalk Area of N.E. Surrey,' Proc. Geol. 

 Assoc., vol. xix, 1906, pp. 188-219; 'Chalk Area of W. Surrey,' ibid., vol. xx, 

 1908, pp. 422-455; Dibley, G. E., 'Zonal Features of the Chalk in Rochester, 

 Gravesend and Croydon Areas.' ibid., vol. xvi, 1900, pp. 484-499; vol. xxix, 

 1918, pp. 68-105. 



