GEOLOGY 



and somewhat flaggy ' Melbourn Rock ' above, which forms the base of 

 the Middle Chalk. This rock may be described as a hard, somewhat 

 splintery, white chalk, with a curiously irregular wavy bedding and part- 

 ings of grey marl, markedly in contrast with the evenly bedded grey marls 

 below. The Melbourn Rock passes up into somewhat softer strata, but 

 deposits of similar character form the whole 200 feet of the Middle 

 Chalk. This chalk is full of fossils ; though the great majority of the 

 fragments belong to a single species of bivalve shell, Imceramus mytiloides 

 (or /. labiatus), fragments of which form a considerable portion of the 

 bulk of the rock and are easily recognizable from their peculiar fibrous 

 structure. The upper part of the Middle Chalk is characterized by a 

 small Terebratulina commonly referred to T". gracilis. 



The Melbourn Rock projects as a distinct ledge in the face of the 

 Chalk escarpment, or crowns projecting spurs, especially near East- 

 bourne ; this platform is often selected as a favourable site for building. 

 The rest of the Middle Chalk forms the steep face of the escarpment, 

 capped by the flinty chalk above ; it also stretches in long tongues up 

 some of the valleys, as well as appearing in others as inliers. One series 

 of these inliers, seen in the valley bottoms between Lewes and Patcham, 

 is brought up by a prolongation of the anticline which passes through 

 Beddingham and Kingstone. Another similar undulation brings Middle 

 Chalk to the surface in the east and west valley between East Dean and 

 Singleton ; and it is possible that Middle Chalk may also be exposed in 

 the centre of the anticline near Littlehampton, though there the country 

 is so covered with gravel, and the foreshore is so sandy, that it is difficult 

 to examine the Chalk below. 



The junction of the Middle and Upper Chalk in many parts of 

 England is marked by a band of hard, splintery chalk known as the ' Chalk 

 Rock.' This however is non-existent in Sussex, though the character- 

 istic fossils, principally small gasteropods, can be found at Beachy Head. 

 Beyond the gradual appearance of flints, there is no very obvious dis- 

 tinction between Middle and Upper Chalk, as seen in these vertical 

 cliffs. The Upper Chalk is purer, whiter, softer, and contains fewer 

 marl partings than the Middle Chalk. FHnts are almost confined to it, 

 though a few scattered nodules can be found 10 or 20 feet below the 

 base. The lowest zone (including the Chalk Rock) is characterized by 

 the echinoderm Holaster planus and a group of other fossils that are 

 always found associated with it, but the lower limit of the zone seem- 

 ingly does not quite coincide with the base of the Chalk Rock ; the 

 thickness is about 50 feet. The next zones are specially characterized 

 by different forms of Micraster, echinoderms which have lately yielded 

 to Dr. A. W. Rowe ^ evidence of the gradual evolution of each succeed- 

 ing form from an older one, so that by slight variations in the shape and 

 pattern of the shell he can tell to within a few feet from what part of the 

 Chalk a handful of these sea urchins was obtained. One result of this 

 study is to show, as might have been expected, that when the missing 



1 Quart. Joui-n. Geol. Soc. Iv. 494 (1899). 



