WHITE CHALK. 13 



part of its range from west to east, the surface occupied by it is very inconsiderable com- 

 pared with that of most of the other strata above and below it, but its horizontal extension 

 becomes greater in proportion as the inclination of the strata diminishes. For this reason, 

 from Alum Bay to Mottestone Down, and from Carisbrook to Culver Cliff, between which 

 intervals the Chalk is nearly vertical, it constitutes a mere ridge of high land, which is scarcely 

 a quarter of a mile broad in Aston Down ; but between Mottestone Down and Carisbrook, 

 where the strata are less inclined, the width of the Chalk exceeds three miles." 1 



" The flints in the Chalk are for the most part irregular in shape, but they sometimes 

 constitute tabular layers coincident with the stratification, or else filling cracks and joints. 

 Those flints which occur parallel with the bedding are of a different age from those filling 

 the cracks and joints. The former are derived from siliceous matter, frequently, and 

 perhaps in most instances, deposited contemporaneously with the calcareous sediment of 

 which the Chalk is composed, around aponges and other organized bodies, the forms and 

 internal structure of which are still preserved. The latter, on the contrary, are of more 

 recent origin, having been carried by percolating water holding silica in solution into cracks 

 and joints formed by the Chalk during or after its solidification. The tabular bands 

 of flint filling cracks and joints are therefore, and as might be expected on the last suppo- 

 sition, unfossiliferous, instead of abounding in fossils, as is the case with the other system 

 of flints." " In the upper part of the Chalk, where the beds are the most highly inclined, 

 the flints, which appear to be whole when viewed in situ, are found, on closer examination, 

 to be nearly all broken so that when extracted from the quarry they fall to pieces." 

 " Shattered flints may be observed in the large chalk-pits south of Newport, and on Arreton 

 Down ; also on Ashley Down, where the Chalk is rather hard (as is most frequently the case 

 where it is inclined at a high angle), dipping 65® in a direction slightly east of north." 2 



The White Chalk contains many species of Echinidaj, of which the most common are 

 Echinocorys vulgaris, Breyn. ; Galerites albo-galerus, Lamck. ; Micr aster cor-anguinum \ 

 Klein ; Cidaris davigera, Konig ; Cidaris scej)trifera, Mant. ; Cidaris subvesiculosa, 

 d'Orbigny, and several other forms, to be figured and described in the following pages. 



The " Upper White Chalk with flints " of English authors corresponds to the Craie 

 blanche of the French, the Obere Kreide of the Germans, and the Mage Senonien of 

 d'Orbigny. 



Besides the localities already mentioned, it is well exposed and very fossiliferous at 

 Lewisham, Grays, Northfleet, Norwich, Brighton, Dover, and other places in the counties 

 Sussex and Kent, and at Flamborough Head, on the Yorkshire coast. 



The following table exhibits at a glance the subdivisions of the Cretaceous formations, 

 with their lithological characters, chief localities, and foreign equivalents, so as to afford 

 an easy reference to the stratigraphical distribution of the species of EcMnidce in each of 

 the beds. 



1 Bristow, on the " Geology of the Isle of Wight," *Mem. of the Geol. Surv.,' p. 28. 



2 Bristow, ibid., p. 31. 



