95 



possible to determine the horizon of the chalk in any small isolated 

 pit by its included fossils. The zonation of the Chalk received 

 adequate attention from Professor Barrois, first in France, where 

 wide downs form the borders of the Paris Basin, and later from 

 him in our own country. The work of others, particularly Dr. 

 Row'e, has confirmed the usefulness of the method and added not 

 merely many details to the chalk fauna, but also considerable know- 

 ledge of the conditions under which the various divisions and beds 

 of the chalk were deposited as well as of the steady flux of evolu- 

 tion which affected species during the vast period required for 

 its accumulation. The best localities for the study of the zonation 

 of the Chalk in our own neighbourhood are the cliffs to the west of 

 Lulworth Cove and the cliffs near Freshwater in the Isle* of 

 Wight. 



The Lower Chalk (Cenomanian) is composed of (i.) the 

 Chloritic Marl at the base, (ii.) Chalk Marl with Ammonites 

 varians, and (iii.) an upper or grey marl with a well marked 

 Belemnitella Marl near the top. It was apparently deposited in 

 a sea which gradually deepened to 270 fathoms. The waters were 

 affected by the silt carried down from the ancient Europe by a river 

 which emptied to the south-east of the basin. The Dorset Lower 

 Chalk seems to have been out of the way of the main currents from 

 this river, for it is comparatively pure limestone, attaining a thick- 

 ness of 149 feet at Ballard Point and rather less at Lulworth. It 

 may be easily seen at West Nothe, Durdle Door and at Compton 

 Bay, in the Isle of Wight. The western shores of the Lower Chalk 

 sea were probably against the highlands of Devon and Wales. 



The Middle Chalk (Turonian) follows the Lower Chalk with- 

 out a break, and was laid down at the still greater depth of 500 

 fathoms in a sea which .probably extended well over what is now 

 Ireland and Scotland. The chalk consists of a fine sediment of 

 shells of Foraminifera and contains between 96 and 99 per cent, 

 of carbonate of lime. Two zones are distinguished — the lower of 

 Rhynchonella Cuvieri, in which the Melbourne Rock is locally 

 feebly developed, and the upper of Terebratula gracilis. Locally 

 it averages 100 feet, and can be easily made out about the Man- 

 o'-War Cove. 



The Upper Chalk (Senonian) is the thickest and most char- 

 acteristic, and is best known as the chalk of the Downs. It shows 

 locally six chief zones, which are distinguished from below up- 

 wards as the zones of Holaster planus, Micraster cor-testudinarium 

 and Micraster cpr-anguinum, forming the ' • Micraster chalk " and 

 Marsupites, Actinocomax quadratus and Belemnitella mucronata 

 forming the " Belemnite chalk." Bands of flints occur in all 

 zones. The Micraster Chalk was laid down on a steadily sinking 

 floor. At the greatest depth of some 650 fathoms the purest chalk, 

 that of the Marsupites and Actinocomax zones, was deposited. 

 After this stage the floor was elevated and the Belemnitella 

 mucronata zone accumulated. Further east, as in Norfolk, higher 

 zones have been preserved, which correspond to comparatively 



