104 MESSRS. A. J. JUKES-BROWNE AND W. HILL : [May 1 896, 



In the first place, it is worthy of note that all the component 

 members of the Cretaceous system are thicker here than at any 

 other place along the south coast. Taking the Gault and Upper 

 Greensand together, they have a thickness of 270 feet at Gore Cliff 

 and of about 230 at Compton Bay, while the Lower Chalk is 200 

 feet thick near Culver Point, and is probably but little less at 

 Compton Bay. 



In the next place, no hard-and-fast line can possibly be drawn 

 between the Gault and Upper Greensand ; there are a set of passage- 

 beds, sandy micaceous clays, which have been referred to the Gault 

 by some authors and to the Greensand by others. As Ammonites 

 rostratus was obtained from these sandy clays by the Survey fossil- 

 collector, it would appear that they belong to the same zone as the 

 overlying micaceous sands, and should not be referred to the Lower 

 Gault. 



"Whether these passage-beds be included in Gault or in Greensand, 

 it is equally impossible to take a definite base-line for the zone of 

 Ammonites rostratus. There is the same kind of passage in other 

 parts of England, and so long as the Amm. rostratus-beds are 

 regarded merely as a zonal subdivision of the Gault-and-Greensand 

 group there is nothing surprising in it. Dr. Barrois has taken the base 

 of the yellow sands near Yentnor as the base of the Cenomanian stage 

 in England, 1 but the bed of sandstone which there forms a convenient 

 base-line has not been detected in other parts of the island. His 

 endeavour to fix the base of the zone of Amm. rostratus and to make 

 it the base of an English Cenomanian is no doubt a logical application 

 of the classification which he had adopted for the beds in the East of 

 Erance, but we think that arrangement was founded on a mistaken 

 view of the chronological value of the typical Cenomanian of 

 Maine and Normandy, a view which may be traced to the erroneous 

 correlations of Prof. Hebert. 



The next point for consideration is the junction of the zone 

 of Ammonites rostratus with that of Pecten asper. This is of 

 importance because some French geologists, notably Prof, deLapparent, 

 are prepared to throw the former into the Albien and to take the 

 latter as commencing the Cenomanian. In the Isle of Wight, 

 however, there is no break or great change of fauna at this horizon : 

 the chert-beds are mere local deposits of organic silica (sponge- 

 spicules), and Pecten asper occurs below as well as above them. 

 No English geologist would think it natural to detach these beds 

 from the Greensand and to group them with the Lower Chalk. 



"When we come to the junction of Chalk and Greensand, however, 

 the case is different. Even here there is a passage and no sharp 

 line of demarcation, but the passage is very rapid and it coincides 

 with a great change in the fauna. Above the highest layer of 

 cherts there are two beds which we should group with the Upper 

 Greensand, and the uppermost of these passes into what we regard 

 as the true Chloritic Marl or basement-bed of the Chalk. The 



1 c Kecherches sur le Terrain Cret. Sup. de l'Angleterre et de l'Irlande,' Lille, 



1876. 



