246 ME. W, HILL ON THE BEDS BETWEEN THE 



It follows that the top of the Middle Chalk must not be looked 

 for in a thin band with peculiar lithological characters, but at a 

 certain palseontological horizon ; this horizon seems to me to be the 

 »one of Hoi aster planus. Inland this zone is somewhat difficult to 

 identify from the paucity of its fossils, but the base of the overlying 

 zone is well marked throughout all the district under consideration, 

 and at Dover by the advent of the Micrasters and by the incoming 

 of an abundance of fossils having characters more closely allied to 

 the Upper than to the Lower Chalk. 



It appears to me that in the neighbourhood of Henley the Henley 

 Eock may be taken as the summit of the zone oi Hoi aster planus and 

 also of the Middle Chalk, although, from the presence of Gasteropods 

 &c. in its upper part, it may be the equivalent of chalk at a higher 

 horizon elsewhere. The white lumpy fossiliferous chalk above the 

 Chalk Eock seen at Henley appears to me to be the equivalent of 

 the '•'• chalh luith many Ilicr asters,'' ozones of M. hreviporus and M. 

 cor-testudinariuryi of Dover, which division I should propose to take 

 as the base of the true Upper Chalk — chalk with many flints — in 

 both localities. 



With regard to the Clialk Roclc of Cambridgeshire, although it may 

 form a convenient line to mark the limit of the Middle Chalk, either 

 from want of continuous sections or the thinness of the rocky band, 

 there has, it appears to me, been included under this head chalk which 

 is really a portion of two zones, viz. that of Holaster planus and the 

 " ChalJc with Micrasters.'" Whether the rock has been reduced by 

 erosion to the thin bed now found there, or whether there is a 

 natural thinning-out of it, I am not prepared to say ; but I would 

 draw attention to the fact that the definite line which marks the top 

 of the rocky layers, and is considered by the authors of the Memoir 

 to be possibly evidence of this erosion, is a feature generally seen in 

 the lower as well as in the higher of these rocky beds. 



I take the Middle Chalk of Dover to be that included between the 

 base of the " Grit Bed " and the top of the zone of Holaster planus. 

 Its thickness and that of the various zones is given below : — 



ft. ft. 



Chalk with many / Zone of Micraster cor-testudinarium ? 66-^ 1 o-.^ 



Micrasters. In „ hreviporus 15 j ^ 



{ 



Holaster planus 22] 



Middle Chalk. \ „ TerebratuUna gracilis 150 I 242 



Bhynchonella Cuvieri 70 J 



Lower Chalk Zon e of Belemnitella plena 6 



The flint line at the upper limit of the zone of Holaster planus is 

 about 6 feet higher than the marl band which formed Mr. Curry's 

 mark when measuring Shakespeare's Cliff. 



The total thickness of the Middle Chalk atDover is slightly in excess 

 of the estimated thickness of it in Cambridgeshire. 



Thus the zones of the Chalk as set forth in the ' Geology of Cam- 

 bridge ' are clearly shown in the cliflfs of Dover, and the main divi- 



