1872.] Geology of the Straits of Dover. ars 
England is the most constant division of the Lower Green- 
sand, and in the Bas-Boulonnais is probably the only 
division of that formation which is represented. ‘The 
Folkestone Beds at Wissant differ only in thickness from 
those at Folkestone; and there is no reason to doubt that 
this division also passes right across the Channel. 
But here our certainty ends. We have seen that the 
Wealden formation 1s of very great thickness on the English 
coast; it is there not less than 1rooo feet thick, and is 
perhaps considerably more. In the Bas-Boulonnais the 
Wealden Beds are probably not more than too feet in 
thickness, and it is quite uncertain which part of our 
English series they represent. Somewhere in the bed of the 
Channel the well-marked division of the English Wealden 
Beds must disappear, and they must gradually put on the 
characters which we find them to possess in the Bas- 
Boulonnais ; but where this change takes place, and whether 
it is gradual or comparatively sudden, we have no means 
whatever of knowing. In the map which accompanies this 
paper I have shown the Weald Clay and the Hastings 
Beds striking out from the English shore in the dire¢tion 
in which, for some few miles, they will probably range. 
On the French side of the Channel I have also shown the 
Wealden Beds ranging for a short distance parallel with 
the Folkestone Beds; but I have not ventured to prolong 
the line far into the Channel. And, notwithstanding the 
fact that in some maps and seCtions which have appeared 
on this subject the lines have been projected with apparent 
confidence, there is not the slightest evidence to show in 
what direction the outcrops run. 
In mid-channel there are two shoals, known as the 
Varne and the Ridge. Itis a matter of great importance 
to know of what rocks these are composed; they will 
probably, however, be Portland or Wealden. From some 
experiments undertaken by M. Thomé de Gamond it seems 
probable that they are the former. The Portland Beds 
form the cliffs at Cap Grisnez, and under them there are the 
Kimeridge Beds. These rocks dip to the north-east; and, 
supposing M. de Gamond’s observations to be correct, 
somewhere between Cap Grisnez and the Varne the 
Kimeridge Clay would wholly disappear. Somewhere to 
the west of the Varne and nearer to the English coast the 
Portland Beds would also disappear, and would be overlain 
by the Wealden series. Probably the Hastings Beds would 
immediately overlie them, and above these the Weald Clay 
