526 PEOF. J. W. JUDD A^^D c. hoieeesham 



TVith them occur a number of partially silicified Foraminifera. The 

 forms usually obtained by the treatment of the chalk-marl with acid 

 are arenaceous types like Bulimina {Ataxophragmium), Textularia 

 (Plecanium) , and Verneidlina. In these a more or less perfect 

 cohesion of the sand grains composing the shells has been brought 

 about by the deposition of siliceous material between them. 



Eut it is with respect to the Jurassic deposits, the existence of 

 which under the London basin was previously unknown, that the 

 evidence afforded by the Eichmond boring is of such great value 

 and interest. 



Since the reading of our paper, in which we endeavoured to define 

 the position and relations of the Upper, Middle, and Lower Oolites 

 respectively in the South-east of England, a new and most valuable 

 piece of evidence has been obtained, which confirms in a verv 

 striking manner the conclusions at which we then arrived. 



During the progress of the Dockyard Extension works at Chatham, 

 H.M. Government have had occasion to sink two deep wells, one of 

 which has penetrated the whole of the Cretaceous strata and reached 

 the underlying rocks. The results have been communicated to 

 Messrs. Whitaker and Topley for the use of the Geological Survey, 

 and we have the courteous permission of the Director-General to 

 publish the interesting details with which the officers of the Survey 

 have furnished us. 



The deepest weU at Chatham attained a depth of 965 feet, and 

 the strata passed through were as follows* : — 



Surface 16 feet above Ordnance datum. 



Depth from 



Thickness. surface, 



ft. ft. 



Alluvium, gravel, and Thanet-sand 27 



Chalk 682 709 



aault 193 902 



Lower Greensand (sandy beds) 41 943 



Dark-blue clay (Oxfordian) 22 965 



The first point which strikes us in this very interesting section is 

 the remarkable way in which the Lower Greensand has thinned out 

 in a distance of 7 miles from Maidstone (at which place it has a 

 thickness of 225 feet, and includes the thick calcareous masses of 

 the Kentish Eag) to a comparatively insignificant and piu-ely 

 sandy representative. 



The blue clays underlying the Lower Greensand were not un- 

 naturally taken for Wealden in the first instance, but a number of 

 fossils washed from these clays by Mr. Creswick, of the Admiralty 

 Office of TTorks, were found to be suspiciously like Oxford-clay 

 forms, and on being submitted to Messrs. G. Sharman and E. T. 

 N'ewton, E.G.S., of the Geological Survey, they were identified as 

 follows : — 



* A short notice of the strata passed through in this well has already been 

 given by Mr. Whitaker in his ' Guide to the G-eology of London and the Neigh- 

 bourhood ' (4th edition, 1884), pp. 19, 21. A more detailed account of them 

 will be given in a forthcoming pubHcation of the Geological Survey. 



