70 PROCEEDINGS OF THE GEOLOGICAL SOCIETY, 



The boundary-lines of the Neocomian group are now for the most 

 part concealed : any good geological map will, however, show that it 

 must have been defined in places by members of the Oolitic series, 

 and that on the N.E. it passed transgressively over them. That 

 this position was brought about by a process of overlap is clear from 

 many considerations, the most satisfactory one being that the lowest 

 or argillaceous portion is of limited extent compared with the higher 

 or Urgonian division, and yet the whole of the fauna indicates depths 

 of from seven to ten fathoms of water only. There are sections 

 which show that in places the Lower Greensand of our S.E. counties 

 has a thickness of 800 feet, as between Guildford ai^d Dorking, and 

 the whole of this consisting of quartzose sand or shingle. 



The materials which compose the shingle-beds are identical with 

 those of the Farringdon gravels, which have been already described, 

 and I cannot do better therefore than borrow a few lines from that 

 paper : — " The mineral character of the pebbles which compose this 

 gravel suggests considerations of much interest in the history and 

 source of origin of the materials which compose the secondary depo- 

 sits of this country. The pebbles, as a mass, have been derived from 

 altered sedimentary strata — such as shales converted into flinty slates 

 or hornstone, and which must also have contained great subordinate 

 veins of quartz-rock : water-worn crystals of felspar may also be 

 detected, indicating the loose structure of the mass of felspathic 

 granite, or porphyry, from which they were separated. The mine- 

 ralogical character of the coast-line whence the materials of the 

 Farringdon gravel were derived is thus clearly indicated, and is one 

 which must necessarily, from the well-known distribution of masses 

 of crystalline rocks, have existed at some considerable distance from 

 the spot to which the gravel has been transported." (Quart. Journ. 

 Gaol. Soc. vol. vi. p. 458.) These accumulations rest on Kimmeridge 

 clay and Coral rag ; they could only have been produced on a coast- 

 line, and they must have followed that line to their present positions. 



If sections be taken at intervals within the Wealden area, as from 

 Farnham to Maidstone, an insight will be got as to the change which 

 the beds of the Lower Greensand undergo from S. to N., and it will 

 then be seen that the subordinate shingle-bands become coarser north- 

 wards, so that. at the latter place rounded blocks of granite have been 

 met with nearly a foot in diameter. Such a fact as this is of itself suffi- 

 cient to indicate the direction in which the coast-line of the Lower 

 Greensand lay; and, coupling it with the evidence of its thinning-out 

 and disappearance in the Boulonnais, we are enabled to carry on the 

 line of the old axis of Artois, and to feel sure that, in its passage 

 somewhere to the N. of our Wealden denudation, it presented at 

 the period in question a like physical feature, and exercised the 

 same influence there that it did along its continental course. The 

 Farringdon shingle-beds followed this line of coast, from the old 

 Palaeozoic rocks on the E., from which all the materials could be 

 derived. 



It was on such considerations that I had defined the limits of the 

 Lower Greensand group, when I received some information from 



