Vol. 57.] TTPPEE GREENSAND AND CHLOEITIC MARL OF WILTSHIRE. 125 



EXPLANATION OF PLATES III-V. 



Plate IIL 



Geological Map of the District of Mere and Maiden Bradley, on the 

 scale of 1 inch to the mile. 



Plate IV. 



View of Maiden Bradley Quarry, showing the following succession : — 



A = The Chloritic Marl, with the position of the nodule-bed represented 



by the uppermost stippled line. 

 B = The Cornstone-Bed. 

 C = The sand below the Cornstones. 

 D = The Chert-Beds, obscured at the base by fallen d6bris, 



Plate V. 



View of part of Dead-Maid Quarry near the eastern end, where the fossil 

 tree-trunk was found, a portion of which is embedded in the Popple- 

 Bed, a little to the right of the vertical white line. This line indicate* 

 the point where the section given on p. Ill was measured. 



The lowest stippled line marks the uneven base of the Popple-Bed. The line 

 denoting its summit rather exaggerates the actual unevenness, which may 

 be partly due to lateral pressure affecting beds of different degrees of 

 hardness. 



Discussion. 



Mr. Lamplugh remarked that one of the Authors had shown 

 inconsistency in taking a lithological in preference to a palaeonto- 

 logical division in this case, whereas in the parallel case of the 

 zone of Ammonites mammillaris at the base of the Selbornian he had 

 recently taken the opposite course. These constant difficulties in 

 regard to the boundaries of formations showed how local most of 

 such boundaries were, and how arbitrary and conventional they 

 must necessarily be when applied over extended areas. 



The Rev. H. H. Wnrw^ooD and Mr. Whitaker also spoke. 



