258 A. J. JUKES -BROWNE ON THE RELATIONS OF 



Part I. — Stratigraphical Relations. 



§1. The Gault and Chalk-marl in Cambridgeshire. — The rock of the 

 so-called Cambridge Greensand has been well described elsewhere 

 by Mr. Seeley* and by Mr. Bonneyf. Without, therefore, entering 

 into any detail regarding its components, it will be sufficient to state 

 that it consists of a chalky mud or marly clay of a light green colour, 

 except where containing, as in some places it does, such an abun- 

 dance of glauconitic grains or so much Gault mud as to give the 

 bed a darker appearance. 



The " behaviour " of the bed itself, however, does not appear to 

 have been described at sufficient length ; Mr. Bonney, at p. 14 of 

 his paper before mentioned, notices " the mode in which the deposit 

 rests on an eroded surface of Gault, and occupies occasionally pot- 

 holes in it, or is interrupted by shoals of Gault." Mr. Seeley, too, 

 had previously remarked upon " the flat irregular surface of clay 

 which the coprolites rest upon." Again, he says J, " there is nothing 

 in the character of the stratum to suggest the time required for its 

 deposition, or whether it occupied the beginning, the close, or the 

 whole of the Greensand age ; for too much stress must not be laid 

 upon its conformity with the Chalk and its unconformabillty with 

 the Gault, for these terms are only true when used relatively" 

 (The italics are mine.) 



Both observers, therefore, have noticed a line of erosion, though 

 Mr. Seeley seems to think it of little importance. On the contrary I 

 esteem it a fact of great significance, and shall have more to say 

 about it in the sequel. 



To those who have not seen any exposure of the Cambridge phos- 

 phate-bed, a description of what may be seen in any coprolite-pit 

 may perhaps be acceptable. Such a vertical section shows really 

 only two formations — Chalk-marl and Gault. The former of course 

 varies in thickness according to the position of the pit ; but usually 

 12 to 15 feet are shown, consisting of a greyish clayey marl (locally 

 called " chinch ") passing down into marl with green grains, at the 

 bottom of which is the coprolite-bed. The nodules, however, are 

 not confined to this thin layer, which is generally not more than 

 8 inches thick ; but scattered coprolites extend upwards to a height 

 of 3 or 4 feet above the surface of the Gault ; it is to be noticed that 

 these are chiefly small, and of a pale buff colour, while those in the 

 seam are nearly always black ; this is sometimes so marked that 

 the foremen have spoken of the existence of a light vein and a 

 dark vein, the latter being always the lowest. 



There is perfect continuity therefore between the Phosphate-bed 

 and the Chalk-marl ; but there is no such passage into the Gault 

 below ; on the contrary, not only is there a sharp line of separation 

 between them, but the surface of the clay is generally uneven, 



* " Cambridge Upper Greensand," Geol. Mag. vol. iii. p. 302. 

 t "Upper Greensand or Chloritic marl of Cambridgeshire," Proc. Geol. 

 Assoc, vol. iii. no. 1. 



% Geol. Mag. vol. iii. p. 302. 



