part 1] GAIJLT A^D LOWEE GREENSAND NEAR LEIGHTON. 9 



the coarse basement-beds of the Grault presently to be described. 

 The * compound ' structure and general aspect of the nodules,^ as 

 well as their fossil contents, imply that they have been concentrated 

 on the sea-floor during a rather long interval when normal sedi- 

 mentation of the Grault clay was arrested, presumably by current- 

 action. A similar phosphate-bed at about this horizon, formerly 

 worked for ' coprolites,' is described by Jukes-Browne as occurring 

 at several places along the outcrop of the Gault in Bedfordshire 

 and Buckinghamshire. 2 



It is important to note that the band is also nearly on the same 

 horizon as the nodular ' Junction Bed, YIII ' of the Folkestone 

 Grault, respecting which Price's remarks are so apposite that I will 

 recall them. He says : — 



' I would suggest that these lines of nodules, mixed as they are with rolled 

 fossils, occurring so plentifully throughout the deposit, mark the floor of the 

 sea during a period, more or less vast, when great physical changes may have 

 altered the course of the currents, and so borne away the sediment, to be 

 redeposited in another direction ; or they may represent periods of upheaval 

 or tranquillity, when the sea-bed was at rest. It may thus be argued that 

 these nodule-beds, not exceeding 1 inch in thickness, are equivalent to a period 

 of time far greater than was required for the deposition of several feet of clay.'"^ 



The lowest part of the Grault has shown little or no change 

 during the working-back of the pit, except that the impressions of 

 crushed fossils sparingly present in it are rather less obscure than 

 they Avere Avhere the cover was thinner. Traces of the decompo- 

 sition of original pyrites are very apparent, and may partly explain 

 the poor condition of the fossils. The small brown-coated nodules, 

 an inch or two in diameter, which occur in this part of the clay are 

 quite different in aspect from the nodules of the higher bed just 

 described, and have not been concentrated or corroded. A fragment 

 of decomposed pyritous wood, 4 or 5 inches long, associated with 

 a line of the small nodules, had however been bored by marine 

 organisms, and was partly encrusted with small oyster-like shell- 

 scars. The fossils of this portion of the clay will be dealt Avith in 

 the discussion of the supposed overturn (pp. 51, 78). 



Last autumn, no solid limestone Avas visible beneath the Gault 

 in the portion of the pit illustrated. The oehreous ironstone- 

 breccia (3) was, however, in one place very calcareous, and repre- 

 sented the decomposed feather-edge of a tabular mass of limestone 



^ Incidentally the nodules supply proof that the band and its associated 

 deposits are not turned upside down, as supposed by Dr. Kitchin & Mr. Pringle. 

 Most of the larger nodules, and many of the smaller ones, are deeply corroded 

 on their upper surface but comparatively fresh on the under side, this being 

 particularly conspicuous when, as is frequently the case, the nodules are 

 casts of portions of ammonites. The under surface often presents a beauti- 

 fully-smooth bright cast of the shell, whereas on the upper surface the fossil 

 is almost or quite indistinguishable. This condition of the larger fossils is 

 well-known in many deposits of slow accumulation, as for instance in certain 

 parts of the Chalk and other limestones (see Proc. Geol. Assoc, vol. xviii, 1904, 

 pp. 287-89). 



" 'The Gault & Upper Greensand of England ' Mem. Geol. Surv. supra cit, 

 pp. 277-78, 280-82. 



3 ' The Gault ' by F. G. Hilton Price, London, 1879, p. 9. 



