()Q MR. a. "w, LAMPLrCxH o>' THE jr>'CTio>' or [vol. Ixxviii, 



of Dr. Kitchin & Mr. Pringle that the Upper Grault alone is 

 present m sections north of Leighton, and that the irongrit-breccia 

 accompanies this division only, it has been necessary for nie during 

 the past year to re-examine as much of the Gault in the neighbom-- 

 hood as is accessible. The investigation has convinced me that 

 the Lower Grault is continuous over the whole area, and is succeeded 

 conformably by the Upper Gault, but with a long jDause in the 

 sedimentation, marked by a band of phosphatic nodules, at or near 

 the base of the Upper Grault. 



Leaving controversial points for later discussion (pp. 70, 78), I 

 will bj-iefly review the featm-es of the Lower Grault as exhibited 

 in the sections. Takinor the incomins^ of ' Inoceramus sulcatus ' in 

 association with ' Ammonites rosti-atus ' as their upper boundary, 

 the Lower clays (excluding the Mam m ilia t us beds) appear in this 

 neighbourhood rarely to exceed 15 feet in thickness, and may in 

 places diminish to 10 feet, as compared with their 27 or 28 feet at 

 Folkestone. This diminished thickness is maintained for a con- 

 siderable distance southwards, judging from the descrij^tion by 

 Jukes-Browne of a boring at Slapton Lock, 3 miles south of 

 Leighton, in which we read^ : — 



' It would appear that tliere is a 110(11116-1)6(1 atont 10 feet from the base of 

 the Granlt; among the fossils preserred I identified the following, Ammonites 

 interruptus. Am. cristatus, Am. rostratus. Am. varicosus (?), Inoceramv.s 

 sulcatus, Inoc. concentricus, Taoc. tenuis (?)' 



— a similar assemblage to that which the same investigator collected 

 from the nodule-bed in the Heath brickyard (p. 27), and to that 

 of the equivalent nodule-bed at Harris's pit (p. 7). 



This reduction of the Lower Gault as we approach the shelving 

 coast-line of the period has its exact counterpart in the Xoi-th of 

 Urance where, at Wissant, its thickness is only 16 feet (with 

 2 feet of basement Mam m ilia tus beds below, almost exactly as 

 at Shenley) and still less in the counti\vto the east, but expands to 

 over 30 feet in the Ai-gonne region, and to over 100 feet in. parts 

 of the departments of Marne, Haute Mame, and Aube ; remaining 

 thi'ouorhout its ransre, however, alwavs very much thinner than the 

 Upper Gault.- 



The sections afford a reasonable explanation for the thinness of 

 the Lower Gault in the Leighton district. Everywhere, except on 



^ Op. supra cit. ^Jem. Geol. Snrv. p. 279 : the anthors. however, did not 

 recognize that the Lower Gmnlt was comprised in the clays below the nodule- 

 bed, and thought, with som.e misgivings, that it must include some higher 

 clay or marl. In their conjectural classification of another boring, at Gubble- 

 cote, 6 miles south of Leighton, they allow 145 feet to the Lower Gault : 

 but I suspect that they should have confined it to the 22 feet of ' dark 

 brownish clay, becoming sandy and glauconitic below ' which has a ' layer of 

 phosphatic nodules ' above it, and has beneath it. 3 feet of ' blue sandy clay 

 with phosphatic nodules at the base' (ihid. p. 282). 



^ C. Barrois ' Sur le Gault dans le Bassin de Paris ' Ann. Soc. Geol. Xord. 

 vol. ii (187-5) pp. 1-61 ; and ' Terrain Cretace des Ardennes " Ihid. vol. v 

 (1878) pp. 227-487; A. J. Jukes-Browne, 'The Gault & Upper Greensand' 

 Mem. Geol. Surr. 1900, chap, xxs-ii. 



