56 ME. G. Vr. LAMPLrGH OX THE JU^'CTIOX OF [vol. Ixxviii, 



ship to the coarse sands at Aylesbury,^ and among the Lower 

 Greensands in other parts of England, as, for example, aromid 

 Ash Wicken in Xorf oik ^ ; and such beds are generally, perhaps 

 always, devoid of phosphatic concretions, rarely yielding any fossils 

 except bits of plants. The composition and mode of occurrence of 

 these deposits are suggestive of estuarine conditions produced by 

 the local influx of land-waters. 



In the present instance, the deposits indicate a temporary 

 cessation of the strong cun-ents which had heaped the Leighton 

 Sands into banks. The Silty beds rest sharph' on an eroded and 

 somewhat undulating surface of these sands, and appear, from their 

 rapid variation in thickness, to have filled up the broader hollows 

 between the banks. They are confined to an area lying north of 

 a north-west and south-east line drawn from the northern end of 

 the village of Heath ^ to the Shenley Hill pits ; and, although 

 their present termination along this line is irregular, and has to 

 some extent been determined by the renewed erosion which preceded 

 and accompanied the formation of the Basement beds, there are 

 clear indications that the southward limit of their original basin of 

 accumulation lay in this neighbourhood. This limit approximately 

 coincides with the crest of the low arch of the Gault and its 

 Basement beds above-described ; and it is along this axis that all 

 the beds undergo the changes in composition and structure which 

 have lent so much interest to the exposures. 



The core of the belt is composed of cross-bedded ' Silver Sand,' 

 which appears here to have been heaped up in a long bank or plexus 

 of banks forming a shoal, with deeper Avater on both sides, the 

 depression on the northern side being subsequently filled in by the 

 Silty beds. 



It is all along the top of this old shoal that the patches of 

 induration occur, by which the upper part of the Sands is converted 

 in places into massive bosses of iron -grit and quartzite. this con- 

 dition being prevalent in the pits ranging from Miletree (fig. 10) 

 on the east, through Nine Acre (figs. 7, 8) and Garside's (fig. 4) 

 to the old Heath House pit (p. 29) on the west (see map, fig. 1, 

 p. 2). In discussing this peculiar induration in onr previous 

 paper (L.W., pp). 240-41). we showed that it must have been 

 efi:ected, in part at any rate, before the deposition of the Gault, as 

 the breccia below the Gault was composed principally of waterworn 

 fragrments of this material, some encrusted with adherent ovsters 

 and serpulte. The conclusion has been confirmed by all my later 



^ A. M. Davies, ' Contribiitions to the Geology of the Thame Yalley,' Proc. 

 Geol. Assoc, vol. xvi (1899) p. 49. See also ' Special Reports on the jNIineral 

 Hesources of Great Britain, vol. vi, Refractory Materials. &c.' Mem. Geol. 

 Surv. 1911. p. 180. 



2 ' Geology of the Borders of the Wash ' Mem. GeoL Sui-r. 1899, pp. 17-18. 



2 Besides the sections dealt with in § II, the Silty beds are seen to a 

 depth of 6 or 7 feet at the top of a big sandpit (' Stone Lane pit ') 700 yards 

 north-north-east of Claridge's pit (fig. 15) or 250 yards beyond the northern 

 border of the sketch-map (fig. 1), at the turn of the road leading from Reach 

 to Woburn ; they form the capping of a deep section in the coarse Sands. 



