Vol. 59.] A LOWEE-GREENSAND FOSSILIFEROUS BAND. 239 



In all the sections there is a clear line of demarcation between 

 the strongly current-bedded ' silver-sands ' and the overlying more 

 regularly-stratified loamy glauconitic sand, with its layers of iron 

 grit and fossiliferous concretions. This line of junction seems to 

 form a definite floor, and to imply some degree of erosion and 

 unconformity. Yet in a mass so irregularly-bedded as the Lower 

 Greensand, such appearances are not unusual, and need not neces- 

 sarily denote anything more than local erosion and re-arrangement. 



The 'Silver-Sands.' 



The silver-sands are sporadically indurated into irregular lumps 

 and nodules of ironstone, in the larger of which traces of fossil 

 wood, sometimes perforated by borers, may be occasionally detected 1 ; 

 but with this exception our search in them for organic remains was 

 unsuccessful. Lithologically they somewhat resemble the ' Sandring- 

 ham Sands ' 2 of Norfolk, but probably occupy a higher stratigraphical 

 position. The ' Nine-acre Pit/ shown in fig. 2 (p. 236), is entirely 

 in these sands ; and there are numerous other extensive excavations 

 at the same and lower horizons in the tract of Lower Greensand north 

 and west of Shenley Hill, but in none could we find any trace of 

 fossils except wood. There are also very fine sections in ballast-pits 

 on both sides of the branch-railway immediately south of Leighton, 

 showing 20 to 30 feet of strongly current-bedded buff and ochreous 

 sands, coarsely gritty in texture, with very little ironstone ; but 

 these sands are capped by Drift, and their junction with the Gault 

 is not at present seen, although it must be nearly reached. No trace 

 of the fossiliferous band was found in these sections ; nor in the old 

 sand-pits at Egginton, 11 miles east of Shenley Hill, which are also 

 close to the boundary of the Gault. 



A striking feature, both in the i silver-sands ' and in the overlying 

 sand, is the high degree of rounding and polishing exhibited by the 

 larger quartz-grains and lydites. This feature is indeed prevalent 

 in many sandy deposits between the Kimmeridge Clay and the 

 Upper Cretaceous in England ; as, for example, in the Spilsby Sand- 

 stone of Lincolnshire, in the Sandringham Sands of Norfolk, in parts 

 of the Folkestone Sands of Kent, and in the Ironsands of Western 

 Sussex. It has been previously pointed out by one of us, that where 

 beds possessing this peculiarity are present in the Lower Cretaceous 

 Series, a great change is usually observable in the fauna above and 

 below such beds wherever fossil evidence is available ; and the con- 

 clusion has been drawn that sands of this character denote periods 

 of slow deposition on a current-swept sea-floor, whereon the same 

 detritus was drifted to and fro and frequently redistributed, without 

 much addition of new material. 3 



1 The specimen of ' Cycadoidea Yatesii from Leightou Buzzard,' described by 

 Mr. W. Carruthers in Geol. Mag. vol. iv (1867) p. 199, and mentioned by Morris 

 (ibid. p. 457), may have been obtained from these sands. 



2 ' The Borders of the Wash ' Mem. Geol. Surv. (1899) p. 17. 



3 ' Summary of Progress for 1898 ' Mem. Geol. Surv. pp. 142-43. 



