240 MESSRS. G. W. LAMPLTJGH AND J. P. WALKEE ON [May I903, 



By long -continued attrition under current- action, the sand- 

 grains may eventually become nearly as well- polished, though not 

 quite so well-rounded, as in desert blown-sands, to which this 

 polishing has been considered (perhaps too exclusively) to be confined. 

 The researches of Mr. A. R. Hunt upon the smoothly-rounded 

 sands of the Skerries Shoal, off the coast of Devon, may be adduced 

 as a modern illustration of this action. 1 It is especially noteworthy 

 that phosphatic nodules are very frequently associated with these 

 polished sands. Into this side-issue we do not propose to enter 

 further at present, as the matter will be more conveniently discussed 

 on some future occasion, in connection with the re-investigation of 

 the fauna of the Lower-Greensand phosphate-beds. 



The Iron-Grit Bands. 



Additional proof of the slow rate of deposition is afforded by the 

 floors of iron-grit, which are so strikingly developed immediately 

 above and below the fossiliferous band at Shenley. Similar iron- 

 stone occurs sporadically in nodules and irregular masses throughout 

 the Lower Greensand of the Midland counties : its presence in the 

 'silver-sands' of the Shenley sections has already been noted, and it 

 is plentiful also in the sand-pits at lower horizons between Leighton 

 Buzzard and "Woburn. In most cases it has clearly been formed 

 subsequently to the period of accumulation of the sands, through 

 the segregation of the iron-salts by percolating waters. Indeed, in 

 some places the induration is probably still in progress. In a small 

 opening south-east of Garside's Pit (shown in tig. 2, p. 236), where 

 the sands crop out at the surface from beneath a thin wedge of 

 Drift, there is at the top of the section a large mass of iron-grit, 

 2 to 5 feet thick, peculiarly hard and vitreous, and almost black 

 in the middle of the blocks, in which apparently there has been 

 some solution and redeposition of silica as well as of iron. This 

 rock passes out downward, along planes of cross-bedding, into a 

 bright orange-staining of the sands, and though these sands are 

 very different in aspect from the white sands of adjacent sections, 

 they are clearly part of the same mass. Here it is highly probable 

 that the present position of the outcrop is responsible for the 

 induration and staining. 



The persistent undulating floors of iron-grit associated with the 

 fossiliferous belt, however, though somewhat similar to the later 

 concretions in appearance and composition, must have been formed 

 very soon after the accumulation of the sands ; for there is clear 

 proof that they were in existence as rocky bands before the material 

 now overlying them was deposited. Above the lower band are in 

 places patches of coarse breccia, composed of flat fragments of iron- 

 grit ranging up to 3 or 4 inches in diameter, more or less sub- 

 angular, but with worn and polished surfaces, mixed with coarse 

 quartz-grit and small ' lydite '-pebbles in a slightly-calcareous paste, 



1 • The Evidence of the Skerries Shoal on the Wearing of Fine Sands by 

 Waves ' Trans. Devon. Assoc. Adv. Sci. vol. xix (1887) pp. 498-515. 



