94 The Gcohrjii of the Raihccnj Line from 



micaceous clay, with crystals of iron p}Tites ; these being abundant 

 as a rule, render the fossils difficult to preserve, and they should be 

 treated frequently with a solution of gelatine ; there is, however, 

 little pyrites iu Wilts. 



Water percolating thi'ough the upper greensand over the gault, 

 causes the sand to slip over the clay, so that landslips are very 

 frequent over the gault, and they may be seen all the way from 

 Burderop Wood by Wroughton, Broad Town, &c. 



With the gault the upper cretaceous series of rocks begins, and 

 "nith it a great period of subsidence, continued, with the exception 

 of a shallowing during the upper greensand period, through the 

 whole of the upper cretaceous period, until a thickness of several 

 hundred feet had been deposited, varying in different parts of 

 England, but thinning steadily in a westerly and northerly direction. 



The junction of the gault with the upper greensand is to be seen 

 in the cutting beyond Bm-derop Wood, where a bridge crosses the 

 railway. 



We shall here find it difficult to mark the exact place where the 

 one bed ends and the other begins, as the gault becomes more sandy 

 until it passes into the greensand and disappears beneath it. 



But the next cutting — viz., that at Chiseldon — shows much more 

 clearly the southerly diji, and here we shall see no trace of the gault. 



The northern end of the Chiseldon cutting shows the junction of 

 the upper greensand with the chalk. 



The Upper Greensand. 



The greensand can be seen to dip south imder the chalk, and 

 before we go far along the cutting has quite disappeared. 



The jimction of the upper greensand and the chalk is marked by 

 a dark green clayey sandy bed mth dark brown phosphatic nodules. 

 This bed is only about 18in. thick. So numerous ai'e these phos- 

 phatic nodules in Cambridgeshire that they have been worked and 

 made into manure. The upper greensand here is about 60ft. thick. 

 In Dorset, South Wilts, and the Isle of Wight it is over 100ft. in 

 thickness. 



AU sand deposits were laid down at no great distance from the 



