94 



The Geology of the Railway Line from 



micaceous clay, with crystals of iron pyrites ; these being abundant 

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

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

 little pyrites in Wilts. 



Water percolating through 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 fromij 

 Burderop Wood by Wroughton, Broad ToAvn, &c. 



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

 with 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 oJ 

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



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

 in the cutting beyond Burderop 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 sandj 

 until it passes into the greensand and disappears beneath it. 



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

 clearly the southerly dip, and here we shall see no trace of the gault I 



The northern end of the Chiseldon cutting shows the junction o:j 

 the upper greensand with the chalk. 



! 



The Upper Greensand. 



The greensand can be seen to dip south under the chalk, anc 

 before we go far along the cutting has quite disappeared. 



The junction of the upper greensand and the chalk is marked b^ 

 a dark green clayey sandy bed with dark brown phosphatic nodules 

 This bed is only about 18in. thick. So numerous are these phos 

 phatic nodules in Cambridgeshire that they have been worked am , 

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

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

 thickness. 



All sand deposits were laid down at no great distance from th< 



■ 



