558 MESSES. JUKES -BEOWJSTE AND W. HILL ON THE LOWEE PAET 



Dunstable and Tring, beyond the southern termination of the 

 Cambridge Greensand. In that district the basement-bed has a 

 very variable composition, especially as regards the glauconitic 

 ingredient — sometimes it resembles what is elsewhere called Upper 

 Greensand, sometimes it is a laminated calcareous and micaceous 

 silt without visible grains of glauconite. Everywhere, however, it 

 is characterized by the presence of Avicida gryphceoides, and it 

 occasionally contains small phosphatic nodules. The principal 

 difference between specimens from Norfolk and Bedfordshire is that 

 micaceous spangles are more obviously abundant in the latter, the 

 Shouldham bed being lighter in colour, as if more purely calcareous. 



Before passiug to the sections north of Shouldham, we may call 

 attention to the fact that the soft whitish marl which overlies the 

 basement-bed in the Stoke boriug appears to be absent at Shouldham, 

 the hard buff- coloured chalk which is seen at the latter place 

 resembling that which is 16 feet above the glauconitic marl at 

 Stoke. This observation suggests the possibility of some 16 feet of 

 the Chalk-marl having thinned out northwards, or having passed 

 into a more purely calcareous and solid form ; and we think this 

 idea furnishes an explanation of the great changes which take place 

 further north. 



The outcrop of the Totternhoe Stone must emerge from beneath the 

 Boulder-clay somewhere south of Pincham, and skirting the ridge 

 below Fincham Mill, it doubtless runs along the slope which lies 

 on the east side of the road to Marham. "VVe did not notice any 

 traces of the stone in this district, but subsequent experience in the 

 country to the north would lead us to think the Totternhoe Stone 

 would occur at Marham some 50 feet below the Melbourn Rock, 

 which is exposed there, and consequently that its outcrop cannot be 

 far from the church. 



At the foot of Marham Hill a strong spring issues from an horizon 

 which must be low down in the Chalk-marl, and a deep watercourse 

 has been cut for the escaping water along the side of the road which 

 leads to the Turf Fen. In this ditch, at a point somewhat less than 

 a quarter of a mile west of the church, we found a bed of Glauconitic 

 Marl, the microscopical examination of which proved it to be 

 identical witb that at Shouldham and in the boring at Stoke Perry. 



No important exposure of the Chalk-marl or Totternhoe Stone 

 was seen for some miles north of Marham, but at Gayton, in a 

 disused pit just west of the one in which lime is still burnt, the chalk 

 at the base of the exposure is hard, grejdsh and gritty : and as it 

 proved under the microscope to contain green grains, it is probably 

 close to, if not actually the top of, the Totternhoe Stone. 



The remarkable change in the lithological character of the Chalk- 

 marl which we have shown to be gradually coming on as we 

 progress northward is still more marked in the exposure in a pit 

 about half a mile N.W.E. of Roydon Church (for section see p. 550). 



The hard, grey-coloured, gritty chalk seen here has much of the 

 character and appearance of Totternhoe Stone, for which it was 

 taken bv Mr. Whitaker. Beneath it is cream y- white material, very 



