Mr. Jukes-Browne on the Cambridge Gault and Greensand. 413 



The first part of the paper deals with the stratigraphical relations 

 of the bods ; and the author calls attention to the fact that in the 

 numerous artificial sections near Cambridge only two formations are 

 really visible, viz. the Chalk Marl with a pebble-bed of phosphatic 

 nodules at the base, and the stiff dark clay of the Gault, npon which 

 these rest. 



The so-called Greensand or nodule-bed passes up into the Chalk 

 Marl, but rests unconformahly on the Gault below, which presents in 

 fact a surface of erosion ; and there is therefore a break of indefinite 

 length between the Cambridge Gault and Greensand. 



The nodule-bed continues to present much the same characters 

 and fossils through Bedfordshire as far as Sharpenhoe, a village 

 about 3 miles east of Harlington, on the Midland Hail way. Here is 

 situated the most westerly coprolite pit or working in the Cambridge 

 beds ; and beyond this the Gault passes into Chalk Marl without any 

 such scam intervening. 



It is not until we enter Buckinghamshire and reach Buckland 

 near Tring, that any thing like true Upper Greensand appears, and 

 separates the Chalk Marl from the Gault. From this point westward 

 the formation increases in thickness and importance ; but its cha- 

 racters and fossils are quite different from those of the Cambridge 

 Greensand. 



Although in Bucks no coprolites are found between the Gault and 

 Greensand, yet they occur in the Gault itself; and one bed may be 

 traced towards the jST.E., and is found to commence where the Cam- 

 bridge nodule-bed ends, thereby raising the presumption that it 

 becomes con fluent with that bed, and has furnished many of the 

 well-known fossils and nodules it contains. 



A consideration of these facts warrants the following general 

 conclusions : — 



I. That the Cambridge Greensand or nodule-bed has no con- 

 nexion with the Upper Greensand, its actual position being at 

 the base of the true Chalk Marl. 



II. That the same bed rests unconformahly on the clay below, 

 and that its coprolites and fossils have been derived from the 

 Gault. 



II I. That in consequence of this erosion a great gap now exists in 

 Cambridgeshire between the Lower Gault and the Chalk Marl, 

 the whole of the Upper Gault and Upper Greensand being 

 absent. 

 The pakeontological evidence leads to exactly the same con- 

 clusions. The fauna is divisible into 2 groups; and the fossils 

 belonging to the enc are preserved in dark phosphate, and being 

 generally w r aterworn are clearly derived forms, while the others are 

 of lighter colour and belong to the deposit. The former group is 

 chiefly composed of Gault species, 70 per cent, of which belong to 

 the upper stage of that formation ; w r hilc the fossils proper to the 

 deposit are also found in the Chalk Marl above. 



The author therefore feels justified in concluding that strati- 

 graphically the bed is Chalk Marl, while palaeontologically considered 

 iti fauna is mainly derived from the Upper Gault, 



