260 



A. J. JUKES-BROWNE ON THE RELATIONS OF 



undulations and hollows being frequently visible in it. These 

 troughs do not invariably occur, though few pits are entirely without 

 them ; they are not often very deep ; but in them the nodules are 

 generally accumulated to a depth of from 12 to 15 inches, while on 

 the summits of the rolls and ridges the bed almost disappears. 



Erom these facts I conclude that the Cambridge Greensand rests 

 on a surface of erosion, and passes insensibly upward into truo Chalk- 

 marl. In fact, had the coprolites been pebbles of any other substance 

 than phosphate of lime, they would no doubt have been described as 

 forming only a pebble-bed at the base of the Chalk, and not regarded 

 as a separate formation at all. I find that Mr. J. E. Walker takes 

 a similar view of the case. In a letter to Mr. Davidson he says* : — 

 " Now, if the sea-coast consisted of Gault clay which was destroyed 

 by the sea, the nodules, bones, shells, &c. might be washed out (as 

 the denudation of the drift-clay forms beds of gravel) and form the 

 Cambridge bed." 



In common, therefore, with Mr. Bonney and Mr. Walker I believe 

 this unconformity (for it amounts to that) has been produced by the 

 erosion of marine currents, probably in a shallow sea ; and it flows 

 from this as a natural consequence that the upper beds of the Gault 

 are now entirely wanting in Cambridgeshire, though they must once 

 have been well developed over this area. There is therefore a break 

 of indefinite length between the Cambridge Gault and Greensand. 



I must, however, notice another explanation which has been given 

 of the appearances above mentioned by the Rev. 0. Eisherf. He 

 describes and figures the pockets in the Gault, but states that they 

 only occur when the bed is near the surface, and attributes them to 

 lateral pressure produced by land-ice. To this opinion I cannot 

 subscribe. Eirst of all, Mr. Fisher must allow me to correct his 

 statement that the undulations and pockets only occur when the 

 bed is within 3 feet of the surface. I have seen them in workings 

 of every depth, in some places more marked than in others, but still 

 in the deepest and shallowest pits alike. 



Eig. 2. — Section near Horningsea, Cambridge. 



rri^ a 



i;C^^£2==^jO^Tir: 



Lirrrr___ c^ 



L_L_,t-^fi -^_ ] Lt^L^2±^.-L^±l. 



a. Gault Clay. ~b. The Nodule-bed. c. Greenish sandy Marl. 



d. Chalk-marl. e. Thin Gravel and Soil. 



* See Geol. Mag. vol. vi. p. 259 (note). 



t " The Coprolite-pits of Cambridgeshire," Geol. Mag. vol. viii. 



