302 Seeley— Rock of the Cambridge Greensand. 



3. Bittosaria WetherelUi- 



a. Natural size. 



b. and c. Portions mas^nifled. The puncta on the walls appear to have been 

 overlooked by the draughtsman. 



4. Ophiura Wetherellii, Forbes " Tertiary Echinoderms," (p. 32, t. 4, fig. 7), 



from the London Clay, Highgate. 



VI. — The Eock of the Cambribge G-keensand, 

 By Harry Seeley, F.G.S., of the "Woodwardian Musetjm, Cambridge. 



EXAMINING tlie Cambridge sands, fresh from a remembrance of 

 the Undercliff and the grand development of the cherty rock 

 of the Upper Greensand in Southern sections, perhaps its most mar- 

 vellous character is that here it no longer makes a feature in the 

 land, but has dwindled away to barely a foot in thickness, and yet is 

 to be traced through several counties.^ Blending insensibly into the 

 chalk, resembling in structure beds that sometimes occur in the 

 Gault, it appears but as a parting seam between those deposits. Yet 

 so soon as Dr. Fitton made clear the sequence of the Cretaceous beds. 

 Professor Sedgwick recognised in the film of black nodules, glau- 

 conite and chalky marl, the analogue of the sands of the Vale of 

 Wardour and the Isle of Wight. 



Although occurring everywhere below the Chalk and often in out- 

 liers capping the Gault, there are no good natural sections. But 

 there is no lack of opportunities for seeing the superposition, since 

 contractors pay £100 the acre, or more, for the privilege of making 

 sections to dig out the dark nodules of phosphate of lime. 



The plan of working is to sink a trench down to the sands ; it 

 may be one foot or twelve feet, but it rarely pays to work deeper ; 

 then, when the whole of the deposit has been dug out and one side 

 undermined, a line is cut parallel to the perpendicular face of the 

 trench and about four feet behind it, along which, according to the 

 size of the working, from twenty to forty men appear with iron 

 crowbars, which are used vertically till the whole slice gives way, 

 and goes toppling over. Then digging out recommences, and so the 

 work goes on till the field is turned upside down. 



The sections so made, show a layer of nodules imbedded in a soft 

 matrix of marl and glauconite, a stratum varying from six inches to 

 a foot thick, which is all that is left of the Upper Greensand. The 

 nodules rest on a flat irregular surface of clay, into which they do 

 not penetrate ; but above the phosphate bed small stragglers usually 

 extend up into the flaggy Chalk. Sometimes, however, another bed 

 is introduced as in Coton, where a dark unfossiliferous clay, nearly 

 black when wet, comes under the flaggy Chalk and over the layer 

 of phosphatic nodules. 



1 See Ann. and Mag. Nat. Hist., March, 1866, on Torynorvinus, etc. ; Quart. 

 Journ. Geol. See, Nov. 1864, on Hunstanton Eed Eock; Geol. Mag., Vol. II., 

 p. 262, Sequence of Eocks and Fossils; Ann. and Mag, Nat. Hist., Oct. 1864, 

 Fossils of Eed Eock. 



