220 MESSES. W. HILL AND A. J. JTJKES-BEOWNE ON 



more usually occurs, is variable in its thickness ; moreover we find 

 that it sometimes passesdown into grey marly chalk, which graduates 

 into the mass of the Lower Chalk, and sometimes it rests on a well- 

 marked and uneven surface of tough dull white chalk, which exhibits 

 rather different microscopical characters from that which elsewhere 

 underlies the shaly marls. This discrepancy was for a long time a 

 puzzle to us, but we now believe that it is due to the action of con- 

 temporaneous erosion on a much larger scale than has ever been 

 previously suspected to occur in the chalk. 



§ 3. Desceiption of Sections. 



Commencing at Shelford, near Cambridge, the following are the 

 principal exposures of the zone of BelemniteUa plena and the 

 Melbourn Eock, as its outcrop is followed south-westward. 

 At Shelford, in the pit on Steeple Hill, the section seen is : — 



ft. 



Grravel 1 



Much broken rubbly chalk 3 



^j- ,, C Thin-bedded yellowish-white chalk 2 



Melbourn i ^^^^ whitish nodular chalk with wide vertical 



KOCK. • J i. A 



[ joints 4 



^ Grey laminated marly chalk 1 



„ e I Hard white chalk, rocky but with smooth 



Bl^\^ -( fracture H 



' -^ ' Softish laminated chalk 1 



1^ Blocky white chalk 3 



The whole of this exposure is weathered, and the character of the 

 upper portion of the Melbourn Eock not well shown. 



At Maggot's Mount, in a pit west of the Obelisk at Harston, there 

 is seen : — 



ft. 



Soil 1 



Much broken thin-bedded chalk 1 1 



Melbourn f Yellowish chalk with few nodules 3 



E-ock. \ Hard whitish nodular rock 3 



1 Greyish chalk with thin shaly bands in upper 

 portion 2 

 Hard white smooth chalk 2 

 Laminated marly chalk 1 



On the northern side of the pit the grey marly chalk at the base 

 is seen to be two feet in thickness, and its passage into tlie white or 

 lower chalk is not so abrupt as is usually the case. 

 A pit on Eoston Hill gives the following section : — 



ft. 



Soil and much broken chalk 2 



Bedded white chalk, with nodules 4 



( Yellowish chalk, thin-bedded, with thin part- 

 Melboum 1 ings of greyish marly chalk containing no- 



Eock. 1 dules 4 



[ Hard white chalk breaking with rough fracture 3 

 Zone Bel. jplena. Buff-coloured marly chalk 2 shown. 



The chalk here has not been worked for some time, and the upper 



