THE MELBOURN ROCK, ETC. 



219 



out nodules, but nodules occur again in higher beds and have been 

 detected at Hitchin 25 feet from the base of the rook. 



Fig. 2. — Diagram sJioiuinr/ the usual order of succession of the Beds of 

 the Melhourn Rock and the Zone of Belemnitella plena. (Scale 

 -rV in. to 1 ft.) 



Zone o( Rhynehonelln Curieri... ^ 



Melbourn Eock 



Zone of Belemnitella plena 



Lower Chalk, zone of Holasfer 

 suhglobosiis 



Hard rocky yellowish chalk ; less 

 nodular than that b«"low ; fre- 

 quently with marly partings. 



Hard rocky whitish nodular 

 chalk. 



Shaly or marly chalk. 



Hard white rock, occasionally 

 replaced by the marbled rock. 



Greenish-grey marly band pass- 

 ing into the chalk beneath. 



White toughish chalk. 



Returning now to the base of the Melbourn Eock, we find it 

 everj^wbere resting on a baud of shaly marl, which is from 3 to 5 

 feet thick and usually encloses a layer of hard white chalk The 

 laminated mail or shale is of a greyish-buff colour, harsh and gritty 

 to the touch, and is generally divided into two layers or bands by 

 the hard white chalk above mentioned. This chalk splits with a 

 smooth even fracture, and thus differs from the rough nodular'chalk 

 of the Melbourn Eock ; occasionally, however, it appears to be 

 replaced by a very curious bed, which, when broken, presents a 

 mottled or brecciated appearance, consisting partly of greyish and 

 partly of white chalk. This bed we have termed the " ^[arbled 

 Eock," and it closely resembles the bed which was found at a depth 

 of 704 feet in the Eichmond boring and described by Prof. Judd as 

 " brecciated chalk "*. 



The shaly marl below this marbled rock, or the white rock which 



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