218 MESSES. "W. HILL AND A. J. JTJKES-BROWNE ON 



stratum, sometimes thinniag out and sometimes passing into a pecu- 

 liar marbled nodular rock. It appears therefore that the Melbourn 

 Eock (as originally described) falls naturally into two portions — a 

 marly belt at the base and a belt of rocky beds above ; further that 

 the marls were formed during the prevalence of Belemnitella ]plena, 

 and not after its extinction. 



We think, therefore, that these marls should be separated from 

 the Melbourn Rock and regarded as representing the uppermost part 

 of the zone of Belemnitella plena. We shall state our reasons for 

 believing that these marls rest on a ver}'- uneven surface of the 

 underlying chalk, and that the greater portion of the zone to which 

 they belong is absent in this part of England. It is possible, how- 

 ever, that a part of the chalk hitherto referred to the Holaster- 

 subc/lohosiis zone really belongs to that of Bel. plena. 



It follows from this recognition that the name " Melbourn Eock " 

 must be restricted to the rocky beds which lie above the marly belt 

 and which graduate into the zone of Rliynchonella Cuvieri (zone of 

 Inoceramus lahiatus, Barrois). These prove to be thicker than was 

 formerly supposed, and where complete sections are found, as at 

 Ashwell and Hitchin, they are from 8 to 10 feet thick ; but the 

 upward passage is so gradual that no hard-and-fast line can be 

 drawn. They are simply rock-beds at the base of the lowest zone 

 of the Middle Chalk ; but from their hardness they form a conspicuous 

 feature in all quarries and exposures (fig. 1), and were on this 

 account noticed by Mr. Whitaker in Eucks as long ago as 1865*. 



Our recent work therefore leaves the Melbourn Eock where it was 

 formerly placed, namely at the base of the Middle Chalk, but a 

 portion of what was formerly included in it is now cut off and 

 relegated to the Lower Chalk. 



§ 2. General Desceiption (fig. 2). 



For the purposes of general description it will be convenient to 

 take the base of the Melbourn Eock as a datum-line ; because this 

 horizon is always readily discernible in consequence of the rock 

 resting on a band of marl, and because the beds above always exhibit 

 the same general succession, while the beds below appear to differ 

 in different sections and do not present a constant succession. 



First, therefore, to describe the Melbourn Eock : its lowest bed is 

 a hard nodular mass from 3 to 4 feet thick, consisting of small, 

 hard, white nodules set in a matrix of greenish-grey chalk, the 

 nodules varying from the size of a pea to that of a small potato. 

 The overlying beds have generally a yellowish tinge, and the nodules 

 in them are arranged in layers at distances of from 6 to 18 inches 

 apart, only a few nodules being scattered through the mass ; there 

 are two or three such beds with a total thickness of from 5 to 6 feet. 

 There are occasionally partings of marl between the rocky beds, and 

 these also contain small hard nodules. At a height of 9 or 10 feet 

 from the base there is sometimes a band of hard smooth rock with- 



* Quart. Journ. Geol. Soc. vol. xxi. p. 398. 



