THE MELBOTJKN ROCK, ETC. 217 



Bedford, Buckingliam, and Oxford, and has been found to be 

 remarkably constant in its character and behaviour. It is generally 

 found at the top of a well-marked feature or slope, and where it 

 recedes into coombes, these are generally steep-sided ; at the head of 

 such valleys there are frequently powerful springs, which are thrown 

 out by the marly beds at the base of the rock, so that it is of practical 

 importance as a water-bearing stratum. 



While one of us has been engaged in drawing this line of outcrop 

 for the Geological Survey and in noting the exposures along its 

 course, the other has carefully compared these sections, as they were 

 discovered, with those previously described in Cambridgeshire, has 

 collected such fossils as were to be found, and has cut and examined 

 under the microscope numerous slices from the different beds of 

 rock and from the chalk below aud above. In the present commu- 

 nication, therefore, we propose to offer the results of our combined 

 observations on this rocky band and the beds associated with it ; and 

 for permission to make use of the information gained during the 

 official survey of the country, we have to thank the Director- 

 General. 



In the first place we have to announce that we recognize a zone 

 of Belemnitella jpJena separate and distinct from the mass of the 

 Melbourn Eock, and that consequently it becomes necessary to make 

 some modification in the definition originally given of that rock. 



The Melbourn Rock was described as consisting of several courses 

 of hard, yellowish, rocky chalk, separated by layers of greyish lami- 

 nated marl or shaly chalk, one of these marly bands always occurring 

 at its base. It was observed that this basement marl sometimes 

 contained rolled specimens of Belemnitella plena, as well as other 

 fossils, but that otherwise the zone of Belemnitella plena appeared 

 to be absent in Cambridgeshire, unless it was represented by the 

 very uppermost portion of the underlying chalk. The facts, how- 

 ever, seemed to indicate that Dr. Barrels was right in regarding this 

 marly band as a remanie bed, i. e. that the chalk of the Bel. plena 

 zone had been destroyed and sifted by current-action, and that this 

 marl was composed of its rearranged particles and contents. As 

 there were in Cambridgeshire two principal layers of marl separated 

 by a rocky band similar to the rock-beds above, the whole set of 

 beds was regarded as forming one horizon and placed at the base of 

 the Middle Chalk. This view was supported by the fact that the 

 horizon coincided with a palaeontological break, the assemblage of 

 fossils found in the Middle Chalk above being very different from 

 that in the Lower Chalk. 



Our recent researches have, however, disclosed three important 

 facts, which modify the view above given, though they by no means 

 invalidate the conclusion that the rock is at the base of the Middle 

 Chalk, and that it marks the incoming of a new fauna. The facts 

 are : — (1) that the lower marl does, in many places, contain perfect 

 and well-preserved specimens of Belemnitella plena; (2) that the 

 uppermost bed of the chalk on which it rests also contains that 

 fossil ; (3) that the rock between the two marly layers is a variable 



