230 



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



enough to send for examination were identical with those of specimens 

 from these heds. 



Melhourn Rock. — In smoothed hand-specimens from the lower 

 portions of the Melhourn Eock the chalk frequently appears marbled 

 rather than nodular, hut clearly defined nodules are readily found. 

 They are of a size from that of a pea to that of a small potato, and 

 are set in a matrix of greenish grey material, the colour of which 

 may he due in some measure to its decomposition ; for in weathered 

 specimens the greenish colour is more marked than in freshly dug ones. 

 Seen in thin sections under the microscope, the matrix is found to 

 consist of the heavier part of such material as the nodules themselves 

 are composed of. That is to say, recognizable atoms of shell, Eora- 

 minifera, &c., such as are seen in the nodules themselves are to be 

 found in greater abundance in the matrix which contains them. 



In the upper and more yellow chalk of the Melhourn Eock the 

 nodules are usually well defined, but are sparingly distributed 

 through the general mass of the rock. In character they agree 

 with the surrounding matrix, in which, as before, the heavier 

 portions seen in the nodule are to be found in greater abundance. 

 It may be noted that the harder the chalk is, the coarser and more 

 abundant are the shell-fragments. 



It may also be noticed that the atoms of shell &c. are sometimes 

 arranged around the nodule in a manner which suggests either the 

 gradual sinking of the nodule into the soft mud, or that the atoms 

 of shell were deflected from their course by the nodule, while 

 travelling over the sea-bed under the action of a current. That 

 the nodules existed as nodules on the sea-bed is, we think, shown by 

 the fact that small oysters are occasionally found adhering to them. 



Above the yellowish chalk the nodules are hardly to be distin- 

 guished from the matrix which surrounds them, and they appear to 

 become gradually lost in the zone of Bliynchonella Cuvieri. At 

 Hitchin Station, however, nodules may be traced at least 25 feet 

 above the base of the rock. 



Fragments of chalk from the Richmond well-boring, from a depth 

 of 692 to 702 feet which Prof. Judd kindly sent for comparison, 

 agree fairly well with the succession of beds described above, the 

 lower especially bearing a very close resemblance to the Melhourn 

 Rock. 



