378 Mr Keeping, Neocomian deposits of Potton and Upware. [May 3, 



chert, also frequently angular and subangular, and of various 

 colours. These belong to two separate ages, namely : (i) Mountain 

 Limestone chert, some of it very flinty and transparent, and con- 

 taining a number of characteristic fossils ; and (ii) a more opaque 

 and reddish chert with Jurassic fossils. Also {d) a pebble of 

 " devitrified pitch-stone " described by Professor Bonney, who 

 recognizes it as very similar to the ancient Rhyolite lavas of the 

 Wrekin. 



With regard to the original homes of the pebbles the great 

 majority of the phosphatised fragments were derived as pebbles from 

 the ancient cliff's of Jurassic rocks which were being denuded away 

 around Cambridge and Potton along the old shore-line of the Upper 

 Neocomian sea. The new Red Sandstone and older Jurassic rocks 

 of the Midlands probably also supplied some of the pebbles. But, 

 having restored the ancient land surfaces of the period and regarded 

 this as a source of pebble supply, it is found that the small and dis- 

 tant exposures of carboniferous and older Paleozoic rocks in Derby- 

 shire and Charnwood Forest are utterly inadequate, and we are com- 

 pelled to look elsewhere for their origin. From the angularity of 

 the majority of the groups of pebbles and from their dissimilarity 

 to the pebbles in older conglomerates, it is clear that the pebbles 

 must have been derived directly from the parent rock, and not 

 handed on from older pebble beds. 



The ridge of Palaeozoic rocks known as the Harwich axis 

 formed, in earlier Neocomian times, a barrier of land separating 

 the northern (or Anglo-Germanic) from the southern (or Anglo- 

 Gallic) sea ; but gradually it subsided, suffering great denudation 

 from the work of waves and currents, as it became slowly sub- 

 merged. Ultimately the northern and southern seas met and 

 surged together through narrow channels, and these were gradually 

 widened, over Cambridgeshire, until the barrier totally disappeared 

 under water. In this destruction of the old barrier we find an 

 abundant source of Palseozoic pebbles. Recent deep borings have 

 discovered the nature of the old axis rocks at Turnford (Devonian) 

 and at Ware (Silurian), and the relations of these are such as 

 to lead us to expect the occurrence of still older Palseozoic beds 

 further north, near Cambridge. Thus the details of the structure 

 of the ancient barrier, so far as yet known, support the inde- 

 pendent conclusion that the majority of the older Palseozoic 

 pebbles of our Neocomian pebble-beds were derived from this 

 source. 



The pebbles of chert point to the former existence of Jurassic 

 limestone over this area ; but no very satisfactory evidence appears 

 bearing upon the question whether coal-bearing beds also form part 

 of this old Palseozoic axis. 



