602 MESSES. W. HUE AND A. J. JUKES-BROWNE [Nov. 1895, 



previous publications, 1 and we regarded them as portions of some 

 pre-existing deposit of chalk-ooze which had gained sufficient co- 

 herence to be rolled into lumps on the sea-bottom under the influence 

 of the current which distributed the material whereof the enclosing 

 matrix is composed. We have not changed this opinion, but it 

 would seem that the chalk-ooze which yielded the Melbourn Rock 

 nodules differed from the similar chalk which occurs both at lower 

 and higher horizons, either in the fact of its containing radiolaria 

 or in some special property which was favourable to their preser- 

 vation. 



"We have recognized these calcific replacements of radiolaria in 

 the nodular portions of the lower beds of the Melbourn Eock taken 

 from the following localities : — Melbourn and Royston, in Cam- 

 bridgeshire ; many exposures near Hitchin (Herts) ; Leagrave, near 

 Luton, in Bedfordshire ; Pitstone and Tring, in Bucks and Herts ; 

 near Watlington, in Oxfordshire; several localities in Lincolnshire 

 and Yorkshire ; in the deep boring at Richmond ; in the lower part 

 of the ' grit-bed ' at Dover ; also at Sutton Waldron, south of Shaftes- 

 bury (Dorset) ; Bincombe, in South Dorset • and quite recently in 

 nodular chalk which may be considered as the equivalent of the 

 Melbourn Rock, from Bindon Cliffs, near Axmouth, Devon. 



They do not seem to occur in the nodules of the upper part of the 

 Melbourn Rock, or in those of the Chalk Rock ; and though one of 

 us has been constantly cutting samples of chalk from many localities 

 and from various horizons, he has never noticed any such organisms 

 in any other part of the Chalk, except quite recently in nodules 

 from the hard beds of the Chalk Marl in Norfolk, Lincolnshire, and 

 Yorkshire. 



One can hardly imagine that the epochs of the Chalk Marl and 

 of the Melbourn Rock were the only times when radiolaria existed 

 in that part of the Cretaceous sea which covered England and France, 

 and it seems more reasonable to suppose that they were frequently 

 present and were embedded in many portions of the Chalk-ooze, but 

 were usually so rapidly and completely dissolved that no trace of 

 them had been left in the consolidated material. Doubtless they 

 contributed to that solution of silica which furnished the substance 

 of the siliceous concretions which we call flints. 



We conclude that the preservation of traces of radiolaria in the 

 nodules of the Melbourn Rock is due to some specially favourable 

 conditions, but at present we have seen nothing to indicate what 

 these conditions were. The fact, however, agrees with Dr. Rust's 

 experience of their mode of occurrence in the Jurassic and Cretaceous- 

 rocks of Germany. 



In the nodules that we have examined the proportion of radio- 

 larians to the mass of the rock is usually small, but specimens varied 

 considerably. Where these are most numerous the rock may be 

 compared with the calcareo-siliceous deposits of Barbados, which 



1 Quart. Journ. Geol. Soe. vol. xlii. (1886) p. 230, and 'The Geology of 

 London,' Mem. Geol. Surv. 1889, vol. i. p. 521. 



