On the Occurrence of Radiolaria in Chalk

The rarity of radiolaria in rocks of Upper Cretaceous age is a remarkable fact, and one for which it is not easy to account, seeing that they are not uncommon in Lower Cretaceous rocks on the Continent, and that they have been found in beds of Jurassic age in France, Hanover, and Bavaria. One would moreover have expected the physical conditions of the Upper Cretaceous period to have been more favourable to their existence than those of Lower Cretaceous or Jurassic times. It will be seen, from the descriptions which follow, that one cause of their rarity is undoubtedly the facility with which the siliceous skeletons of Polycystina disappear, when they are embedded in rocks which contain a large proportion of carbonate of lime. Our knowledge of Upper Cretaceous radiolaria comes chiefly from Germany. In 1876 Prof. Zittel described six species, occurring in a Greensand of Senonian age in Westphalia and Brunswick. In 1888 Dr. Rüst described some well-preserved forms which he had detected in phosphatic nodules from the Gault of Saxony, Hanover, and Southern France, and in material filling the body-chambers of ammonites from the same formation. He also described Dictyomitra anglica from flints in the Upper Chalk of England ( op. cit. p. 211 & pl. xxviii. fig. 16) and Dictyospyris chlamydea , same occurrence ( op. cit. p. 204). Besides those in the English flints mentioned by Riist there are only two records of the occurrence of radiolaria in English Cretaceous deposits.Prof. W. J. Sollas, in 1873, describing the coprolites