1918] Davis: The Radiolarian Cherts of the Franciscan Group 305 



rence has shown that they may occur in tuffs, marls, clays, 37 chalk, 3s 

 limestones, and in eoprolites. Cayeux 39 has also described radiolaria 

 from fine grained glauconitic rocks of the sandstone type (gaize) of 

 Jurassic and Cretaceous age. These consist of grains of quartz and 

 glauconite with numerous skeletons of radiolaria and diatoms, to- 

 gether with sponge spicules, the whole cemented by opal, chalcedony, 

 and argillaceous material. 



In spite of the wide spread distribution of radiolaria there are 

 only certain types of sediments in which they are numerous: (1) 

 radiolarian earths; (2) very fine grained mudstones ; (3) some dia- 

 tomaceous earths; (4) bedded radiolarian cherts. 



The Radiolarian Earths of Barbados 



A typical example of a fossil radiolarian ooze is found in the 

 radiolarian marls of Barbados and the adjacent islands. 40 



The island of Barbados contains three formations. The basal 

 beds consist of sandstones, grits and shales known as the Scotland 

 Beds. These beds are folded and unconformably above them is the 

 Oceanic series of Miocene age, above which are raised coral rocks 

 and reefs. 



The members of the Oceanic series blend gradually into one an- 

 other, but can be separated into four members. 



1. At the base are calcareous deposits, consisting of white lime- 

 stones, some of which are soft and chalky. These have a thickness 

 of about forty feet. The basal member contains from sixty to eighty 

 per cent of calcium carbonate, largely in the form of shells of fora- 

 minifera. 



Toward the top of the first member the number of siliceous organ- 

 isms begins to increase, until the second member of the series is 

 reached. 



2. The second member, which has a thickness of 130 feet, is com- 

 posed of the remains of siliceous organisms, mostly radiolaria, with 



37 See also Shrubsole, W. H., Notes on the Radiolaria of the London Clay, 

 Quar. Jour. Geol. Soe., vol. 45, pp. 121-124, 1889. 



38 See also Holmes, W. M., On Radiolaria from the Upper Chalk, Quar. Jour. 

 Geol. Soc., vol. 56, p. 694, 1900. 



39 Cayeux, L., Contribution a 1 'etude micrographique des terrains sedimen- 

 taires, Lille, 1897. 



40 Jukes-Brown and Harrison, The Geology of Barbados, Quar. Jour. Geol. 

 Soc, vol. 47, p. 197, 1891; Quar. Jour. Geol. Soe., vol. 48, p. 170, 1892. 



Gregory, J. W., Contributions to the Paleontology and Physical Geology of 

 the West Indies, Quar. Jour. Geol. Soc, vol. 51, p. 255, 1895. 



