1918] Davis: The Radiolarian Cherts of the Franciscan Group 323 



chemical composition and consists in the development of coarsely 

 crystalline quartzite from the cryptocrystalline silica of the chert. 

 This recrystallization of silica is accompanied by the formation of 

 minute scales of biotite, produced by the combination of the impuri- 

 ties of the matrix. The purer silica which fills the radiolarian tests 

 contains no material for the formation of biotite and in consequence, 

 the radiolarian remains may be much more conspicuous in slightly 

 altered rocks than in the unaltered parts. 



These radiolarian cherts are believed to represent accumulations 

 of radiolarian ooze. It was thought by Hinde, and his idea was 

 accepted by Dr. Peach, that they were of deep sea origin, deposited 

 at depths greater than 2000 fathoms. Hinde 66 says : 



With the exception of the Eadiolaria very few other organisms can be recog- 

 nized in the sections of this chert rock. There are one or two spicules of Hexacti- 

 nellid sponges, readily distinguishable from the detached Beloid spicules by their 

 larger size and distinctive forms, and I have met with a few minute toothed 

 plates and detached denticles, which bear a certain resemblance to the radulae of 

 naked molluscs; there are further numerous almond-shaped hollow bodies about 

 1 millim. in length with imperforate siliceous walls, of whose nature I am quite 

 ignorant. This Orodovician chert may therefore be fairly considered to be due 

 to the accumulation of the tests of Eadiolaria, and is thus a pure Eadiolarian 

 rock, equally as much as the Tertiary beds of Barbados and the Nicobar islands, 

 which according to Haeckel, correspond to the recent Eadiolarian ooze, ' ' and are 

 certainly of deep sea origin, having probably been deposited at depths greater 

 than 2000 fathoms." If this same conclusion is applicable to this fossil chert, it 

 represents, as Prof. H. A. Nicholson has already pointed out, a true deep-sea 

 deposit in the Palaeozoic period, the existence of which in the geological series 

 has of late been disputed. The beds of fine-grained red and green mudstones 

 associated with this chert likewise favor the same view of its origin in deep 

 water. 



Jukes-Brown 67 has taken exception to this idea of deep sea depo- 

 sition. He regards these cherts as entirely different from the radio- 

 larian earths of Barbados which contain chert nodules. He notes the 

 interbedding of cherts with graptolite shales of terrigenous origin and 

 points out that at Tannylaggie, west of Bladennoch, the radiolarian 

 cherts are interbedded with graywackes and grits. These facts force 

 him to the conclusion that the cherts are not radiolarian oozes, such 

 as are now accumulating in the abyssal depths of the ocean. He cites 

 several recorded occurrences which go to prove that radiolaria are 

 preserved in deposits which are not abyssal but were formed in com- 

 paratively shallow water. He believes the graptolite shales are the 



so Ann. Mag. Nat. Hist., (6), vol. 6, p. 40, 1890. 



67 Jukes-Brown, A. J., The Building of the British Isles, pp. 12, 72, Lon- 

 don, 1911. 



