REPORT ON THE RADIOLARIA. clxix 



exhibit very primitive structural relations. The occurrence of fossil Polyeystina in the Carboniferous 

 formation of England has been incidentally mentioned by W. J. Sollas : " In the carboniferous 

 beds of North Wales pseudomorphs of Eadiolaria in calcite occur, along with minute quartz crystals " 

 (Ann. and Mag. Nat. Hist., 1880, ser. 5, vol. vi. p. 439) ; and in the siliceous slate-beds of Saxony 

 Rothpletz has shown the existence of a few Sphseroidea (Zeitschr. d. Deutsch. Geol. 

 Gesellsch., 1800, p. 447). 



245. Abundance of Radiolaria in the Various Rocks. The relative quantity of 

 well-preserved or at all events recognisable Radiolaria in the different rocks is very 

 variable. In this respect three different degrees may be distinguished, which may be 

 called shortly " pure, mixed, and poor " Radiolarian formations. The pure Radiolarian 

 rocks consist for the greater part (usually much more than half, sometimes even more 

 than three-quarters) of closely compacted often calcined masses of siliceous Polycystine 

 shells. To this category belong the pure Miocene Polycystine marls of Barbados ( 246), 

 the Tertiary Polycystine clay of the Nicobar Islands ( 247), and the Polycystine quartz 

 of the Jura ( 248). All these pure Radiolarian rocks may be regarded as fossil Radio- 

 larian ooze ( 237), and are certainly of deep-sea origin, having probably been deposited 

 at depths greater than 2000 fathoms. Their palseontological character also is in favour 

 of this view, for the abyssal Osculosa ( 235) are more abundant and richer in species 

 than the pelagic Porulosa ( 233). The elevation of this deep-sea layer above the surface 

 of the sea appears to have taken place but seldom ; it has only been observed on a large 

 scale at Barbados and in the Nicobar Islands. The mixed Radiolarian rocks are much 

 more common ; they were probably deposited at much less depths, or perhaps are not 

 true deep-sea formations at all. The siliceous shells of Polyeystina always constitute less 

 than half (sometimes less than one-tenth) of their mass, and are less prominent than 

 other siliceous remains (Diatoms), or calcareous remains (Foraminifera), or in some cases 

 than the mineral constituents (pumice, &c.). To this group belong many of the above- 

 mentioned Tertiary marls and clays (especially the Mediterranean Tripoli), also many 

 flints, cherts, and other quartzites from Mesozoic strata (especially from the Jura), and 

 probably also some palaeozoic quartzites. The marine ooze from which they have 

 originated may have been deposited at very various, even at slight, depths of the ocean. 

 Formations poor in Radiolaria, which contain only a few species of SPUMELLARIA and 

 NASSELLAEIA mingled with other fossil remains and mineral particles, occur in all forma- 

 tions and are probably very widely distributed. Further careful examination of thin 

 sections (especially of coprolites) will yield here a rich harvest of new forms. Both the 

 mixed and the pure Radiolarian formations may be divided according to their petrographic 

 characters into three groups, which, however, are connected by intermediate varieties 

 (1) soft, chalky marl ( 246), (2) plastic clay ( 247), and (3) hard, flinty quartz 

 ( 248). 



(ZOOL. CHALL. EXP. PART XL. 1887.) Rr y 



