clxiv THE VOYAGE OF H.M.S. CHALLENGER. 



CHAPTER X.— GEOLOGICAL DISTRIBUTION. 



(§§ 241-250.) 



241. Historical Distribution. — Radiolaiia are found fossil in all the more important 

 groups of tlie sedimentary rocks of the earth's crust. Whilst a few years ago their well- 

 preserved siliceous skeletons were only known in considerable quantity from Cainozoic 

 marls (§ 242), very many Spumellaeia and Nassellaria have recently been found in 

 Mesozoic and a few in Palaeozoic strata. By the aid of improved modern methods of 

 investigation (especially by the preparation of thin sections of very hard rocks) it has 

 been shown that many hard siliceous minerals, especially cryptocrystalline quartz, contain 

 numerous well-preserved Eadiolaria, and sometimes are mainly composed of closely com- 

 pacted masses of such siliceous shells ; of this kind are many quartzites of the Jura (§ 243). 

 These Jurassic quartzes (Switzerland), as well as the Tertiary marls (Barbados) and clays 

 (Nicobar Islands), are to be regarded as " fossil Radiolarian ooze " (§ 237). Dense masses 

 of compressed Spumellaria and Nassellaria form the principal part of these rocks. 

 Isolated or in smaller quantities, fossil Polycystina, belonging to different families of 

 Spumellaria and Nassellaria, also occur in other rocks, and even in some of Palaeozoic 

 origin. Since specimens have also been recently found both in Silurian and Cambrian 

 strata, it may be stated that as regards their historical distribution, Eadiolaria occur in 

 all fossiliferous sedimentary deposits, from the oldest to those of the present time. 



242. Cainozoic Radiolaria. — The great majority of fossil Radiolaria which have 

 hitherto been described, belong to the Cainozoic or Tertiary period, and in fact, to its 

 middle portion, the Miocene period. At this period the richest and most important of 

 all the Radiolarian formations were deposited, such as the pure " Polycystine marl " of 

 Barbados (see note A), also that of Grotte in Sicily (see note B), and the clay of the 

 Nicobar Islands (see note C). Besides the above-mentioned deposits, which may be 

 designated " pure " fossil Radiolarian ooze, many deposits containing these organisms 

 have recently been discovered in widely separated parts of the earth, partly of the nature 

 of tripoli or marl, partly resembling clay. Among these may be mentioned in the first 

 place many coasts and islands of the Mediterranean, both on the south coast of Europe 

 (SicUy, Calabria, Greece), and the north coast of Africa (from Oran to Tripoli). The 

 extensive layers of tripoli which are found in these Mediterranean Tertiary mountains 

 Ijelong to the upper Miocene (Tortona stage), and consist partly of marl rich in calcareous 

 matter, and resembling chalk, partly passing over into plastic clay or "Kieselguhr" 

 (§ 246). The quantity of Radiolaria contained varies, and is more conspicuous the fewer 

 the calcareous shells of Foraminifera present. Similar Tertiary Polycystine formations 

 occur in some parts of America (see note D) ; probably they have a very wde distribution. 

 In their general morphological characters, the Tertiary Spommellaria and Nassellaria 



