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



Grotte in Sicily (L. N. 35) that its Eadiolarian fauna is much richer than Ehrenberg supposed. 

 The same is the case in the Tripoli of Caltanisetta, and also in the Baden marl of the Vienna basin. 

 The richest deposit appears to be the pure Kieselguhr-like Tripoli from Oran ; a small specimen, 

 which was recently sent to me by Professor Steinmann of Freiburg, i. B., contained many hitherto 

 undescribed species, and was at least as rich as the purest Barbados marl. 



247. Radiolarian Clays. Among the Radiolarian or Polycystine clays we include 

 the firm, often plastic, formations, which contain a larger proportion of Radiolaria than 

 of other organic remains. The first of these to be mentioned is the Cainozoic formation 

 of the Nicobar Islands in Further India, which rises to a height of 2000 feet above the 

 level of the sea, and consists for the most part of coloured masses of clay of varying 

 constitution ; on Car Nicobar these are mostly grey or reddish, on the Island of Camorta 

 they are partly strongly ferruginous and red and yellow (e.g. at Frederickshaven), partly 

 white and light, like meerschaum (e.g. at Mongkata). The latter varieties appear to pass 

 over into pure loose Polycystine marl like that of Barbados, the former into calcareous 

 sandstone. Although the Polycystine clays of the Nicobar Islands are as yet only very 

 incompletely known, it may be concluded with great probability that they are true 

 deep-sea formations and nearly allied to those recent forms of red clay, which by their 

 abundance in Radiolaria most nearly approach the Radiolarian ooze, such for example 

 as the red clay of the North Pacific between Japan and the Sandwich Islands (Stations 

 241 to 245, compare 229 and 239). With this view agrees also the greater or less 

 quantity of pumice dust and other volcanic products. Probably Radiolarian clays like 

 those of the Nicobar Islands occur also in other Tertiary rocks ; part of the Barbados 

 marl passes by gradually increasing content of clay into such ; and in this case also the 

 amount of included pumice is often considerable. Many mixed Radiolarian marls of the 

 Mediterranean (e.g., of Greece and Oran) also appear to pass over at certain points into 

 Radiolarian clay. 



The Eadiolarian clays of the Nicobar Islands are unfortunately very incompletely known both 

 as regards their geological nature and their palaaontological composition. The communications of 

 Eink (Die Nikobaren-Inseln, eine geographische Skizze, Kopenhagen, 1847) and of Ehrenberg 

 (L. N. 6, p. 160 and L. N. 25, pp. 116 to 120) leave many important questions unanswered. The 

 latter has only figured twenty-three species in his Mikrogeologie (L. N. 6, Taf. xxxvi.). In his 

 tabular list of names (L. N. 25, p. 120) he only incompletely records thirty-nine species, although 

 in 1850, immediately after the first examination of the Nicobar clay, he had distinguished " more 

 than a hundred species, partly new, partly identical with those of Barbados " (L. N. 16, p. 8). I 

 have unfortunately been unable in spite of many efforts, to obtain for investigation a specimen of 

 Nicobar clay. The only microscopical preparation (from Ehrenberg's collection), which I was able 

 to examine, contained several hitherto undescribed species. A thorough systematic examination of 

 these important Eadiolarian clays is a pressing necessity, especially as they seem to be markedly 

 different from those of the Mediterranean (from ^Egina, Zante, &c.). 



