clxx THE VOYAGE OF II.M.S. CHALLENGEK. 



246. Radiolariaii Marl. — Those soft, friable rocks, which contain a large quantity of 

 calcareous matter, but consist for the most part of the shells of Spumellapja and 

 Nassellaria, are called Radiolarian or Polycystine marl, often more correctly Polycystinc 

 tripoli ; the best known example of them is the chalky marl of Barbados in the Antilles 

 (§ 242). The Tertiary mountain system of this island, which in Mount Hillaby rises to 

 a height of 1147 feet and includes about 15,800 acres, consists almost exclusively of 

 these remarkable masses of rock. Most of it appears as a soft, earthy, often chalky marl, 

 with a considerable but Vcnriable amount of calcareous matter. Those specimens, the 

 creater half of which is composed of well-preserved siliceous shells of Polycystina, and 

 which contain little lime, approach the tripoli and "Kieselguhr." Those specimens, however, 

 which contain the largest amount of calcareous matter resemble common writing chalk in 

 consistency, and consist for the most part of shells of Foraminifera and their fragments ; 

 of these there are only few species but large numbers of individuals, generallj^ in small 

 fragments with a fine calcareous powder between them. They may be regarded as fossil 

 Globigerina ooze (§ 238). In a third group of specimens from Bai'bados the quantity of 

 fragments of pumice and other volcanic matters predominates ; the amount of clay is also 

 very considerable ; these deposits pass over partly into actual clay partlj^ into volcanic 

 tuff. A fourth group exhibits relations to a coarser, often ferruginous material, and 

 although the shells of Polycystina are less abundant in it, still it may lie shown to be 

 composed largely of fragments and metamorphosed remains of them. The colour of this 

 deposit, which in some places passes over into sandstone, in others into clay, is usually 

 rather dark, grey, brown, sometimes red and occasionally black (bituminous). The 

 Radiolarian marls of the first two groups, which sometimes approach the white chalk, 

 sometimes the Kieselguhr, are grey, or even pure white (see note A). The same con- 

 stitution is exhibited by the yellowish or white, very light and friable Polycystine marls 

 of Sicily, which in Caltanisetta approach the chalk, and in Grotte the Kieselguhr. In 

 Greece (iEgina, Zante, &c), on the other hand, they pass over into plastic clay, and the 

 same occurs in the Baden marl of the Vienna basin. In North Africa, however, on the 

 Mediterranean shores of which the Radiolarian marl seems to be very widely distributed 

 (from Tripoli to Oran), it sometimes becomes changed into actual firm j)olishing slate, 

 sometimes into pulverulent Kieselguhr or tripoli (Terra tripolitana, see note B). Most of 

 these Radiolarian marls appear to date from the middle Tertiary (Miocene) period, and 

 to be deep-sea formations. 



A. The Polycystine marl of Barbados appears at different parts of the island to present greater 

 variations in its petrographical and zoographical composition than would appear from Ehrenberg's 

 description (1875, L. N. 25, pp. 106-116). Through the kindness of one of my former students. 

 Dr. Dorner, to whom I take this opportunity of expressing my thanks for the favour, I received a 

 large number of specimens of Barbados rock, taken from various parts of the island, and they 

 exhibit very great variations in their external appearance, their chemical composition, and the 



