MUSEUM OF COMPARATIVE ZOOLOGY. 45 



mineral particles seldom exceeded 0.1 mm. in diameter, but near shore 

 they were very much larger, and fragments of rucks and pebbles were 

 frequently dredged. Altered fragments of plagioclase, basalts, and dia- 

 base were rather frequent. 



The percentage of carbonate of lime in these deposits was usually 

 very high, being frequently 70 or 80 per cent, and in the case of a chalk 

 rock 90.24 per cent. Where the shores were composed of volcanic or 

 other rocks not calcareous, the debris of these made up the larger part 

 of the deposits, which might be called volcanic muds. But the majority 

 of the deposits should be termed Pteropod or Globigerina oozes, owing 

 to the large number of these organisms present in them. It should be 

 remembered, however, that both in the size of the mineral particles and 

 in the nature of a large number of the calcareous particles, these de- 

 posits differ considerably from similar deposits found far away from land 

 in the open ocean and called also Pteropod and Globigerina oozes. 



The siliceous organisms never make up more than four or five per 

 cent of the whole deposit, and consist of Radiolaria, Sponge spicules, and 

 a few Diatoms. 



Fragment of White Chalk. — From 994 fathoms, off Nuevitas, Cuba, 

 there was obtained a fragment of white chalk coated on the surface with 

 streaks of peroxide of manganese. This chalk contained 90.24 per cent 

 of carbonate of lime. The sections showed the rock to be composed of 

 crystalline grains of carbonate of lime, which however were not the 

 result of precipitation. A few sections of Globigerina and Textularia 

 were observed, but no other organisms could be recognized. After dis- 

 solving away a considerable quantity, small fragments of quartz and 

 hornblende, Sponge spicules and Radiolarians were observed in the resi- 

 due. It is impossible to be certain that this rock was formed in the 

 position from which it was dredged, though there are reasons for sup- 

 posing that it was. The ooze which came up from the same place was 

 of a reddish or brownish tinge, and contained an immense number of 

 Pteropods, Heteropods, and pelagic Foraminifera ; the percentage of 

 lime was not so high as in the white chalk rock, and the residue was 

 much darker in color. 



Concretions. — Off the Birbadoes in 221 fathoms (St. 280) a very 

 hard calcareous concretion was obtained, which showed perfectly how 

 the rock was formed by crystallization of cai'bonate of lime around the 

 shells of Foraminifera and other centres. A zone is seen around the 

 shells, composed of fibro-radiate calcite ; the crystals of calcite, coming 

 from the various centres, abut against each other, and frequently leave 



