Hi EXPEDITION OF THE " CHALLENGER " 79 



instrument came up filled with a very fine cream-coloured paste, 

 which scarcely effervesced with acid, and dried into a very light, 

 impalpable, white powder. This, when examined under the 

 microscope, was found to consist almost entirely of the frustules 

 of Diatoms, some of them wonderfully perfect in all the details 

 of their ornament, and many of them broken up. The species 

 of Diatoms entering into this deposit have not yet been worked 

 up, but they appear to be referable chiefly to the genera Fragil- 

 laria, Coscinodiscus, Chcetoceros, Aster omphalus, and Dictyocha, 

 with fragments of the separated rods of a singular silicious 

 organism, with which we were unacquainted, and which made 

 up a large proportion of the finer matter of this deposit. Mixed 

 with the Diatoms there were a few small Grlobigerince, some of the 

 tests and spicules of Radiolarians, and some sand particles ; but 

 these foreign bodies were in too small proportion to affect the 

 formation as consisting practically of Diatoms alone. On the 

 4th of February, in lat. 52, 29' S., long., 71 36' E., a little to 

 the north of the Heard Islands, the tow-net, dragging a few 

 fathoms below the surface, came up nearly filled with a pale yellow 

 gelatinous mass. This was found to consist entirely of Diatoms 

 of the same species as those found at the bottom. By far the 

 most abundant was the little bundle of silicious rods, fastened 

 together loosely at one end, separating from one another at the 

 other end, and the whole bundle loosely twisted into a spindle. 

 The rods are hollow, and contain the characteristic endochrome 

 of the Diatomacece. Like the Globigerina ooze, then, which it 

 succeeds to the southward in a band apparently of no great 

 width, the materials of this silicious deposit are derived entirely 

 from the surface and intermediate depths. It is somewhat 

 singular that Diatoms did not appear to be in such large num- 

 bers on the surface over the Diatom ooze as they were a little 

 further north. This may perhaps be accounted for by our not 

 having struck their belt of depth with the tow-net ; or it is 

 possible that when we found it on the llth of February the bottom 

 deposit was really shifted a little to the south by the warm 

 current, the excessively fine flocculent debris of the Diatoms 

 taking a certain time to sink. The belt of Diatom ooze is 

 .certainly a little further to the southward in long. 83 E., in 



