THE DEPTH AND MARINE DEPOSITS OF THE PACIFIC OCEAN. 37 



The tuff does not contain any organisms, either calcareous or siliceous ; 

 in all the specimens manganese oxide grains are irregularly distributed 

 throughout the mass, and some specimens contain large patches of the 

 oxides. 



From this same station the trawl also brought up pieces of pumice, one 

 to two inches in diameter ; a flat piece of black volcanic glass, very much 

 decomposed, but still recognizable, coated with a continuous layer of the 

 oxides of manganese and iron, the decomposition proceeding apparently 

 along cracks, so that with a little effort the fragment can be broken be- 

 tween the fingers; rounded pieces of decomposed rock, of high specific 

 gravity, are also coated with the oxides, the rock itself having turned yel- 

 lowish-brown through decomposition. 



Hardened Deposit. 



From Station 4693 (lat. 26° 30.1' S. ; long. 105° 45.2' W. ; depth, 1142 

 fathoms ; 14th December, 1904), angular blocks of consolidated Globigerina 

 Ooze, more or less covered and impregnated with manganese and iron oxides, 

 were brought up. These blocks (see PI. 3, fig. 6) vary in size from a frac- 

 tion of an inch to four inches in diameter. In general appearance they look 

 very much like slag, the outer surface being rough, vesicular, and mammil- 

 lated. Of the specimens, the poorest in manganese oxide is a nearly pure 

 white agglomerate of Foraminifera with irregularly distributed patches of 

 the oxides inside the fragment, whilst the outside has the slaggy surface 

 alluded to. Its specific gravity is low. Another sample, also chiefly com- 

 posed of Foraminifera, is more coherent, and has a higher specific gravity. 

 In more advanced stages the proportion of the oxides increases and is more 

 equally distributed throughout the mass, giving it a rusty brown tinge, while 

 the density increases. Other samples show a similar further increase in the 

 proportion of the oxides of iron and manganese, but in none is a complete 

 replacement of the carbonate of calcium by the oxides to be seen ; even the 

 parts purest in manganese still give a marked effervescence with acid. 



As regards structure, there is no tendency towards an arrangement in 

 concentric layers around a common nucleus. The very few mineral par- 

 ticles present in these concretions are very small, and belong to the chlorite 

 group and other products of the decomposition of ferruginous minerals. 



Though the above concretions are not actual manganese nodules, they 

 mark at least one step towards the formation of nodules, and this under 



