144 THE DEPTH AND MARINE DEPOSITS OF THE PACIFIC OCEAN. 



the shagreen appearance met with elsewhere, but in some cases there are 

 minute mammillae between the large protuberances. The specific gravity 

 is higher in the small nodules than in the large ones, but is never very high. 



A number of nodules were cut, but no nucleus was detected, nor any 

 concentric arrangement of manganese layers ; the manganese-iron oxides are 

 distributed throughout a ground-mass of lohite material, which is certainly the 

 same thing as the ashy substance mentioned above, but harder. The 

 proportion of this substance to that of the oxides varies so much that in 

 some cases it is difficult to say whether one is dealing with a nodule or 

 with one of the lumps referred to above, but in the more perfect nodules — 

 that is, the small ones — the proportion of oxides predominates over that of 

 the palagonite ground-mass. 



To conclude : the very interesting point about these nodules is their 

 mode of formation ; they were not formed by the successive deposition of 

 oxides round a nucleus, but are due to the impregnation of ash cakes by 

 manganese-iron oxides without any increase in volume. Worm-burrows 

 running through the hard nodules demonstrate this clearly, for the worms 

 could not have burrowed through such a hard substance, so that their 

 burrows existed before the deposition of the oxides. 



It is also interesting to note that the white ground-mass of the more 

 perfect nodules is harder and more compact than that of the nodules still 

 poor in manganese-iron oxides. 



Station 13, 5th September, 1899. 

 Lat. 9° 57' N. ; long. 137° 47' W. ; depth, 2690 fathoms. 



At this station a large quantity of nodules was dredged, one of which is 

 figured in Dr. Alexander Agassiz's preliminary report.^ They somewhat re- 

 semble those of Station 4711 of the 1904 cruise. Here again they obey 

 a law to which we have often had occasion to refer, all the nodules being 

 formed under definite but unknown conditions, which brought on a definite 

 type of structure. 



They average 4 to 6 inches in diameter, and are all irregularly spherical 

 or irregularly cubical, and never tend to form slabs. Their most conspicuous 

 feature is the high degree of mammillated structure ; the whole surface is 

 covered with large protuberances, which may be as much as one inch in 

 diameter. Sometimes these protuberances may be compounds, that is, formed 



' Mem. Mus. Comp. ZooL, vol. XXVI., Plate, Manganese Nodules, fig. 1. 



