'38 



OR JOHN MURRAY AND MR ROBERT IRVINE ON THE 



In some cases it was observed that large light-yellow-coloured patches occurred in the 

 red and chocolate clays, arising apparently from the decomposition of some organic body 

 at the discoloured spot, so that even in these abysmal deposits the same deoxidation 

 changes do take place as in the Blue Muds, and there may be a tendency to the pro- 

 duction of manganese nodules, on or near the surface, at the expense of the dioxide of 

 manganese in the deeper layers. We know that many of the deep-sea nodules were 

 formed on the very surface of the deposit, and even projected above it, the upper portion 

 of the nodule giving attachment to Hydroids, Polyzoa, Annelids and other organisms. 



All the manganese nodules from the dredging or trawling at any one of the "Challenger " 

 stations have a strong family likeness, being similar in their general form and size. So 

 marked was this resemblance, that Mr Murray, or his assistant, could at sight with 

 certainty indicate the station from which any given specimen was procured. 



Sometimes they w T ere all flattened and of an oval shape, 

 with a well-marked upper and under surface ; nodules 

 of this shape were evidently formed on the immediate 

 surface of the deposit. At other stations they were pear- 

 shaped, and in these instances the small end was embedded 

 in the mud, while the large end projected above the deposit. 

 In other localities all the nodules were round, at one station 

 being about half an inch, and at another one or two inches in 

 diameter. At one place in the South Pacific the surface of 

 a Red Clay had been covered by a fall of volcanic ashes over 

 an inch in depth, the whole deposit had then become a 

 hardened mass, and subsequently was broken into fragments 

 by some disturbance at the bottom. Large slabs, consisting 

 of the upper layers of this deposit, were brought up in the 

 trawl, and it could be seen that many of the manganese 

 nodules had been situated on the very surface of the Red Clay previous to the fall of ash, 

 while only a few had been completely embedded in the deeper layers of the Red Clay. 

 There is, then, much evidence to show that even in the abysmal regions the manganese 

 nodules are more abundant in the surface, than in the deeper, ]ayers of the deposit. 

 That these nodules do, however, occur at least a foot beneath the surface is proved by 

 some small ones having been found at that depth in the sounding-tube. 



General conclusions with reference to the Manganese Dioxide in Marine Deposits. — 

 From the foregoing considerations it may be inferred that the manganese of the dioxide 

 present in marine deposits was originally combined with the silica in the crystalline rocks 

 of the earth's crust. Through the alteration and decomposition of these crystalline rocks 

 the manganese was converted into bicarbonate. In terrestrial rocks and in the beds of 

 streams this bicarbonate is deposited as dendrites and coatings of manganese dioxide. 

 If any bicarbonate of manganese reaches the sea in the waters of rivers it is almost 

 immediately deposited as manganese dioxide on meeting the alkaline sea-water. The 



Fig. 8. — Large tooth of Oosyrhina 

 {Oxyrhina trigonodon ?), about the 

 largest specimen taken during the 

 cruise (natural size). Station 276 ; 

 2350 fathoms. South Pacific. 



