308 
THE VOYAGE OF H.M.S. CHALLENGER. 
XXIX., but these figures show rather the structure of the nodules themselves than the 
inicrostructure of the mineral. It may be seen from the figures that the manganese is 
disposed in fine concentric layers, marked ofi’ by black opaque or brown lines. The fine 
black uiidulatory zones may be recognised as having a concretionary arrangement even 
with high })owers, as represented in PI. XXIX. fig. 3. The appearance of nodules with 
many centres, and other peculiarities of structure, are well shown in many of the micro- 
scopic sections. In PI. XXVIII. fig. 3 there are several centres of concretion, organic 
and inorganic — fragments of teeth, palagonite, and other volcanic rocks ; the deposition 
has commenced around each of them, and they ultimately became united into a single 
nodule by successive layers of manganese. In PI. XXIX. fig. 2 another concretion is 
represented containing several nuclei ; near the upper part of the figure there is a section 
of a shark’s tooth as one of the principal centres, and immediately below this another 
hirge nucleus consisting of a volcanic lapilli containing green augite and plagioclase. 
PI. XXIX. fig. 4 shows again the zonary arrangement of the manganese around two 
centres composed of altered material and their subsequent envelopment in one nodule by 
the continuous deposition of manganese, which has enclosed at the same time the clayey 
matters with fragments of minerals and organisms. 
The microscopic as well as the macroscopic examination shows a well-marked zonary 
structure, always combined with a dendritic arrangement. In PI. XXVIII. fig. 1 the 
arborescent form of the black manganese is directed parallelly to the radii of the nodule, 
and is intercalated in yellowish brown muddy materials. This is quite the ordinary 
aspect of dendrites of this mineral ; the figure also shows the zonary structure indicated 
by curved bands of a deep brown colour. PI. XXVIII. fig. 2 shows likewise the dendritic 
arrangement, but not so well marked ; the large ovoid body occupying the centre of the 
figure was probably the primary form of the original nucleus, now mostly transformed 
into manganese. In PI. XXVIII. figs. 4 and 5 variations of the same zonary and 
dendritic structure are represented. 
It may be concluded from the study of the microscopic sections that the deposition 
of the manganese has been continuous, for the manganese oxides can be seen to ramify 
across the earthy or clayey layers, and are thus continuous with the manganese in the 
purer and darker zone.s. In all respects the microscopic examination confirms the views 
arrived at from a macroscopic examination as to the structure of the nodules and their 
varied nuclei. 
Chemical Composition . — To what mineral species or ore of manganese are these 
nodules to be referred ? 'I’he numerous analyses given below prove that we have to do 
with a hydrated oxide of manganese, mixed with variable quantities of limonite, clay, 
and (dher earthy and sandy matters. Among the associated substances there are several 
whicli arc in a way jteculiar to the concretionary and reniform manganese ores, for 
in.“tance, copper, cobalt, nickel, &c. The general mass of the substance of the nodules 
