REPORT ON THE DEEP-SEA DEPOSITS. 
351 
alternating zones give to the nodule a well-marked concretionary and shelly structure. 
What would appear to have been the original nucleus of pumice has likewise assumed a 
concentric arrangement. Two processes probably took place : the one a deposition of 
manganese layers on the outside in successive bands, and a simultaneous alteration of the 
nucleus, which likewise produced a concentric arrangement. The external zones of the 
nodule are not so dark coloured as those towards the centre, and the fine, black, undu- 
lating, concretionary lines are less numerous, but the whole face of a nodule like this one 
takes on a beautiful, black, metallic lustre when polished with the hand or with a piece 
of cloth. The demanganesed portion represented in the right half of the figure is of a 
whitish colour, and easily pulverised into an impalpable powder. The dark shaded 
portions in the left-hand figure represent the zones in which the manganese is most 
abundant, and these appear on the right-hand figure as empty spaces on the surface 
treated with concentrated hydrochloric acid. PI. II. fig. 4 represents another of these 
nodules in section (natural size). In this case there are several nuclei, all probably highly 
altered fragments of pumice, surrounded by concentric layers of manganese, and the whole 
cemented into one large nodule. This figure shows again the concretionary and shelly struc- 
ture, the nodule frequently breaking up into successive scales like an onion. PI. II. fig. 2 
represents still another nodule from this station, the central parts of which are occupied 
by a siliceous Sponge {Farrea). Although in some places portions of the skeleton appear 
to have been removed in solution, still on the whole it is very well preserved ; it is every- 
where surrounded by the manganese depositions, and the manganese has even penetrated 
into the canals of the spicules. In the stalk-like portion at the lower part of the figure 
there were numerous Sponge spicules. Fig. 2a represents a magnified portion of the 
Sponge skeleton, which retains its vitreous and brilliant appearance. Among these large 
rounded nodules there were several tube-like bodies composed of manganese, 4 to 5 cm. 
in length and 1 cm. in diameter, with a hollow centre in which were many spicules of 
sDiceous Sponges. PI. I. figs. 5 and 6 represent (natural size) the appearance of a good 
many nodules from this station. The nuclei consist of pumice, much decomposed, espe- 
cially on the surface in contact with the enveloping layers of manganese, which vary from 
a millimetre to several centimetres in thickness. In fig. 5 the pumice at the centre of 
the fragment is white, and retains nearly all its characters, but close to the manganese 
layers decomposition is much more advanced and it assumes a brown colour. When 
examined under the microscope with reflected light, the pores of the pumice are seen to 
be filled with an earthy matter, which forms casts of the little vesicles. They do not 
disaggregate under the action of hydrochloric acid, but simply undergo discoloration ; 
sometimes these granules give a black cross with polarised light, in fact they have a 
great resemblance to certain casts of Foraminifera observed at Station 176, 1450 fathoms, 
South Pacific. In fig. 6 the pumice has undergone greater alteration than in the specimen 
represented in fig. 5, and is surrounded with a thicker deposit of manganese. 
