366 
THE VOYAGE OF H.M.S. CHALLENGER. 
to be much more limited, and have chiefly been found in 'more or less close proximity 
to volcanic islands. There are, however, indications of an approach to the condition 
of the chocolate-coloured clays of the Pacific in the deep water about 20° N. and 50° 
W. in the Atlantic. 
If now we attempt to summarise the foregoing descriptions, it may be said that these 
concretionary masses of manganese assume a great variety of forms in modern deep-sea 
deposits. Sometimes the oxides cover consolidated masses of tufa, fragments of rocks, 
portions of the deposit, branches of Coral, and remains of other calcareous organisms. 
At other times fragments broken off from what must have been huge concretionary 
masses were obtained in the trawl and dredge ; this was especially the case in the 
shallower waters near to, or on the slopes of, volcanic islands. The prevailing con- 
cretions, however, were more or less rounded nodular masses, from 1 to 10 or 15 cm. 
in diameter, and hence resembling all concretionary bodies formed in a plastic or liquid 
medium. As will have been noticed in the descriptions of the nodules at each station, 
they may present great variations in the dredging, but as a rule the nodules at any one 
station have a family resemblance, and differ, in size, form, and internal structure, from 
those at another station ; so much so that now, after a detailed study of the collections, 
it is usually possible for us and our assistants to state at sight from which Challenger 
station any particular nodule had been procured. 
In a great many cases the external form depends on the shape of the nucleus, but 
there are a number of minor peculiarities which afibrd indications of the station to which 
the samples belong. The great irregularity of some nodules depends on the fact that the 
nucleus is not simple. The concretionary depositions have commenced around several 
adjacent foreign bodies ; by the increase of the successive layers around the several 
nuclei, the little nodules have come in contact with each other, have become united, and 
finally have developed into a large single nodule with several protuberances, assuming 
the aspect of double, triple, or quadruple nodules. In the case of very large nodules 
the multiple origin becomes for the most part obliterated at the external surface. The 
surfaces of the nodules are covered by all sorts of asperities and mammillae, these being 
generally more pronounced on the under surface which had been immersed in the deposit. 
Sometimes there is no apparent nucleus, and nodules of this character usually 
contain more manganese, being dark brown or black to the very centre, and take on a 
briglit metallic lustre when polished with chamois leather or a piece of cloth. Almost 
always, however, there are one or more recognisable nuclei, around which the manganese 
and iron have concreted. It may be remarked that there is no chemical relation between 
the manganese and the nucleus to initiate the depositions, for the nuclei may be in- 
dilfercntly carbonates, phosphates, silicates, or .silica. Any solid body sufiices for the 
support of the original and subsequent concretionary deposits. Basic and acid silicates 
