REPOKT ON THE DEEP-SEA DEPOSITS. 
301 
be transformed into argillaceous matter. Some manganese nodules have tlieir nuclei 
almost entirely composed of little angular fragments of basic glass, most of which are 
entirely transformed into palagonite. It is barely necessary to observe that this trans- 
formation would take place much more rapidly and more completely the smaller the 
vitreous splinters, so that it is only in those stations where the specimens are large that 
the traces of the unaltered glass are preserved in consequence of their forming the nuclei 
of the manganese nodules ; the minute splinters distributed throughout the deposit, being 
entirely converted into palagonite, can only be recognised by the aid of the microscope, 
after having followed the transitions which these specimens of different sizes undergo 
in the process of decomposition. These basic vitreous fragments, and the palagonitic 
particles resulting from their decomposition, are so frequently associated with numerous 
nodules of peroxide of manganese in deep-sea deposits, as to at once suggest a relation of 
cause and effect, which is confirmed by the analyses showing the presence of manganese 
in the unaltered vitreous fragments. 
When these fragments of basic glass are examined in thin sections under the 
microscope, the vitreous part is seen to be perfectly transparent, with more or less deep 
colours ranging from grey-brown to brown and yellowish brown ; it is perfectly isotropic 
and with a homogeneous structure, but is occasionally traversed by more or less irregular 
lines of fracture, which indicate vaguely a perlitic structure. These fractures can be 
seen in PI. XVII. fig. 3, and the general aspect of the sections is shown in PL XVI. 
figs. 1-4. 
The minerals observed in the vitreous base are olivine and plagioclase, often separate, 
rarely associated ; augite is relatively rare, and so also is magnetic iron. In addition to 
the species distinctly recognisable, there are frequently very large numbers of crystal- 
lites, whose accumulation in certain sections of the rock masks or renders opaque the 
vitreous matter enclosing them. The mineral most frequently met with in these sections 
is olivine, which is observed in the form of very minute, very regular, and generally 
almost colourless, crystals. Often their proportions are so very small that, in spite of 
the thinness of the preparations, they are still entirely encased in the vitreous matter. 
The faces of these crystals are generally ooP, 2Poo, ooPoo, ooPco. Frequently they have 
exactly the same form as fayalite. In a certain number of cases they occur as skeletons of 
crystals. It is possible to follow in thin slides all the transitions between these embryonic 
forms and the sharply-terminated crystals, the latter almost always containing a rather 
large number of inclusions of a brownish, glass, similar to the surrounding base. 
Sometimes these inclusions are so large that the crystal forms around it merely a 
simple border. On PI. XVI. the various peculiarities presented by olivine in these 
basic glasses are represented in detail. 
After olivine, the mineral most frequently occurring is plagioclase-felspar ; it is some- 
times found in the form of lamellae, similar to those observed in eruptive rocks of the 
