REPORT ON THE DEEP-SEA DEPOSITS. 
305 
taking the form of the elongated internal nucleus. The internal zones are brownish, 
marked by deeper coloured lines of separation, due to infiltration of manganese ; then the 
palagonite affects a whiter tint, followed by other well-marked zones of a greenish colour. 
Finally the external zones become more vague, take on a soapy aspect, and become 
charged with matters containing manganese, but through this brownish mass the ill- 
defined external limits of the original vitreous fragment can be traced. These various 
zones are especially well seen under the microscope, but, as we have already pointed out, 
they can be observed on a macroscopic examination. 
When these vitreous fragments in process of decomposition are studied in transmitted 
light, the stages of the alteration are still more evident. The altered substances are 
not only seen to penetrate and to modify the vitreous mass, but the decomposition only 
attacks the easily alterable glass, and does not generally affect the embedded crystals. 
This is well shown in PI. XVI. figs. 3 and 4, where the palagonite is seen with its 
characteristic tint to advance into the glass while leaving the crystals of olivine intact 
and in place. PI. XVII. fig. 3 gives a similar example, but here little lamellae of 
plagioclase remain as witnesses of the primitive nature of the substance in which they 
are enveloped. This figure, which rejDresents the nucleus of a manganese nodule, from 
Station 302, 1450 fathoms. South Pacific, shows the zonary aspect of the brown pala- 
gonite formed at the expense of a brownish homogeneous volcanic glass, shown in 
the lower two-thirds of the figure, this glass being especially rich in small crystals of 
plagioclase. In the interior it has not undergone alteration, but the fractures are seen 
to be infiltrated with manganese. In the palagonitic zones the same felspathic lamellae 
are observed as in the vitreous portion ; at the lower part of the figure a crystal of 
plagioclase is seen, one-half of which is in the altered and the other half in the 
unaltered portion of the rock. In this figure the striking contrast between the 
glass without structure and the concretionary texture of the decomposed portion is well 
represented. 
In certain cases the alteration has reached a much more advanced stage, as repre- 
sented, for instance, in PI. XVII. fig. 1. The vitreous matter, which formerly occupied 
the whole of the space, has been so far decomposed that there now remain only two 
isolated vitreous fragments, characterised by their greyish tint ; all the other portions 
of the specimen have been transformed into yellowish palagonite. In the same figure 
manganese is seen to be infiltrated into the fractures, and presents, especially at the upper 
part of the figure, a dendritic arrangement. It also often happens that the secondary 
substance has penetrated to the very centre of the vitreous fragment, as represented in 
PI. XVII. figs. 2 and 4 ; in these cases nothing remains of the basic glass, and it is 
observed that the perlitic structure is sometimes developed in a remarkable manner. No 
better example of this could be produced than that shown in PI. XVII. fig. 4, where each 
of the sinuous and more or less curvilinear fissures preserves its parallelism ; the fissures 
(deep-sea DEPOSITS CHALL. EXP. 1891.) 39 
