306 
THE VOYAGE OF H.M.S. CHALLENGER. 
are marked by infiltrations of manganese bringing out very characteristic undulations, 
which may be aptly compared to a transverse section of mahogany. 
^^^len the vitreous matter is areolar, it is evident that the alteration of the glass into 
palagonite must progress more rapidly than in the case of a more compact glass, and, as 
might therefore be expected, the primitive vitreous material has almost completely disap- 
peared, and the fractures due to the perlitic structure are so pronounced that the entire 
fragment must have fallen to pieces were it not held together by the enveloping man- 
ganesCi PL XVIII. fig, 4 shows the decomposition that takes place in some specimens 
of these basic glasses, and the same thing is well represented on the left hand side of 
figure 3 on the same plate. In these preparations, which must be made rather thick to 
prevent the nucleus falling to pieces, the palagonite is divided into little fragments 
following: the sinuous lines of fracture. We often find in the sediments numerous 
splinters of palagonite with rounded or curved surfaces, along with zeolitic globules 
formed of the circular microscopic geodes, which probably once filled the circular pores of 
an original areolar fragment of rock, as shown in PL XVIII. fig. 4 ; PL XVII, fig. 2 also 
ofiers an excellent example of the same palagonitisation of the mass. PL XVIII. fig. 1 
represents a still further advanced state of alteration ; in this, throughout the funda- 
mental mass of manganese, triangular, elongated, or irregular splinters of altered 
glass are observed. The sections of these are yellowish, often zonary near the borders ; 
to a primary zone another succeeds of a deeper tint produced by the interposition 
of a black manganese pigment ; then, near the centre, the colour becomes lighter, and 
often there exists an internal concretionary zone surrounding an empty space. In 
])olarised light these sections of volcanic glass still show traces of a black cross and of 
chromatic polarisation ; they are almost entirely transformed into zeolitic matter. Where 
the centre has disappeared, it is possible that in polishing the thin sections the hard 
centre wa.s eliminated from the softer surrounding mass, but we are not disposed to 
admit this supposition as probable from our examination of specimens in reflected light 
previous to preparation. The view is rather held that the decomposition has advanced 
so far that the centre was reduced to an almost earthy mass, and has thus been elimi- 
nated, the border composed principally of zeolitic matter alone remaining. 
In a .sub.scquent chapter we shall return to the role played by zeolitic substances in 
these b.'usic glasses, but it may be pointed out here that in the pores of the less compact frag- 
ments the colourless zeolites with radiate fibres give the black cross of spherulitic aggregates. 
Tliese zeolites are arranged in the interior of the vacuoles upon a layer of carbonate of 
iron, and it is sometimes po.ssible to observe crystals like phillipsite, brevicite, or even 
chabasite, but in the great number of cases these zeolitic aggregates are finely zonary, 
or at the same time zonary and fibro-radiate, as may be observed in PL XVIII. fig, 4. 
The part of the ])orcs not filled with zeolites is generally invaded by manganese and the 
mud of the deposit, filled with microscojiic concretions of peroxide of manganese. 
