THE VOYAGE OF H.M.S. CHALLENGER. 
a04 
In the sections of these basic glasses that have been examined, augite among the 
basjiltic elements plays the least important role. If this mineral be present its form is 
not sharply defined ; its sections are greenish, sometimes violet coloured. Its cleavages 
are not distinct, and the crystals are small, containing inclusions of the vitreous mass. 
Magnetite is generally absent. 
The various minerals enumerated above are not found together in all the preparations, 
but it cannot be denied that we find in them the whole series of transitions from these 
basic glasses to felspathic magma basalts. However, in these vitreous fragments the 
substance which serves as a base is incomparably better developed than in those basalts, 
and the crystals disseminated in the glass are A'^ery small in comparison with those 
observed in typical basaltic rocks ; besides, the crystallites are as numerous in the basic 
glasses as in the basaltic rocks. It might be said that when the basic vitreous rock is 
porous, the crystalline elements are better developed, and that the transition to the 
basalts or limburgites takes place rather through the types of areolar basic glass than 
through the types of compact structure. 
The progress of the decomposition of the basic glasses into palagonite can be distinctly 
followed in thin sections under the microscope. The unaltered part, as already stated, is 
in transmitted light characterised by a great homogeneity of structure ; no trace of 
perlitic scaling can be observed ; the colour is clear, brownish or greyish in different 
specimens. The palagonite, on the contrary, is but little homogeneous ; the zonary 
and perlitic structures are sharply accentuated ; the colour is a beautiful red, sometimes 
remarkably brilliant, and may pass into reddish yellow, yellow, dirty brown, green, 
and finally to a milky white, in which last case transparency gives place to semi- 
opacity. This resinoid substance is still further distinguished from the unaltered 
vitreous matter by optical properties : while the latter is always isotroj)ic, the 
former presents between crossed nicols the phenomena of chromatic polarisation ; the 
})alagonite is coloured brilliant yellow mixed Avith red ; the tints have a wavy disposi- 
tion, and the layers are seen to be formed by an aggregate of crystalline fibres dis- 
posed more or le.ss perpendicularly to the surface of the lapilli in process of decomposi- 
tion. This fibrous arrangement is also evident by the well-marked traces of the black 
cross of spherulitic aggregates observed in the palagonitic zones. This secondary substance 
is seen to line or to infiltrate all the fi.ssures of the vitreous fragment, more or less pro- 
foundly according to the degree of alteration ; it surrounds all the external borders, and 
is generally zonarj', but the zones are capricious, sometimes imitating those of concre- 
tionar}’ minerals, like the zinc blends or certain agates. One of the best examples in this 
respect is the specimen figured on FI. XIX. fig. 3, as seen by reflected light. The 
fragnient is enveloped in black-brown opaque manganese, seen on the right, left, and 
bottom of the figure. Directly in the centre is a fragment of black-grey, homogeneous, 
unaltered basic glass ; around this nucleus arc found the various zones of decomposition, 
