312 
THE VOYAGE OF H.M.S. CHALLENGER. 
plagioclase, augite, and magnetite — with or without a vitreous basCi Generally they are 
fine grained ; rarely they are seen to have the structure of dolerites. A great number 
are scoriaceous, the vesicles being filled with zeolites, or carpeted by zeolitic zones, as 
in the case of the palagonitic lapilli. They are often associated with volcanic ashes, 
in which the mineralogical elements of basalts predominate. Their alteration is less 
advanced than in the case of the basic glasses, which is undoubtedly due to the facts 
that they contain more crystalline elements, and are more compact. However, if they 
possess a vitreous base, or are porous, the hydrochemical decomposition appears to have 
advanced rapidly from the periphery towards the centre. 
This decomposition attacks not only the base, but also the olivine and augite, 
transforming them into secondary products ; the plagioclases, however, offer a greater 
resistance to this alteration, as may be seen by reference to PL XIX. figs. 2 and 4. 
The two fragments there represented come from Station 276 in the South Pacific; it 
will be at once seen how much they are altered, and how much their structure is masked 
by deposition of secondary products. Fig. 4 shows a felspathic basalt with a decomposed 
vitreous base coloured by manganese ; the lamellae of plagioclase alone seem to have 
remained intact, and they are sharply marked off from the fundamental mass. The 
pyroxene enclosed in the base is entirely decomposed, and often transformed into deles- 
site ; in order to recognise this mineral, it is necessary to clean the preparation with an 
acid, when the form of the sections of pyroxene is revealed, but the optical properties 
are effaced. 
Olivine is the element in which the decomposition is most advanced ; to such a point 
is this the case that it would be even impossible to recognise it except for the cleavages 
and the form of its sections. These sections are shown in PI. XIX. fig. 4, where they 
appear regularly terminated with geometrical inclusions of the vitreous base with a 
fibrous structure, and covered with the red colour of hematised olivine. In the basalt 
represented in PI. XIX. fig. 2, the alteration has attacked the olivine to such an extent 
that the crystal is almost destroyed ; not only is it hematised, but it seems to be broken 
up and everywhere invaded by infiltrations of manganese disposed in large brown or 
opafjue jiatches. No better exami)le could be given of the state of decomposition of the 
volcanic rocks and minerals so frequently met with in deep-sea deposits. 
The vitreous base of some of the basaltic specimens is the first portion of the rock 
to undergo alteration, and it generally j)rescnts the same palagonitic ajopearance as the 
ba.sic glasses, forming irregular patches of variable dimensions, or thin veins Ijetween the 
crystallised minerals. When thus altered this base assumes a zonary structure, presenting 
a l>eautiful red tint, and reacting between crossed nicols ; in fact it behaves in every 
way like j)alagonite. Generally manganese infiltrations follow with the progress of the 
decomposition, and the structure of the base is then entirely masked, as may be observed 
by reference to PI. XIX. fig. 4. When it happens that this fundamental vitreous 
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