300 
THE VOYAGE OF H.M.S. CHALLENGEK. 
between 2 ’8 and 2 '9 ; the greyish green powder is very magnetic, and always gives 
the reaction of manganese. There is a marked contrast in the hardness between the 
vitreous centre, which has a hardness of about 5, and the altered palagonitic envelope, 
which when dried may have a hardness of about 4 ; but when freshly taken from the 
sea this resinoid secondary substance can be cut with a knife like new cheese. The 
powder of the vitreous glass is attackable by acids, with separation of gelatinous silica, 
leaving a residue formed principally of minerals which had been enclosed in the vitreous 
mass. These minerals, however, are rarely visible to the naked eye, and consist of 
minute yellowish grains of olivine, augite, and some small lamellae of plagioclase. 
When seen by the naked eye, the central vitreous mass is almost always bordered 
or penetrated by the yellowish brown, resinoid, slightly transparent, palagonite. This 
secondary substance has not such a brilliant aspect as the glassy interior, sometimes even 
presenting an earthy aspect ; it forms zones around the vitreous nucleus, and each zone 
is distinguished by a different colour, marking the progressive transformations of 
the basic ghiss. The unaltered centre is usually more or less irregular, but presents 
a form in relation with that of the whole fragment. It is evident that the decomposi- 
tion has taken place from the periphery towards the centre, and that each stage is 
marked by the diflferent coloured zones, which may be black-brown, red, red-brown, pale 
yellow or gi'een, yellowish white, or almost colourless, the uncoloured portions having a 
soapy aspect. The external zones are in general paler and more delicate than those 
towards the interior ; they are so thin in some cases that they can only be recognised 
under the microscope. The specimens collected at Station 302, 1450 fathoms, in 
the South Pacific, present this zonary character due to the decomposition of the basic 
glass in the greatest perfection, so that they resemble the finely-zoned structure of some 
agates. In vesicular specimens the progress of this decomposition is not so well shown 
as in the compact specimens ; generally they are entirely transformed into palagonite, 
and no trace of the original glass has been left. 
It has been already stated that these fragments frequently form the nuclei of 
manganese nodules, and it is interesting to observe that they are very rarely found 
without a more or less thick coating of manganese, whilst lapilli of felspathic basalt, of 
augitic or hornblendic andesite, or fragments of ancient rocks like gneiss and granites, 
are often found in the same deposits without any, or but a very slight, coating of 
manganese. This might be interpreted by supposing that these basic fragments had 
lain for a much longer time u])on the bottom of the sea than the other fragments, and 
that the manganese had thus had time to deposit round them in much thicker layers, or it 
may be held that they are more rapidly altered, and yield in the process the concretionary 
manganese which covers them. It may even be, we think, that traces of these highly 
alterable basic glas.ses have been preserved in consequence of the manganese coatings 
having held together in position the fragments which, were they free, would crumble and 
