310 
THE VOYAGE OF H.M.S. CHALLENGER. 
fmgmeuts of these palagonitised glasses, on which the chemical action would continue to 
be exercised more easily as the materials became more and more subdivided. We should 
also expect to find, in a more or less isolated condition, the crystals formerly imbedded in 
the glasses, as these ofler more resistance to decomposition than the indeterminate 
silicate which formed their base. This is what is actually observed, for in the free state 
in the deposits there are found minute lamellar crystals of plagioclase, often in the form 
of rhombic tables, sometimes augite, more rarely olivine, magnetite, and fragments of 
the zeolitic bands just referred to. 
It is true that many of these minerals, such as plagioclase, augite, and magnetite, 
might be projected as volcanic cinders, or be derived from floating pumice, and being 
surrounded with a vitreous coating this might subsequently be transformed into pala- 
gonite ; but when these fragments are associated with fibro-radiate and globiform zeolitic 
minerals, it seems fair to conclude that the great majority of them have been derived 
from the disintegration in situ of brecciaform vitreous lapilli. The other part of the 
residue, that is to say, the palagonitic matter itself, must necessarily be reduced by 
transformation into argillaceous matter' more or less charged with iron and manganese. 
The rocks of this type on laud surfaces show transformation on a large scale into red 
argillaceous matter, as, for instance, the argillaceous deposits of Iceland, and the red 
earths of a large number of islands in the Pacific. We shall have to speak again of 
these transformations when describing the chemical deposits at the bottom of the sea, 
l)ut it may now be pointed out that the hydrochemical modifications indicated for basic 
glasses must, in a certain measure, hold good also for the basalts with a vitreous base, 
which are so closely allied to' the basic glasses. 
It is difficult to offer an opinion as to the geological age of the eruptions which gave 
origin to these tufas, for nothing is known with regard to their stratigraphical relations ; 
all we do know is that they are spread out on the superficial layer of the sea-bed at the 
points from which they were dredged. Remembering the profound analogies between 
these tufas and palagonitic tufas, some of which belong to the Tertiary Period, and their 
a.ssociation in the deposits of the Pacific with sharks’ teeth and earbones of Cetaceans, 
some of them similar to those of Tertiary species, it is probable that these tufas go back 
JUS far as the Tertiary Period. There is no certainty, however, on this point, since 
eruptions giving rise to basic glasses analogous to those described still take place in the 
region of the Pacific, where the submarine tufas collected by the Challenger are best 
rejjresented. The smallest palagonitic particles found free in the deposits may possibly 
have come from eruptions much more recent than the large lapilli of the same substance, 
for minute vitreous pjirticles would, on account of their microscopic dimensions, more 
rapidly undergo decomposition into hydrated silicate tluin would be possible for the 
more voluminous fragments enclosed jus nuclei in some manganese nodules or free in 
the deposits. 
