THE VOYAGE OF H.M.S. CHALLENGEK. 
*J98 
the fragment at first sight like a striated felspar, or even like the remains of certain 
microscopic organisms. In following under the microscope the contours of these vitreous 
splinters, it will be observed that they are generally terminated by curved lines, and 
present a riddled appearance, all the sinuosities being curvilinear.^ They also present in 
most cases near the vesicles the optical phenomena of tension, analogous to those observed 
in “ Rupert’s drops.” From the differences in the form of the vesicles, and the nature 
of the fracture, it is even possible in some eases to determine the variety of pumice from 
which the minute splinters have been derived. 
It must be pointed out that the trituration of floating pumice, and the decomposition 
of pumice at the bottom, both tend to bring about a more or less complete isolation of 
the individual minerals which formed an integral part of the pumice rock at the time of 
its eruption ; these volcanic minerals are thus spread over the bed of the sea in the same 
way as the vitreous splinters of the pumice. It may therefore be very difficult to recognise 
a difference between volcanic dusts projected as ashes from a crater, and the pulverulent 
debris derived from the wear and tear of floating pumice at the surface of the ocean and its 
disintegration in the deposits. In the case of showers of ashes some mechanical processes 
may, however, produce a sorting of the mineral particles from the areolar vitreous particles, 
as for instance when these are transported by winds or marine currents. On account of 
their lightness, the vitreous particles are carried to greater distances than the fragments 
of crystallised minerals coming from the same eruptions. It follows from this, that at 
any given point the pumiceous particles may quite well predominate in a marked manner 
over the minerals. This may explain why in some deposits the vitreous particles appear 
to mask, by then* number, the volcanic minerals with which they are associated. 
As examples of the aspect and of the characters which these minute particles of 
pumice affect in the sediments, we have represented on PI. XXVI. fig. 4 the residue 
(mineral particles) 6f a Blue Mud from the South Pacific, Station 303, 1325 fathoms. 
Almost the whole field of the figure is here occupied by pumice particles. Two varieties 
can be distinguished : first, basaltic (designated in our Tables brown vesicular glass), the 
minute fragments of which are perfectly characterised by their slightly brownish tint, by 
their leas lengthened structure, and by the relatively small number of vesicles. The 
little pumice fragments of the more acid variety are distinguished by their elongated 
fibres and pores, their very irregular borders, and their almost colourless tint, which ma)’^' 
be said to be more or less greyish. This figure gives a good idea of the characteristic 
appeaninces just described ; it also shows the predominant part taken by microscopic 
splinters of jmmice among the mineral particles of this deposit. PI. XXVII. fig. 3 shows 
the extremely fine mineral particles in a Red Clay from the North Pacific, Station 240, 
2900 fathoms. The splinters of pumice figured liere belong especially to the acid variety. 
> A. Penck, “ Htiulien iiber lockere vulknninche AuHwiirflinge,” ZritHchr. d. d. geol. Geselhrh., 1878, jiji. 97-129 ; 
.1. .S. Diller, “Volcanic Snnd which fell at Uiinhalnshka, Alaaka, Oct. 20, 1883,” Science, vol. iii. p. 651. 
