294 
THE VOYAGE OF H.M.S. CHALLENGEK, 
of a fall of volcanic ashes, likewise obtained in the dredges, it is often impossible to say 
whether they came from a submarine or a subaerial eruption. 
Pumice . — On account of its abundance and the wide area of its distribution, pumice 
merits the fii*st consideration among the volcanic materials of marine deposits. This 
rock is merely a vesicular variety of a number of lithological species, and the wide distri- 
bution of this variety is dependent on its spongy structure. Mr Murray was the first to 
point out the important role played by pumice in the formation of pelagic deposits and 
in the origin of the soil of coral atolls,^ and the same considerations, with further develop- 
ments, were afterwards dwelt upon in a joint paper by ourselves.^ 
During the Challenger Expedition fragments of pumice were frequently taken in the 
tow-nets while floating on the surface of the sea, and were often found to be covered with 
Cirripeds and other marine animals. Long lines of pumice fragments were also ob- 
served on coral reefs just above high-water mark. From New Zealand, South America, 
Japan, and other countries, immense quantities of pumice are carried to the ocean by 
rivers. The Expedition, however, did not meet with any of those prodigious fields of 
floating pumice which have many times been recorded by voyagers as being so vast as to 
impede the progress of their ships, — for instance, after the famous eruption of Krakatoa 
in 1883,® and in the South Pacific in June and July 1878 by Captain Turpey, and by 
Captain Harrington in March 1879.* The pumice sent to us by Captain Turpey was dark- 
green in colour, and was believed to have been derived from a submarine eruption. The 
fragments of pumice, which float on the surface in great fields, or in long parallel lines, 
are carried enormous distances by oceanic currents, and, being rubbed and knocked 
against each other by the action of the waves, they ultimately assume a rounded appear- 
ance, as if they had been rolled like river pebbles. While this rubbing, knocking against 
each other, and rounding goes on, a very large number of the triturated fragments that 
are broken away from the outer surfaces fall as minute splinters to the bottom of the sea, 
contributing largely to the formation of pelagic deposits. The larger aud smaller 
fragments of pumice slowly become waterlogged and sink to the bottom. Mr Murray 
* Murray, Proc. Roy. Soc. Edin., vol. ix. p. 247. 
* Murray and Renard, Proc. Roy. Sac. Edin., vol. xii. p. 495. See also Helge Biickstrdm, “Ueber angeschwemmte 
Bimsteine und Schlackcn der nordeurojmischen Kiisten,” Bihang till. K. Svemka Vet. Ak. Handl., Bd. xvi. Afd. 11, No. 5. 
® Tlie Bay of Ijampoong, in the Strait of Sunda, was blocked by a vast accumulation of pumice projected in a few 
hours by the eruption of Krakatoa. This floating barrier of pumice had a length of 30 kilometres, a breadth of 
1 kilometre, and a depth of 3 to 4 metres ; it was raised about 1 metre above, and plunged 2 metres below, the 
surface of the water. These numbers indicate that at this point 160,000,000 cubic metres of volcanic matters were thus 
accumulated. This elastic and moving wall undulated with the flux and reflux of the waves, and the fragments of 
which it was formed were carried by currents to thousands of miles from the eruption, and scattered finally over the 
surface, an<l, as we now know, also over the bottom, of the ocean {Gomptcs Rendus, tom. xc. p. 1101, 1883). See also 
Charles Meldrum, Brit. A»s. Rrjiort for 1885, pj). 773-779, 1880; S. M. Rendall, Nature, vol. xxx. p. 288, 1884. 
* Cn])tnin Turj»ey says that in some parts of the sea these pumice stones were in such large numbers that the small 
lioaU which the ship drew after it rose out of the water and were drawn along as if on a bed of rocks. Captain 
Harrington says that many of the patches assume<l the apj)earance of islands, ami were large enough to retard the pro- 
gress of the Teasel considerably, their appearance being alanning. 
