290 
THE VOYAGE OF H.M.S. CHALLENGER. 
glass, with numerous closed, often elongated, vesicles, and with a few individualised 
mincralogical elements. This variety is rich in silica, and hence is referred to the litho- 
logical types of liparite or trachyte. The minerals which are embedded in the vitreous 
mass, or project from the weathered surfaces, are sanidine, plagioclases, black mica, 
augite, and magnetite, and with the microscope many microliths belonging to the same 
species can be observed. Quartz is very rare ; sometimes rhombic pyroxene is present. 
A second variety, but less abundant than the liparitic variety, is that known as 
andesitic pumice. In external characters andesitic pumice stones nearly approach the 
liparitic ones, and might at first sight be confounded with them ; they have the same 
grey colour, are sometimes fibrous, and decompose in the same way. They are especially 
distinguished by the minerals which they contain, the most important being augite, 
plagioclases, and magnetite, while microliths of augite and hornblende are sometimes 
seen in the transparent base. Olivine is absent, and the silica in this andesitic pumice 
is estimated at 60 per cent. A specimen of this variety, from 2300 fathoms in the North 
Pacific, was analysed, with the following results : — 
Station. 
Depth in 
Fathoms. 
No. 
Loss on 
Ignition. 
SiO^ 
AlgOg 
FejOg 
MnOg 
CaO 
MgO 
KgO 
NagO 
Total. 
241 
2300 
80 
4-95 
60-95 
15-97 
9-08 
g.tr. 
2-92 
1-40 
1-61 
2-34 
99-22 
The surface of the fragment analysed was extremely friable — almost earthy. Beneath 
this decomposed layer the centre was still formed of a whitish mass, having the appear- 
ance of a fresh rock, but, as showm by the analysis, especially by the percentage of 
water, which rises to nearly 5, this central portion had also undergone a profound 
alteration. 
A third variety of pumice met with in deposits is basaltic or basic pumice. In a 
large measure we owe our knowledge of the nature of this variety of pumice to the 
investigations of Cohen' on the lavas of the Hawaian Islands and of Niafou in the 
Friendly Islands, as well as on some floating pumice collected between New Britain and 
New Ireland. He has pointed out that volcanic products from several areas of the 
Pacific far removed from each other have a true basic character, and belong to the 
ba.saltic glas.ses. The pumice stones collected from numerous stations in the Pacific 
during the voyage of the Challenger have exactly the same characters as those described 
by Cohen. These are vitreous vesicular rocks of a rather deep colour, yellowish or 
approaching to bottle-green. The pores affect in general a more rounded or spherical 
form than those of the preceding varieties of pumice. The vitreous partitions between 
the vesicles arc not very thick, and when the specimens are but little altered they show 
* E. Cohen, “Uel>er Lnven von Hawaii und einigen anderen Inseln des grosaen Ocean, etc.,” Neues Jahrbuch fiir 
.^finrraloyie, etc., Jahrg. 1888, Bd. ii. p. 2.3. 
