REPORT ON THE DEEP-SEA DEPOSITS. 
297 
iridescent colours on the fractures. Under the microscope there can be seen in the 
dark-green transparent glass the skeletons or sharply-terminated individuals of olivine, 
augite, and plagioclase ; there is little or no magnetite, but sometimes black or opaque 
concretions. The percentage of silica is on an average about 50, therefore much less 
than in the acid varieties. The following is an analysis of one of these deep-coloured 
specimens from a dredging in 1400 fathoms in the South Pacific. After having washed 
the fragment with oxalic acid, to take away traces of manganese which covered the 
specimen, and then repeatedly with boiling distilled water, to extract the sea-salts and 
to detach the mud adhering to it, the following results were obtained : — 
Station. 
Depth in 
Fathoms. 
No. 
HoO 
SiO^ 
TiO^ 
FeO 
MnOg 
CaO 
MgO 
K^O 
NagO 
Total. 
184 
1400 
79 
1-70 
50-56 
0-80 
10-30 
4-95 
7-59 
0-14 
9-35 
9-27 
1-24 
2-81 
98-71 
On comparing the above results with those obtained by Cohen they present a perfect 
analogy, within the limits that may be expected for analyses of rocks of the same family, 
so that this specimen represents the basaltic lavas of Hawaii in its mineralogical composi- 
tion, and the same may be said of all the dark-coloured specimens that have been dredged 
from the sea-bottom in the Pacific, and to a less extent in other oceans. 
Minute fragments of the different varieties of pumice noted above can be detected in 
all marine deposits, and in some areas the greater part of a Ked Clay, or of the residue of a 
calcareous ooze after removal of the carbonate of lime by dilute acid, may be made up of 
minute fragments and splinters of pumice. These microscopic fragments may be derived 
from the trituration of floating pumice, or from its disintegration on the sea-bottom, 
or, again, they may have been ejected as showers of ashes from subaerial or submarine 
eruptions, and have been widely distributed by aerial or marine currents. 
In general, when mineral particles are reduced to infinitesimal dimensions, and are 
irregularly fractured, they lose their distinctive characters ; the crystallographic form 
and the optic properties are no longer recognisable, but with vitreous pumice fragments 
the recognition of the particles is still possible when they are even less than 0'005 mm. 
in diameter. The most reliable diagnostic character of these pumice particles is to be 
found in their peculiar structure. The enormous numbers of vesicles in the pumice, 
due to the expansion of the dissolved gases in the original magma, present a special 
structure and characteristic fracture which can be recognised even in the minutest 
fragments. This property can easily be tested by pulverising a piece of pumice in an 
agate mortar, when it will be noticed on examination under the microscope that the 
minutest particles bear the impress of this vesicular or filamentous structure. The 
appearance arising from several pores being drawn out so as to be mere streaks, renders 
(deep-sea deposits chall. exp. — 1891.' 38 
