REPORT ON THE DEEP-SEA DEPOSITS. 
313 
mass is still sufficiently intact to permit observation with high powers, it is sometimes 
seen to be devitrified by globulites, by trichites, and other embryonic crystalline elements. 
In short, the basalts met with in deep-sea deposits do not offer any peculiarities of 
structure or of composition different from those met with in similar rocks found on sub- 
aerial surfaces ; that which more especially characterises them is their advanced state of 
alteration. 
Fragments of limburgitey more or less palagonitised and filled with whitish zeolites, 
are also found under the same conditions in deep-sea deposits as the lapilli already referred 
to, from which they are distinguished by their mineralogical composition. In these, 
crystals of porphyritic augite and olivine are observed in an altered vitreous base. When 
the fundamental mass is still fresh it offers all the characters of the glass in the basic 
lapilli ; when it is altered it presents all the characters of palagonite. These fragments of 
hmburgite are relatively rare, and are easily confounded with the basic glasses and the 
vitreous basalts into which they pass. We have only found them sharply characterised 
at a few stations, and especially at Station 157, 1950 fathoms, in the Southern Indian 
Ocean, where they were associated with pebbles of ancient and recent volcanic rocks. 
The fragments of augite-andesite, or of andesite with rhombic pyroxene, occur very 
frequently, and with the same external characters as the lapilli of the preceding types. 
They may be distinguished by their composition, the absence of olivine and the presence 
of sanidine and quartz separating them from the basaltic lapilli. Sometimes they contain 
rhombic pyroxene, and are associated frequently with tufaceous andesitic cinders. In 
the Tables of Chapter II. they are stated to occur at Stations 147 and 150, in the Southern 
Indian Ocean; Station 214, among the Philippines; Stations 276, 293, and 295, in the 
South Pacific. Some rare lapilli of hornblende-andesite were also met with ; these had 
a fundamental mass with a fine greyish black grain, enclosing plagioclases, hornblende, 
and sometimes sanidine. 
It may be said that, generally speaking, fragments of acid rocks are especially rare 
among the recent volcanic rocks in the Challenger dredgings. An exception must, how- 
ever, be made with respect to pumice, for we have seen that its mode of transport may be 
quite different from that of the fragments and lapilli just referred to. Just as the lapilli 
of basic rocks are abundant in certain regions of the Pacific, for example, so, with the ex- 
ception of pumice, are trachytic and liparitie lapilli rare in these same positions. At certain 
stations, however, the nature of the mineral particles, the relative abundance of sanidine and 
of hornblende, the occasional presence of quartz, and especially of splinters of acid glass, all 
indicate that eruptions of trachytic cinders and lapilli must have taken place at the bottom 
of the sea. But it is difficult to be quite certain upon this point, for after what has been 
said above with reference to the distribution of pumice, its origin, and its disintegration, 
it may quite well happen that what is regarded as trachytic ashes from a submarine 
eruption may be nothing else than the residue derived from the disintegration of liparitie 
(deep-sea deposits chall. exp. — 1891.) 40 
