3828 SAMOAN ISLANDS. 
portion of a single side of the crater remains. The dip of the layers, 
and their composition, are proofs that the islands were formerly craters. 
Nuutele is the largest and highest of the four. The others are from 
a half to two-thirds of a mile long, and are not over three hundred feet 
in height. Namu’a and Tapu-tapu are connected with the shores of 
Upolu by the coral reef. Nuutele and Nuulua stand isolated, with 
deep water around them. The tufa is mostly composed of fine basaltic 
or volcanic earth, with rarely an imbedded pebble. It has a compact 
earthy appearance and colour, and is often friable. Coarse varieties, 
or conglomerates, are uncommon, but isolated masses of basalt are 
sometimes imbedded in the tufa. 
Besides the volcanic materials, the tufa also contains an occasional 
fragment of coral, or coral limestone. I collected some specimens of 
imbedded limestone pebbles from the tufa near the top of Namu’a, 
two hundred feet above the sea. ‘The pebbles were as white as the 
coral rock of the reefs, and some of them were as large as walnuts. 
Tested with an acid, they afforded a brisk effervescence. They were 
undoubtedly enclosed within the tufa as a part of it, and not subse- 
quently carried up to their present place. 
The facts here adduced show that these islands are not the result 
of lava eruptions. The ejections of lava, if there were such, were 
submarine. The islands have been formed by ejected volcanic 
earth or mud falling over the rising walls of the crater, which 
they consequently overlap, inclining both inwards toward the cen- 
tral vent, and outward down the slopes of the cone. ‘The opening 
of these submarine vents probably followed some movement in the 
neighbouring volcanoes, and there may have been an ejection of lava 
beneath the sea from the fissures thus formed. But subsequently the 
eruptions consisted of loose cinders and comminuted lava. The pre- 
sent position of the craters in a sea which covers, to a considerable 
height, their tufa sides, (for the tufa extends with its even dip some 
distance below the surface,) and so low that the bottom of the crater, 
as in Nuutele, is not above the level of the sea, is some evidence that 
these are the results, in part, if not wholly, of eruptions from vents 
beneath the water. ‘The fragments of coral rock imbedded in the 
tufa, lead us to the same conclusion ; or at least, they show that the 
vent was liable to incursions of the sea, through some opening below, 
which carried in the coral pebbles. The coral pebbles were not sub- 
jected to a high heat, for they retain their original freshness outside as 
well as within. 
