316 SAMOAN ISLANDS. 
The tufa and basaltic conglomerates have few peculiarities. The 
latter are like those of Tutuila, (page 311.) The tufa is a very fine- 
grained earth-coloured rock, without lustre, and fragile. It is abun- 
dant at the eastern extremity of Upolu, and in the adjacent islands, 
and will receive farther attention on a future page. A variety of 
a brick-red colour, sprinkled with white points (feldspathic) is used 
as a paint by the islanders. I have not seen this variety in place, 
and have suspected that it may be decomposed rock proceeding from 
the second variety of porphyritic basalt (var. 6, b.) The dissemina- 
tion of the whitish points is very similar to that of the feldspar in this 
rock. ‘The tufa resembles a red ochre, and owes its colour to iron. 
Structure.—a. A lamellar structure characterizes the rock in some of 
the cliffs. Just west of Lauli, this structure is finely developed ; the 
rock (var. 4) is divided into layers from half an inch to a foot in 
thickness. The layers separate with difficulty, although very distinct 
on the broken surface of the cliff. ‘The layers are often contorted or 
curved. 
The same structure, I am told, is exhibited by recent layers of the 
black lava, on the south side of Upolu. Layers from an inch to six 
inches thick, are slowly cleaving off from the roofs of some of the 
caverns in that part of the island. 
6. A concentric structure is not common in the basalt and basaltic 
lavas of the island. Three miles east of Apia, along the coast, this 
structure is imperfectly developed. ‘The centres of the concentric 
masses have a dark greenish-black colour and are compact. The 
rock adjoining the central mass is altered to an ochre yellow or 
reddish-brown colour; the latter is the external colour of the two, and 
is apparently the result of a farther decomposition and a more com- 
plete development of the iron in the constitution of the rock. This 
structure may be seen at other places along the shores; but I have 
observed no additional facts respecting it worthy of note. 
c. Columnar basalt, though common in imperfect forms, is still, ac- 
cording to our observations, of rare occurrence in regular prisms. In 
the high peaks around Fangaloa the same vertical cleavages may be 
detected as in the hiils of Tutuila. Some of the thin layers of basalt, 
or basaltic lava, along the coast, are broken, by perpendicular frac- 
tures, into columnar masses, from two to five feet in breadth. The 
fractures generally produce curved surfaces, and the masses or columns 
stand half an inch or an inch apart. 
Along the rocky coast beyond Lauli to the eastward, the surface 
