296 SOCIETY ISLANDS. 
Pacific generally, the luxuriant vegetation is much in the way of geolo- 
gical exploration. ‘The thickness of the layers near the shore varies 
from six to twenty feet. 
Ascending the valleys towards the interior, the layers become 
thicker, and five or six miles from the sea, bluffs of a thousand 
feet constitute apparently a single continuous bed; or at least, there 
is no line of demarcation separating it into parts. Not unfrequently 
the whole height exhibits a continuous columnar structure throughout. 
These facts are well observed in the Papenoo Valley. The enclosing 
ridges of this valley have a rounded contour and sloping sides for the 
first few miles, through which the thin stratified structure continues ; 
but beyond, they are vertical, and a division into layers is rarely 
distinguished. 
In the lofty peak of Orohena, this massive structure is still more 
remarkable. In the view of it from the top of Aorai, a surface of three 
to four thousand feet is exposed to view, almost bare of vegetation, 
and throughout it no trace of layers was detected. Instead, indica- 
tions of a columnar structure were observed. It was owing, appa- 
rently, to this even continuity of surface, that the usual amount of 
vegetation was not spread over it, for there was only here and there 
a crevice that could sustain even a bush. ‘The contrast is very 
striking between the appearance of these heights and the basaltic 
bluffs of the island of Madeira, where the stratified structure is dis- 
tinct to the summits, and as apparent, even many miles distant, as in 
a cliff of secondary limestone. ‘The same contrast would be observed 
in the island of Tahiti, could the valley sections of any of the ridges 
near the shore be stripped of the soil which covers them; for the 
successive layers, where they may be examined, usually stand out 
with great distinctness, owing not only to the alternations of soft and 
wearing tufas with the firmer basaltic rocks, but as much to the open 
spaces or rugged cavities which separate successive beds of the latter. 
The interior of the island differs from the circumferential portion not 
only in general structure, but also in the texture of the rocks. While 
the latter are usually vesicular, the former are compact, or have only 
minute cellules, if any. From these central mountains come the 
gray basalt, containing large crystals of chrysolite, and the trachytic 
and syenitic varieties. The first four varieties of rock described 
appear to be most abundant. 
An imperfect columnar structure is very common throughout the 
island, and may often be detected in the thinner beds along the shores. 
