298 SOCIETY ISLANDS. 
seldom attended with any disturbance or uplifting of the layers. 
We met with no tilted rocks in Tahiti. The slopes are regular and 
uniform. ‘The only exception to this noticed is at One Tree Hill, 
where the tufa is much inclined. But of this we speak on a follow- 
ing page. 
Minerals.—With the exception of the essential constituents of the 
rocks already mentioned, we observed nothing of much interest in 
the rocks of Tahiti. Small geodes of stilbite and analcime were 
found in pebbles in the Papenoo Valley; and at One Tree Hill the 
basalt of the dikes contains minute cavities which are filled, apparent- 
ly, with natrolite. Particles of iron pyrites are occasionally found 
in the basalt. ‘The augite crystals have the simple form represented 
in figure 1, of the author’s ‘Treatise on Mineralogy. They vary in 
length from a quarter to three quarters of an inch, and when broken, 
usually present grayish-brown and brownish-black colours, often with 
a slight iridescence. ‘The chrysolite has the usual dark apple-green 
and grass-green colours. ‘The large crystals, two inches in length 
and an inch broad, are quite brittle, and show no distinct faces of 
crystallization, yet usually afford rectangular sections. 
6 Decomposition of the rocks.—The basaltic rocks of the island are 
subject throughout to rapid decomposition. ‘This may be observed 
on every part of the island; from the very summits of the peaks to 
the seashore, the rock is almost invariably covered with earth. Where 
the top of the mountain ridge was a mere edge, but three or four feet 
wide, there was seldom a spot without soil and a growth of bushes; 
it is owing to the fact that the shrubbery shuts out the dangers either 
side and affords some support, that the mountain travelling is at all 
practicable. 
On the declivities back of Matavai and in many places elsewhere, 
the rocks may be seen in the process of decomposition ; and often the 
soil still retains the form of the original layer, rising into rough points 
and craggy hillocks, looking like the rock itself; yet so soft as to be 
gathered up by the hand like so much earth. 
In the progress of decomposition, the chrysolite appears to yield 
first. It turns iridescent, and as the change goes on, becomes 
finally, in the half-altered rock, an ochre-yellow or brownish-yellow 
earth, very soft and pulverulent. The compact base next crumbles, 
and, unless it contains iron or is largely charged with chrysolite, it 
usually retains a dirt-brown colour, until finally pulverised, when its 
colour is modified by that of the altered chrysolite. Other beds con- 
