q 
a eAGHIOL 0" Te. 301 
3. CONCLUDING REMARKS ON THE GEOLOGY OF TAHITI. 
In this place, I simply state the most probable view with regard to 
the formation of the island of Tahiti, reserving for a future chapter 
the general course of argument bearing upon the subject. 
There is abundant evidence that the material constituting the island 
has been in fusion, and that the main vent of eruption from which the 
beds of rock of the northern peninsula flowed out, was situated near 
the centre of the same. ‘The inclination of the rocks outward from 
the centre, is proof of this fact ; as well as the uniformity of the angle 
of dip varying only between three to twelve degrees. 
Why the shore portions for five to eight miles inward differ so 
strikingly from the interior, the former consisting of thin layers of 
basaltic lava and conglomerate, the latter of thick beds, in some places 
more than a thousand feet in depth, may be explained on the same 
hypothesis. This centre was the centre of heat, the fountain of the 
liquid rock that flowed over, and produced the beds of the outer por- 
tion of the island; and the cooling of this liquid centre, or the influ- 
ence of the central heat, may have given the solid compact character 
to the interior. 
Have we no remains of the great centre? ‘There is a striking re- 
semblance in the grand amphitheatre at the head of the Punaavia 
Valley to the central basins described as characterizing other volcanic 
islands. Von Buch dwells upon this point in support of his own 
peculiar views, and instances, besides, some of the Canary Islands, 
Barren Island in the Gulf of Bengal, Santorini in the Grecian Archi- 
pelago, and others, as examples of it. If this supposition be correct, 
the peaks of Aorai and Orohena stand on the edge of the pit; and the 
ridge partially enclosing the plain may be the remains of its circum- 
ference. But we do not give this as our decided opinion. It is 
possible that the whole plain at the head of the valley may be the 
result of subsidence. 
Whatever view be held, Tahiti, as it is, is but the ruins of what it 
was. The vast volcanic mountain has been reduced to a collec- 
tion of ridges and profound valleys, and Orohena and Aorai stand as 
monuments, marking, though imperfectly, the amount of degradation 
and subsidence which has taken place to bring out its present features. 
The causes of these changes rest with fire and water, and will be dis- 
cussed in the sequel. 
76 
