364 PACIFIC OCEAN. 
Solid Centre of Volcanic Mountains.—One peculiarity in the large 
volcanic mountains remains to be considered. ‘Tahiti is one of these 
mountains laid open by extensive degradation. We find that while 
the circumferential portions consist distinctly of a series of layers 
from different eruptions, alternating occasionally with conglomerates, 
and dipping gently outward, the interior peaks are solid to their 
summits, and imperfectly columnar. The same is the case at 
Rarotonga, as I was informed by Mr. W. C. Cunningham. On 
Kauai, the layers become very thick towards the interior, as in Ta- 
hiti, a depth of several hundred feet occurring six or eight miles 
from the shores; but the centre of the island not having been exa- 
mined, we cannot say how far it corresponds with Tahiti. Similar 
facts are stated by von Buch and others; and it seems, therefore, 
to be a general characteristic of the larger lava mountains. More- 
over, it appears to be ascertained that these central portions are more 
feldspathic in character, often more crystalline in texture, and quite 
compact without a trace of cellules. Clinkstone and varieties of por- 
phyry with syenite or an allied rock, are the usual materials constitut- 
ing them. 'These same feldspathic rocks, including even the syenite, 
inclosing hornblende, occur upon Tahiti.* On Oahu, the compact 
varieties are found on the Waianae plains, which lie near a former 
centre of a volcanic mountain. They occur among the lower layers 
of the eastern Oahu range. At the summit of Hale-a-kala, Mount Kea, 
and also of Mount Loa, clinkstones appear at the surface, and in some 
instances they have throughout a crystalline texture. ‘Thus feldspa- 
thic rocks seem to prevail at the centres of all the mountains examined, 
though basaltic rocks constitute the exterior surface and the flanks of 
the mountain. And when the mountain has been broken through by 
any cause, the structure at the middle is not stratified, but solid and 
compact; while in the valleys leading to the interior, there are layers 
of the different kinds of graystone and basalt, often containing chryso- 
lite, or augite, or both these minerals. 
This difference of structure admits of explanation in accordance 
with the facts which have been described. It would seem that the 
vast amount of material constituting the solid nucleus must have 
been simultaneously in fusion, in order to produce so uniform a struc- 
ture. The hypothesis strikes the mind as almost incredible. But 
* Although the locality of the syenite was not ascertained, I obtained a kind of trachyte 
on Aorai, one of the central peaks. 
