294 SOCIETY ISLANDS. 
common. During the rainy season from December to March, in 
which rain sometimes falls incessantly for weeks, the streams are very 
much swollen, “the low lands overflowed, fences washed away, and, 
unless great care is taken, many plantations destroyed.”* It strikes 
the traveller as remarkable that little debris is to be found at the foot 
of the precipices or mountain ridges. 
I can say but little of the southeastern or smaller peninsula of 
Tahiti. Its mountains have some general similarity to those we have 
described. The shores are mostly bordered by a narrow plain, which 
is said to be noted for its beauty and fertility. 
The narrow isthmus between the two peninsulas is described in the 
Journal of Mr. Couthouy, who visited it from the Vincennes, as “a 
low narrow belt of sand, chiefly coral detritus and comminuted shells 
thrown up by the surf, with a small portion of black, ferruginous sand, 
and minute fragments of olivine, derived from the decomposition of 
volcanic rocks.” 
GEoLocicaL StructureE.—The rocks of Tahiti, with the exception 
of the coral of the shores, are wholly of igneous origin. Dark gray 
basalt and basaltic lavas are most common. ‘These alternate occasion- 
ally with beds of conglomerate, consisting of the same basaltic material, 
or a finer tufa of similar origin. 
We notice a few of the varieties before proceeding with remarks on 
the position and character of the beds they constitute. 
1. A compact and tough grayish-blue or dark gray rock, with small 
disseminated grains of chrysolite. 
2. The same with small black crystals of augite. 
3. The same with chrysolite in crystals, one or two inches long, 
with smaller grains disseminated. 
4. The same without augite or chrysolite. 
5. The same, porphyritic, with numerous thin tables of feldspar ; 
crystals mostly compound. Sometimes containing also augite. 
6. Brownish-red, with brownish-black augite crystals. 
7. Black approaching pitchstone. 
8. A light-grayish trachytic basalt, porphyritic with a few crystals 
of feldspar. 
9. A grayish-white syenite, containing acicular crystals of horn- 
blende. 
With the exception of the last three, which were not observed in 
* Ellis’s Polynesian Researches, vol. i. p, 28. 
