166 
GEOLOGY. 
bery varied with groves and grottos, in which the sporting fish display their infinite 
variety of vivid and resplendent colouring. 
List of Specimens. 
Atnygdaloidal trap, containing crystallized zeolite, chabasie, mesotype, analcime ; — 
olivine ; — soap-stone mica ; — chalcedony ; — jasper, and carbonate of lime. — C. 
TAHITI. (OTAHEITE.) 
The Island of Tahiti, like those of Gambier, is formed of volcanic rocks and corals. 
The chief body and centre of the island is composed of the former, while its base is sur- 
rounded by a flat zone of the latter, extending in some places from the sea beach to the 
mountain foot, a distance of three miles, or even more ; whilst at others it is interrupted 
by the ridge of the sloping mountain jutting out into the sea. 
The declivities of the mountains are diversified by numerous knolls rising above 
each other, and variegated with the long grass of the saccharum fatuum, the scanty 
covering of the polypodium pedatum, a few shrubs of the metrosideros spectabilis, and 
the dodonea viscosa, interrupted by bare and brick-coloured tufa. Their tops are fre- 
quently rounded, in a few places uneven and precipitous, but seldom peaked, and are 
covered with a deeply verdant vegetation. The mountain streams flow rapidly down 
the deep ravines, receiving accession to their waters from numerous cascades on each 
side, and carry along with them the lighter volcanic matters to be strewed on the sur- 
face of the coral plain, or blacken with their sand the whole line of the beach. On this 
mixture of lava and coral may in some measure depend the richness of the bordering 
plain, abounding with cocoa nuts, bread-fruit, sugar-cane, &c. varying from 200 yards 
to a quarter of a mile. 
At unequal distances from the beach a coral reef is raised round the greater part of 
the island, almost, and in some places altogether, to a level with the surface of the sea. 
In several places this line of circumvallation has not yet been raised to within several 
fathoms of the surface, affording at these points an entrance to vessels ; whilst there is 
room and perfect safety for more than all the navies of Europe to ride in calm water 
within the reef. These entrances or breaks in the wall are observed to be generally 
situated opposite the mouth of some river, and have been attributed to the influence of 
the fresh water preventing the usual growth of the corals. To me it appears more 
probably dependent upon the original inequality of the bottom, on which these natural 
artificers have raised their structure. 
The island is chiefly made up of basaltic lava of different degrees of porosity. 
The most solid, formerly used for hatchets or adzes, is frequently met with in nearly 
horizontal strata, and contains very few imbedded minerals. The more porous lavas 
are full of basaltic hornblende, olivine and zeolites. In numerous excursions up the 
ravines, I met with columnar basalt, in one place only, about nine miles up Matavai 
river, the columns being about twelve inches in diameter, here it forms a perpendicnlar 
and picturesque cliff, diversifying its front with natural bendings of the pillars at the 
