ab ZN Yel Abs 293 
above the sea. From the officers of the Vincennes who visited it, I 
learned that it is confined, on three sides, by steep mountain de- 
clivities, and on the fourth opens to the southward. It appears to 
be only the commencement of a valley which continues to the sea, 
differing in no respect from the common character of the Tahiti val- 
leys at their head. A low ridge fifty feet high forms a barrier 
between the lake and the continuation of the valley below. According 
to Lieutenant Emmons, the lake is about half a mile long, and in the 
middle is ninety feet deep. ‘The waters have no outlet except by 
some subterranean passage. 
Lieutenant Emmons agrees with Lieutenant Collins, of Captain 
Beechey’s Expedition, in supposing that this low barrier was formed 
by a slide from the declivities.* 
Tahiti, though mountainous quite to the water’s edge in a portion 
of its circuit, is bordered in many parts by a plain, raised six to 
twelve feet above high water level, and in some places nearly a 
mile in width. These plains are the sites of the principal villages. 
The situation and general features of these plains render it probable 
that many of them rest on coral reefs that have become covered with 
soil from the adjoining hills. ‘The present fringing reefs, in many 
places, extend out from the shores from one to two hundred yards, 
and, at low tide, they are usually left bare; if filled in with stones, and 
covered with earth, many acres of land would be added to the island. 
The barrier reefs extend from half a mile to a mile from the shore, 
and are continued, with few interruptions, around the whole island. 
From Matavai to Papiete there is a narrow and intricate ship channel 
between the barrier and fringing reefs, (page 41.) 
The streams of the island are mostly small, and in many of the 
narrower valleys, they often disappear entirely among the porous or 
cavernous rocks which form their bed. The only streams of much 
size in the region that came under my observation are those of the 
Papenoo and Punaavia Valleys. Springs are frequent along the 
shores, proceeding from the streamlets that have taken a subterranean 
course high up among the valleys, or from the waters which must 
every where be absorbed by rocks of the cellular structure here so 
* We observe in the Journal of Tyerman and Bennett, an account of a slide in the 
Matavai Valley, “that dammed up the channel, till the water had spread into a broad 
pool, which threatened, when it should burst by accumulation, to devastate all the lower 
lands.” ‘The water opened, however, a slow vent, by which it was finally drained off.— 
Vol. i. p. 101. 
74 
