514 Proceedings of the Royal Society 
Between 250 and 350 yards from the edge of the reef there 
was generally a very steep and irregular slope ; about 100 fathoms 
was got at the latter distance, and the angles between these last- 
mentioned distances often exceeded 45 degrees. The talus here 
appeared to be composed of huge masses and heads of coral, which 
had been torn by the waves from the upper ledge and piled up 
on each other. They were now covered with living Sponges, 
Alcyonarians, Hydroids, Polyzoa, Foraminifera, &c.* 
From 350 to 500 yards from the edge of the reef, we had a 
slope with an angle of about 30°, and made up chiefly of coral sand. 
Beyond 500 yards the angle of the slope decreased till we had at a dis- 
tance of a mile from the reef an angle of 6°, a depth of 590 fathoms, 
and a mud composed of volcanic and coral sand, Pteropods, Pelagic 
and other Foraminifera, Coccoliths, &c. 
In the lagoon channel the reefs were found to be fringed with 
living coral, and to slope downwards and outwards for a few feet, 
and then plunge at once to a depth of 10 or 16 fathoms. Many 
portions of these inner reefs were overhanging, and at some places 
overhanging masses had recently fallen away. Everywhere much 
dead coral rock was exposed to the solvent action of the sea 
water. The reefs of Tahiti are at some places fringing, at other 
places there is a boat passage within the reef, and at Papiete there 
is a large ship channel with islets within, and the outer edge of 
the reef is a mile distant from the shore. The island itself is 
surrounded with a belt of fertile low land, frequently three or four 
miles wide ; this shows that the island has not in recent times 
undergone subsidence; there are, indeed, reasons for supposing it 
has recently been slightly elevated. Everything appears to show 
* This ledge and steep slope beyond where a depth of 30 or 40 fathoms was 
reached, was characteristic of a large number of atoll and barrier reefs, and 
seemed due to wave action. Experiments had been made with masses of 
broken coral, and it was found that these could (on account of their rough 
and jagged surface) be built up into a nearly perpendicular wall by letting 
them fall on each other. A talus formed in water deeper than 40 fathoms 
where there was little if any motion would be different from one formed on 
land. In the latter case the disintegrating forces at work always tended to set 
the talus in motion; in the former case everything tended to consolidate and to 
fix the blocks in the positions first assumed. A removal of lime in solution 
would take place from the blocks forming this steep slope, but, except in very 
deep water, this would not be sufficient to check the outward extension of the 
reef. 
