Page:A Treatise on Geology, volume 1.djvu/348

 332 oval or irregularly rounded at their circumference: of this form are the group called by Cook the Friendly Isles, consisting of numerous islands, the majority of which are tabular.

There are also many crescent-shaped reefs, with the most convex portion of their arc the highest, often denoting themselves to the mariner only by the breaking of the waves, and here and there a rock above the level of the ocean, while the horns of the crescent are depressed, and gradually lost in the greater depth: in a few instances, as at Gambier's Island, they are sufficiently raised to have become verdant and inhabited.

Of those which form long narrow strips of land, Mr. Stutchbury refers to Tehuro, a few leagues from Tahiti, and the great reef which takes the course of the north-eastern shore of New Holland, which Captain Flinders describes as being more than 1000 miles in length: in the course of which there is a continued portion exceeding 350 miles with scarcely a break or passage through it.

Of the last group of coral islands, or rather reefs, encircling elevated land, the Society Islands, including Tahiti, offer striking examples; being often surrounded by coral reefs, generally situated 400 or 500 yards off shore, with a deep channel between, having numerous openings, through which ships can enter and lie at anchor in perfect safety. These breaks in the coral barrier are, in most instances, opposite the mouths of freshwater rivulets.

The islands Raiatea and Tahaa (Ulietea and Otaha of Cook) are divided by a strait, by which ships can enter at the windward side of the islands, and get to sea again through the leeward channels. These two islands are entirely surrounded by one coral reef, extending throughout the circumference of both; the openings through the reefs are, in most cases, denoted by the points being rather higher and more verdant, having trees, principally cocoa nut trees, planted by the natives upon them. The passage is seldom more than 100