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Pisonia grandis trees, casting so complete a shade that no other plants grow beneath them, save only a single cocoanut palm, which was probably planted by Governor Tilly's party about fifteen years ago. This forest forms a nearly symmetrical dome, the leaves and branches on its confines extending quite to the ground. The largest trees are near the southern end of the grove, and about three feet above the ground one of these trees had a girth of 25 feet 7 inches, and was about 80 feet high. The ground under these trees is covered with a rich chocolate-colored humus, which is of considerable depth near the southern end of the grove.
Apart from this grove of pisonia trees and a half dozen cocoanuts planted by Governors Tilly and Terhune in 1902 and 1920, there are only two other species of plants upon the islet. These have been identified by Professor William A. Setchell and are a pink-flowered creeping Boerhaavea diffusa with stems rarely more than 3 feet long; and a thick-stemmed succulent Portulaca n. sp. with small yellow flowers. Both of these plants grow fully exposed to the sun on the coral breccia and calcareous sand which surrounds the pisonia grove, and none are found under the shade of the trees.
On the south side of Rose Islet the sand beach is reduced to from 1 to 5 feet in width at low tide, and cliffs of coquina from 5 to 8 feet high front the sea. A few feet inland this rocky ledge rises to a height of about 11 feet above high tide level. The pisonia grove appears to be confined to this region of coquina rock and does not appreciably extend out over the loose calcareous breccia which has been washed in upon the islet in time of storm.
The tree-covered rocky center of the islet is composed of a coquina consisting chiefly of wave-worn fragments of lithothamnium, and also rare and occasional pieces of broken coral such as Favites, Porites, Symphyllia, Pocillopora, and still more rarely Acropora. Imbedded in it are many wave-worn half-valves of Tridacna, and Gasteropod shells, and spines of Echini such as Cidaris were found, as was also the much-corroded ulna and part of the skull of a small Cetacean about the size of a black-fish, the latter being embedded in the coquina about 8 feet above high tide level. A large amount of organic matter dark brown in color and derived
from the decomposed roots of the pisonia trees permeated this coquina to a depth of several feet. All of the fossils found imbedded in the coquina are forms now living on the reef flat. Professor C. B. Lipman found that the coquina in contact with the soil contains 12.05 per cent. of phosphoric acid.
On the wave-washed southeastern shore of Rose Islet some modern beachrock has been formed and projects a few inches above high tide level; but this is more recent than the rocky matrix of the islet, which is now elevated about 11 feet above high tide level.
Sand Islet, which lies north of Rose Islet, is a mere accumulation of fragments of lithothamnium, shells, and broken coral, and is devoid of vegetation, and only about five feet above high tide level. The sea must wash completely over it in time of storm.
Several hundred boobies (Sula), most of which had half-grown young, were nesting on the coral breccia of Rose Islet, while others had constructed nests of sticks high among the branches of the pisonia trees. A few boatswain birds with eggs were also nesting in the trees, and several nearly grown young of the noddy (Anous) were running over the ground, while adult noddies and sooty terns visited the island at night. Frigate birds were hovering over the island, but none were nesting. Wilkes states that the noddies and sooty terns were nesting on Rose Islet on October 7, 1839, and these species were still nesting when Governor Terhune visited the island on January 10, 1920.
A small brown-gray rat was abundant and specimens of it were presented to the Bishop Museum in Honolulu, where they were identified by Mr. J. F. G. Stokes as being a Malayan form which appears to have become widely spread over Polynesia, being probably introduced by the early Polynesians themselves, who esteemed them for food, and took much delight in hunting them for sport. Apart from these very tame and abundant rats, the only other animals we observed were a small brown short-tailed lizard, identified by Dr. Thomas Barbour as Lepidodactylus lugubris, which is abundant in Polynesia, and the larva of a sphynx moth (Celerio Oken) feeding upon the portulaca. A few gnats and an occasional house fly, which may have been introduced from the U. S. S.
Fortune, which cruised continuously around the island, there being no anchorage, were the only insects we observed.
The upper surface of the atoll rim which encircles the lagoon is a hard smooth-floored flat, with but little loose sand upon it, and in most places it is awash at low tide, although in others it projects as a ledge about a foot above low tide of the reef tides.
This hard, smooth surface of the atoll rim is veneered everywhere by a layer of lithothamnium, as is characteristic of the wavewashed surface of offshore and barrier reefs. The condition over the fringing reefs of the Pacific is quite different, for here loose fragments are washed inward from the seaward edge and backed up against the shore. Thus the whole surface, excepting only the wave-washed outer edge, is covered with small loose fragments which could not remain upon an atoll rim or a barrier reef, for they would soon be washed off into the lagoon. This loose nature of the material forming the shoreward parts of fringing reefs at once distinguishes them from offshore reefs. Professor W. M. Davis's attempt, following Darwin, 1842, to institute a class of "offshore fringing reefs" is not justified, the structure of the two forms of reefs being widely different. As a matter of fact, reefs along shores are either barrier reefs or fringing reefs, and one is never in any doubt in distinguishing the one from the other.
Hundreds of large blocks of limestone, of the sort called "negro heads" on the barrier reef of Australia, lie scattered over the flat wave-washed rim of Rose Atoll. These loose boulders are quite uniformly about 51⁄2 feet high, and only when tilted are they any higher. In addition to the loose boulders there are a few others. which are mushroom-shaped and still remain attached to the floor of the atoll rim, of which, indeed, they form an integral part. One of the most remarkable of these mushroom-rocks lies to the eastward of Rose Islet, and is supported upon so slender a pedicel that it would seem as if the next storm must cause it to topple over. In many places over the flat wave-washed floor of the atoll rim one finds remnants of pedicels which once supported "mushrooms." In addition, some of the boulders have become secondarily cemented to the floor of the flat by the growth of lithothamnium around their
bases. The largest boulder we observed lay loosely upon the reef flat east of Rose Islet and was somewhat tilted by being jammed against another rock. . It was 12 feet 5 inches long, 8 feet wide, and 7 feet 6 inches high, and as its specific gravity was 2.3, it apparently weighs 46 tons.
The appearance of these boulders supports the view that the atoll rim was once about 6 to 8 feet higher than at present, and has been cut down to present sea level in recent times; most of the mushroomrocks having been completely undercut so that they now lie loosely upon the floor of the flat.
It can be seen that the surface of the present reef flat consists chiefly of lithothamnium, a beautiful bright pink variety of which forms a veritable veneer over its surface. Professor Alexander H. Phillips made an analysis of this lithothamnium and found it to contain 74.4 per cent. of calcium carbonate and 19.47 per cent. magnesium carbonate. Also rock from the solid floor of the atoll rim west of the main entrance to the lagoon gave 83.86 per cent. of calcium carbonate and 14.36 per cent. of magnesium carbonate; while a large loose boulder from the same region consisted of 77.28 per cent. of calcium carbonate and 18.3 per cent. of magnesium carbonate. Also, Professor C. B. Lipman found that the largest loose boulder on the reef flat east of Rose Islet contained 79.5 per cent. calcium carbonate and 14.54 per cent. magnesium carbonate. It will be recalled that Högbom found the magnesium carbonate in various species of lithothamnium to range from 3.76 to 13.19 per cent., and Clarke and Wheeler1 found from 10.93 to 25.17 in 15 species, and thus the Rose Island species seems to be peculiar in possessing a fairly high magnesium content.2
It thus appears that the loose boulders lying upon the atoll rim have the same general chemical composition as the solid rock of the rim itself and are remarkable in that they contain a large amount of magnesium. In fact, these boulders are only remnants of the old rim which was once about 6 or 8 feet higher than at present, but has been almost entirely planed down to the lowered level of the 1 U. S. Geol. Survey, Prof. Paper No. 102, 1917.
2 See J. W. Judd, Funafuti Report, 1904, 1934, p. 377.
present surface of the ocean, leaving only an occasional mushroomrock on a pedicel as a vestigial remnant of the old rim.
Inspection shows that the solid rock of the atoll rim and also. the boulders lying upon it consist chiefly of lithothamnium compacted into a dense mass of chalky whiteness superficially resembling dolomite, and having a specific gravity of about 2.3, thus being higher than that of pure coral limestone, the specific gravity of which would range from 1.85 to 2. A pure dolomite containing 45.65 per cent. of magnesium carbonate should have a specific gravity of about 2.9.
There are a few fossil corals, chiefly Pocillopora, imbedded in the rock of the atoll rim and the boulders, but the whole visible rock of the atoll consists so largely of lithothamnium that we may call it a "lithothamnium atoll" rather than a "coral atoll."
The flat upper surface of the atoll rim is in most places planed off nearly to low tide level, but it is veneered with a vigorous growth of a beautiful pink lithothamnium which has been provisionally determined by Professor W. A. Setchell as Porolithon related to P. craspedium. In most places this lithothamnium forms irregular, more or less connected, patches growing on the smooth hard floor of the flat. West of the main entrance to the lagoon it grows in long nearly parallel, flat-topped, over-arching ridges all parallel with the line of the wave fronts of the breakers as they surge over the reef. These ridges are about 6 inches high and from 6 inches to several feet in width, and with channels of similar width between them.
Lithothamnium grows in greater profusion over the reef rim of Rose Atoll than in any other Pacific reef I have seen; but apart from the single species of pink lithothamnium there are remarkably few organisms growing in the shallows of the reef flat. Occasionally we find a pale-olive-green Porites, allied to P. solida, and there are a few small stocks of Favites or Symphyllia; but Acropora and Pocillopora, which are the dominant forms in most breaker-washed reef flats of the Pacific, are practically absent from Rose Atoll, except at the extreme edges of the atoll rim fronting the lagoon on the sea, where a few stunted specimens of these genera occur.