3o8 



NATURE 



[July 26, 1900 



fathoms, and 35° at 2475 fathoms; about 120 miles from the 

 Marquesas. 



Our deep-sea nets not having reached San Francisco at the 

 time we sailed, we limited our pelagic work to surface hauls, 

 of which we generally made one in the morning and one in the 

 evening, and whenever practicable some hauls with the open 

 tow nets at depths varying between 100 and 350 fathoms. The 

 results of these hauls were very satisfactory. The collection of 

 surface animals is quite extensive, and many interesting forms 

 were obtained. As regards the deeper hauls with the Tanner net, 

 they only confirm what has been my experience on former ex- 

 peditions, that beyond 300 to 350 fathoms very little animal 

 life is found, and in the belt above 300 fathoms a great number 

 of many so-called deep-sea crustaceans and deep-sea fishes were 

 obtained. I may mention that we obtained Pelagothuria at 

 about 100 fathoms from the surface. 



On our way to Tahiti from the Marquesas we stopped a few 

 days to examine the westernmost atolls of the Paumotus. 

 Striking Ahii we made for Rairoa, the largest atoll of the 

 Paumotu group. Skirting the northern shore from a point a 

 little west of Tiputa Pass, we entered the lagoon through 

 Avatoru Pass, anchoring off the village. 



We made an examination of the northern side of the lagoon 

 between Avatoru and Tiputa Passes. The lagoon beach of the 

 northern shore is quite steep, and is composed of moderately 

 coarse broken coral sand at the base, and of larger fragments 

 of corals along the upper face, which is about 5 to 6 feet above 

 high-water mark. These coral fragments are derived in part 

 from the corals living on the lagoon face of the northern shore, 

 and in part of fragments broken by the waves from somewhat 

 below the low-water mark. The ledge which underlies the 

 beach crops out at many places on the lagoon side of the 

 northern shore ; we traced it also along the shores of Avatoru 

 Pass, and about half-way across the narrow land running between 

 Avatoru and Tiputa Passes. It crops out also at various points 

 between them in the narrow cuts which divide this part of the 

 northern land of the lagoon into a number of smaller islands. 

 These secondary passes leave exposed the underlying ledge, 

 full of fossil corals. 



It became very evident, after we had examined the south 

 shore of the lagoon, that the ledge underlying the north shore 

 is the remnant of the bed, an old Tertiary coralliferous limestone 

 which at one time covered the greater part of the area of the 

 lagoon, portions of which may have been elevated to a consider- 

 able height. This limestone was gradually denuded and eroded 

 to the level of the sea. Passages were formed on its outside 

 edge, allowing the sea access to the inner parts of the lagoon. 

 This began to cut away the inner portions of the elevated lime- 

 stone, forming large sounds, as in the case of Fiji atolls, and 

 leaving finally on the south side only a flat strip of perhaps 

 2500 to 3000 feet in width which has gradually been further 

 eroded on the lagoon side, and also on the sea face to leave 

 only a narrow strip of land about 1000 feet in width and perhaps 

 10 to 14 feet in height, the material for this land having come 

 from the disintegration of the ledge of Tertiary limestone, both 

 on the sea face and the lagoon side. 



The underlying ledge is not the remnant of a modern reef ; 

 its chaiacter is identical with that of the elevated limestones of 

 Fiji, which are of Tertiary age, and the rock is in every respect 

 the same as that I observed on many of the elevated islands of 

 Fiji. The atoll of Rairoa is in a stage of denudation and 

 erosion very similar to that of Ngele Levu, in Fiji, only in 

 Ngele Levu the elevated limestone attains a height of about 

 60 feet. Our visit to the south shore of the lagoon, both on 

 the lagoon side and on the sea face, left us no doubt regarding 

 the character of the underlying ledge of the north shore. As 

 soon as the south shore was sufficiently near, as seen from the 

 lagoon side, for us to distinguish its character, we could see 

 that the entire shore line was formed of a high ledge of lime- 

 stone, honeycombed, pitted and eroded, both by atmospheric 

 agencies and the action of the waves, in its lower parts both on 

 the lagoon side and on the sea face. The great rollers of the 

 weather side broke through between the columnar masses of 

 the ledge into the lagoon, and as far as the eye could reach 

 there extended a more or less continuous wall. 



Crossing over to the weather side of the southern land of 

 Rairoa in one of the passages between two of the islands, we 

 came upon the limestone ledge, from 12 to 14 feet high and 

 about 40 to 50 feet wide, which formed the sea face of the 

 islands and islets, and extended far to the westward as a great 



NO. 1604, VOL. 62] 



stone wall more or less broken into distinct parts. We found 

 this ledge to consist of elevated limestone as hard as calcite, 

 full of corals, honeycombed and pitted, and worn into countless 

 spires and spurs, and needles and blocks of all sizes and shapes, 

 separated by deep crevagses or potholes, recalling a similar 

 scene in Ngele Levu on the windward end of the lagoon. 

 In the passages the parts of the ledge which had not been 

 eroded extended as wide buttresses, gradually diminishing in 

 height till they formed a part of the lagoon flat and extended 

 out below the recent beach rock which covered it in short 

 stretches. 



The amount of water which is forced into such a lagoon as 

 Rairoa is something colossal, and when we observe that there 

 are but a small number of passages through which it can find 

 its way out again on the leeward side, it is not surprising that 

 we should meet with such powerful currents (7 to 8 knots in 

 several cases) sweeping out of the passages on the lee sides. 



The islands and islets of Rairoa are fairly well covered with 

 low trees and shrubs and large groves of palm trees. 



It was with great interest that we approached Makatea, as it 

 is the only high elevated island of which Dana speaks as occur- 

 ring in the western Paumotus. For though he mentions some 

 others as possibly having been elevated 5 to 6 feet, yet he con- 

 sidered them all, as well as Makatea (Metia or Aurora, of 

 Dana) as modern elevated reefs. Yet, from the very description 

 given by him of the character of the cliffs and of the surface of 

 Makatea, I felt satisfied that it was composed of the same 

 elevated coralliferous limestone so characteristic of the elevated 

 reefs of Fiji, and which, from the evidence of the fossils and the 

 character of the rock, both Mr. Dall and myself have been led 

 to regard as of Tertiary age. 



The cliffs had the same appearance as those of Vatu Leile, 

 Ongea, Mango, Kambara, and many other elevated islands of 

 Fiji. There were fewer fossils, perhaps, but otherwise the 

 petrographic character of the rock was identical with that of 

 Fiji. 



The south-western extremity of the island sloped gradually to 

 the sea, and showed two well-defined terraces. The lines of 

 these two terraces could, as a rule, be traced along the faces of 

 the vertical cliffs by the presence of caverns along the lines of 

 those levels, similar to the lines of caverns indicating the line 

 of present action of the sea at the base of the cliffs. 



During our stay in Papeete some time was spent in examining 

 that part of the barrier reef of Tahiti which had been surveyed 

 by the Challenger. We found the condition of the outer slope 

 of the reef quite different from its description as given in the 

 Challenger narrative. The growing corals were comparatively 

 few in number, and the outer slope showed nothing but a mass 

 of dead corals and dead coral boulders beyond 16 or 17 fathoms, 

 few living corals being observed beyond 10 to 12 fathoms. 



We also made an expedition to Point Venus, to determine, 

 if possible, the rate of growth of the corals on Dolphin Bank 

 from the marks which had been placed on Point Venus by 

 Wilkes, in 1839, and by MM. Le Clerk and de Benaze, of the 

 French navy, in 1869. We found the stones and marks as 

 described, but on examining Dolphin Bank in the steam launch 

 I was greatly surprised to find that there were but few corals 

 growing on it. I could see nothing but sparsely scattered 

 heads, none larger than my fist ! the top of the bank being 

 entirely covered by Nullipores, although we sounded across the 

 bank in all possible directions and examined it thoroughly. It 

 is greatly to be regretted that Dolphin Bank was not examined, 

 neither in 1839 nor in 1869, and notes made of what species of 

 corals, if any, were growing on its surface ; for an excellent 

 opportunity has been lost to determine the growth of corals 

 during a period of 60 years. The choice of this bank as a 

 standard to determine the growth of corals was unfortunate, as 

 it is in the midst of an area comparatively free from corals. 



From Papeete we steamed back to Makatea, and examined 

 the island more in detail. We crossed the island from west to 

 east, the path leading down from the summit of the cliffs 

 bordering the island into a sink at least 40 to 50 feet lower than 

 the rim of either face of the island. The sink occupies a little 

 more than one-third the length of the island. It is deeper at 

 its southern extremity, where it is said to be 75 to 100 feet 

 below the rim of the adjoining cliffs. 



It is difficult to determine if this sink is the remnant of the 

 former lagoon of the island, or of a sound formed during its 

 elevation, or if it has been formed by the action of rain and 

 atmospheric agencies. The amount of denudation and erosion 



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