JULV 26 IQOO] 



NATURE 



309 



to which this island has been subjected is very great, as is 

 clearly indicated by the small caiions, pinnacles, and walls of 

 limestone, as well as by the crevasses which occur in the sur- 

 face of the basin in all directions. The extent to which this 

 action has penetrated into the mass of the island is also plainly 

 shown by the great number of caverns which crop out at all 

 levels along the sea face of the cliffs, some of which are of great 

 height, and extend as long galleries into the interior of the island. 

 From Makatea, we visited Niau, Apataki, Tikei, Fakarava, 

 Anaa, Tahanea, Raroia, Takume, Makemo, Tekokota, Hikuero, 

 Marokau, Hao, Aki-Aki, Nukutavake, going as far east as 

 Pinaki, when we turned westward again and made for the 

 Gloucester Islands. These, as well as Hereheretue, proved 

 most interesting ; they formed, as it were, an epitome of what 

 we had seen on a gigantic scale in the larger atolls of the 

 western and central Paumotus. We could see at a glance in 

 such small atolls as Nukutipipi and Anu-Anurunga the con- 

 nection between structural features which, in an atoll of 40 

 miles in length and from lo to 15 miles in width, it was often 

 dirticult to determine. 



The deepest sounding among the Paumotus was on the line to 

 the northward of Hereheretue in the direction of Mehetia, where 

 we found a depth of 2524 fathoms, and a continuation of the red 

 clay characterising the soundings since we left Pinaki. 



We have seen nothing in this more extended examination of 

 the group tending to show that there has anywhere been sub- 

 sidence. On the contrary, the condition of the islands of the 

 Paumotus cannot, it seems to me, be explained on any other 

 theory except that in their present condition they have been 

 formed in an area of elevation — an area of elevation extend- 

 ing from Matahiva on the west to Pinaki in the east, and 

 from the Gloucester Islands on the south to Tikei on the north. 

 All the Paumotu Islands we have examined are, without ex- 

 ception, formed of Tertiary coralliferous limestone which has 

 been elevated to a greater or less extent above the level of the 

 sea, and then planed down by atmospheric agencies and sub- 

 marine erosion, the greatest elevation being at Makatea (about 

 230 feet), and at Niau, where the Tertiary coralliferous limestone 

 does not rise to a greater height than 20 feet. At Rairoa it was 

 15 to 16 feet high. At other islands it could be traced only as 

 forming the shore platform. 



The appearance of the old ledge and of the modern reef rock 

 is so strikingly different that it is very simple to distinguish 

 the two, even where only comparatively small fragments are 

 found. 



In the Paumotus, the islands have been elevated to a very 

 moderate height, and probably to nearly the same height, for the 

 old ledge forming the base of the modern structure is found ex- 

 posed nearly everywhere at about low-water, when it cannot be 

 traced at a slightly greater elevation. This would readily 

 account for the nearly uniform height of the islands throughout 

 the group. 



But there is another element which comes into play in this 

 group, and has an important part in shaping the ultimate con- 

 dition of these atolls. At the Fijis we have seen the submarine 

 erosion continue until there is little left of many of the atolls 

 beyond the merest small i-let or rock to indicate its structure. 

 In the Paumotus, in the great atolls which are evidently only the 

 exposed summits of parts of ridges or spurs of an extensive 

 Tertiary coralliferous limestone bed, the rim of the atoll is, 

 after having been denuded to the level of the sea, again built up 

 from the material of its two faces, which is thrown up on the 

 wide reef flats both from the sea face and from the lagoon side. 

 Many of the lagoons are filled with shoals or ledges awash 

 or a few feet above the sea-level. These shoals are parts of 

 the old ledge which have not as yet been eroded, and the dis- 

 integration of which has gone far to supply the material for the 

 land of the outer rims of the atolls. 



The lagoons of these atolls have a general depth of 13 to 20 

 fathoms. In some cases they are somewhat deeper, as is stated, 

 but there are no measurements, the greater depths, 30 fathoms 

 or more, being due to orogenic conditions. Some of the atolls 

 are quite shallow, as at Matahiva, as well as Pinaki, where the 

 lagoon is not more than 2 to 3 fathoms, and Takume, where it 

 is from 5 to 6 fathoms deep. Some of the smaller islets we 

 visited, among which are Tikei, Aki-Aki, and Nukutavake, have 

 no lagoons. 



The only atoll we have seen the lagoon of which is entirely 

 shut off from the sea is Niau. In this case the old ledge form- 

 ing the rim of the land, which surrounds the nearly circular 



NO. 1604, VOL. 62] 



lagoon, is about a third of a mile in width and sufficiently high, 

 15 to 20 feet, to prevent any sea from having access to it except 

 in case of a cyclone. It is very difficult in this case to decide 

 whether this lagoon has been gradually filled up after elevation, 

 or whether it is merely a sink on a more or less uneven limestone 

 surface. 



Dana and other writers on coral reefs mention a great number 

 of lagoons as being absolutely shut off from the sea. I take it 

 these statements are due to their descriptions being taken from 

 charts, many of which, as in the cause of the Paumotus, are 

 very defective. For nothing is easier than to pass at a short 

 distance by the wide or narrow cuts which give in so many 

 cases the freest access to the sea to the interior of the lagoons, 

 and described as closed because they have no boat passages. I 

 could mention, as instances of such lagoons, those of the atolls 

 of Takume, Hikuero, Anaa, &c., which may be said to be 

 closed, yet into which a huge volume of water is poured at 

 every tide over low parts of the encircling reef flats. 



The character of the coral reefs of the Paumotus is very 

 different from that of other coral reef regions I have seert. No- 

 where have I seen such a small number of genera, so many 

 small species, and such stunted development of the corals. 

 None of the great heads of the genera so charaAeristic of the 

 West Indian regions, or of the Great Barrier Reef of Australia, 

 are to be seen, with the exception of a couple of species of 

 alcyonaria they are absent, so far as our experience shows, and 

 there are but few sponges and gorgonians to be found among 

 the corals. 



The same paucity of animal life seemed to extend to the 

 deep-water fauna. All the hauls we made off the islands, in 

 from 600 to 1000 fathoms, usually the most productive area of 

 a sea slope, brought nothing, or so little that we came to grudge 

 the time spent in trawling on the bottom, as well as towing 

 on the surface or near it, a great contrast to the conditions in 

 the Atlantic in similar latitudes, and very different from our 

 anticipations. 



From Papeete we steamed to Aitutaki, Niue, and for the deep 

 hole of the Tonga- Kermadec Deep, about 75 miles to the 

 eastward of Tonga-Tabu, and in 4173 fathoms made a haul 

 with the Blake beam-trawl, by far the deepest trawl haul yet 

 made. We found in the bag a number of large fragments of a 

 silicious sponge belonging probably to the genus Crateromorpha, 

 which had been obtained by the Challenger in the Westerrt 

 Pacific, but in depths less than 5CX5 fathoms. We also brought 

 up quite a large sample of the bottom ; it consisted of light 

 brown volcanic mud mixed with radiolarians. 



On our way back to Papeete from the Paumotus we examined 

 the eastern coast of Tahiti, and from Papeete studied the 

 western coast as far as Port Phaeton, at Tararoa Isthmus. We 

 also examined, in a general way, the Leeward Society Islands j 

 Murea, Huaheine, Raiatea, Tahaa, Bora-Bora, M-otu Iti and 

 Maupiti. There are excellent charts of the Society Islands, so 

 that it was comparatively simple to examine the typical points 

 of the group and to gain an idea of their structure so far as it 

 relates to coral reefs. The Society Islands are all volcanic 

 islands edged with shore platforms, some of great width, upon 

 which the barrier or the fringing reefs of the islands have grown. 

 The structure of the reefs of the Society Islands is very similar 

 to that of the Fiji reefs round volcanic islands. A comparison, 

 for instance, of the charts of Kandavu, Viti Levu, Mbengha, 

 Nairai, and of other volcanic islands in the Fijis, with those of 

 the Society Group, will at once show their identity. Huge 

 platforms of submarine denudation and erosion characterise both, 

 with fringing and barrier reefs determined by local conditions. 

 Perhaps it is easier to follow the changes which have taken • 

 place m the Society Islands ; and such islands as Tahaa and 

 Bora-Bora, where we anchored, as well as Maupiti, are 

 admirable examples and epitomes of the structure aiTd mode of 

 formation of the coral reefs of that group. 



The only island of the Cook Group which we examined was 

 Aitutaki, as Atiu is composed of elevated limestone, and Raro- 

 tonga is volcanic ; I hoped we might find that atoll to be in 

 part volcanic and in part composed of eleva-ted coralliferous 

 limestone ; we found it to be volcanic, an island with the 

 structure of Bora-Bora on a smaller scale. 



We anchored at Niue, an island composed of elevated 

 coralliferous limestone showing three well-niarked terraces, 

 the lowest of not more than 5 to 10 feet and in many places 

 disappearing completely, the limestone cliffs -rising vertically 

 from the sea well mto the second or even the third terraces. 



