264 THE SPONGES OF THE WEST-CENTRAL PACIFIC 



to the fringing reef to study its sponge fauna. Truk lies between 7° and 8° 

 north latitude, between 151° and 152° east longitude. 



Kuop Atoll, a short distance south of Truk, is as typical an atoll as are 

 most of those in the Marshall Islands. I found it very interesting to study. 



Ponape has not sunk so deeply as Truk, and it follows that much of the 

 area of the valleys between the volcanic peaks is still above sea level. There are 

 drowned valleys, however. The lagoon around Ponape is, of course, much 

 narrower than that about Truk, usually only 1 to 5 kilometers wide. The main 

 land mass is about 20 kilometers in diameter. Because of its heavy rainfall and 

 proportionately large land area, the Ponape Lagoon must have exceptionally 

 rich content of whatever organic and inorganic material may be dissolved out 

 or washed down from the land. Ponape is about 7° north latitude and a little 

 more than 158° east longitude. 



The Marshall Islands, exclusive of Eniwetok (162° 30' east) lie between 

 165° and 175° east longitude, and from nearly 5° to about 12° north latitude. 

 No volcanic rock occurs naturally in any. (This is to say that, if each was 

 originally a volcanic cone with a fringing reef, the cone has sunk clear out of 

 sight and only the living, growing reef remains.) The Japanese geologist, 

 Risaburo Tayama, who studied this region carefully, writes (1935) that the 

 coral reefs are distributed with such lack of uniformity in the Marshalls, 

 Carolines, Palaus, and Marianas that he is reluctant to accept Daly's glacial 

 control theory. He prefers the venerable subsidence theory of Darwin. I find 

 Tayama convincing in this regard. 



The Marshalls include a few small isolated islands, such as Mejit. Per- 

 haps these had volcanic bases which never more than barely reached the 

 surface. Typically, however, the unit in the Marshalls is a large atoll. There 

 is a lagoon upwards of 25 kilometers in diameter, surrounded by a rim of 

 reef and islets. In a few cases, notably Namorik, no deep channel pierces the 

 ring, but usually there are one or a few deep channels through which ships 

 may enter the lagoon. The rim is usually less than half islet, more than half 

 reef. The islets are often as little as 100 to 300 meters wide but may be sev- 

 eral kilometers long. The reef which connects the islets, like the string on 

 which a necklace of beads is strung, is often so near the surface that at low 

 tide one could wade from one islet to the next. 



The deeper portions of these reefs are probably chiefly calcium carbon- 

 ate in a variety of conditions varying from solid limestone to colloid. On the 

 outer side, the wall is perpendicular or even overhanging, especially on the 

 southwest or lee side. At a depth of 50 meters or so, however, a slope curves 

 outward and at great depths becomes more and more gentle. On the inner 

 side the reef yields to a gradual slope until in the lagoon a depth of 20 to 40 

 meters may be reached. Occasional small islands of live coral rise nearly to 

 the surface of the lagoon, but most of its bottom is covered with dead calcar- 

 eous debris, foraminiferal sand, fragments of coralline algae, and bits of 



