R. A. Daly — Problems of the Pacific Islands. 169 



for a number of years ; secondly, because the listing of many 

 questions connected with this one subject will serve to suggest 

 the great number of questions attaching to every one of the 

 major Pacific problems. The latter conception is so essential 

 to a full understanding of the need for systematic exploration, 

 that I trust you will forgive the tediousness of the enumera- 

 tion. 



Subsidence theory of coral reefs. — The older theory is 

 famous because of its masterly presentation by Charles Darwin 



Fig. 3. 



Kilometers 



Fig. 3. A. North-south, section of the island of Kauai. Water shown in 

 solid black ; vertical and horizontal scales the same. 



B. Section of the same island after assumed subsidence, with the growth 

 of a (barrier) reef upward and outward on its own talus (T). Between the 

 reef and central island is the "moat," the upper part of which is occupied 

 by the waters of the lagoon. The detrital filling of the "moat" is not 

 shown. 



C. Section of the same island after further assumed subsidence, with 

 further widening of the "moat" and the formation of an atoll. 



and James Dwight Dana. The accompanying diagram, copied 

 from Darwin's book, will serve to recall this conception of reef 

 origin (tig. 2). A volcano, originally built up above the sea- 

 level, Z, becomes colonized by corals, which quickly construct 

 the fringing reef, F. The island is now supposed to share in 

 a slow downward movement of the sea-floor; the corals grow 

 upward to form a barrier reef, JS, when sea level is at L '. 

 Between the reef B and the central island is a depression or 

 " moat," tilled with sea-water and bottom deposits of the 

 so-called lagoon. When sea-level has reached the position L" , 

 the volcano has already disappeared and the atoll, A, is all that 

 is left to betray the former existence of the volcanic island. 



Am. Jour. Sci.— Fourth Series, Vol. XLT, No. 242.— February, 1916. 

 12 



