Davis. — S/f/nifjcanf Feaf tires of Reef -bordered Coasts. 



27 



difficult. It is of importance that tlie observer who has opportunity of 

 examining a dissected reef should locate the structural details that he may 

 discover with respect to the total reef-mass ; it is also important that he 

 should bear in mind the expectable structures of reefs formed according to 

 the several chief theories of reef-origin, as shown in figs. 7 and 8, for he 

 will thus be led to make special search for critical structures in their 

 appropriate locations. 



Fig. 7. 



Thus if the great body of an elevated reef consist, as in fig. 7, [of 

 steeply sloping layers of reef detritus mostly free from admixture with 

 volcanic sands and gravels, resting conformably upon a non-eroded volcanic 

 slope, T, and more or less complicated by slides, the reef should be 

 explained as a product of outgrowth during a prolonged still-stand period. 

 Darwin clearly recognized the possibility of reef-formation in this manner, 

 but regarded it as seldom occurring, because it would not result in the 

 formation of a reef-enclosed lagoon from 20 to 40 fathoms in depth. 

 Murray attempted to overcome this difficulty by assuming, as Semper 

 had before him, that the lagoon-cavity would be excavated by solution ; 

 but the assumption is not supported by the features of lagoons, as has 

 been noted above. 



On the other hand, an elevated reef may show a three-part structure, 

 as in fig. 8. The steeply dipping, exterior strata, T, may be formed of 

 detritus chieflv derived from the reef, but with some fine sands and silts 



Fig. 8. 



from the central island. The slanting layers may be sometimes compli- 

 cated by slide -structure as in the ])receding case ; they may rest on a heavy 

 deposit of volcanic detritus, D, which should be associated with a buried 

 cliff. The intermediate wall-like structure, R, should contain much coral 

 in place, as well as large and small fragments. The outward or inward 

 slant of the wall appears to be dependent on the rate of subsidence during 



