AGASSIZ: THE GREAT BARRIER REEF OF AUSTRALIA. 121 



and a half to over two fathoms of water, being separated by lanes 

 of coral sand or by accumulations of fragments of dead corals. On 

 the reef flat proper the coral heads are reduced to diminutive heads, 

 and the greater part of the flats inside of the line of breakers, 

 which carries from four to five feet of water at average low tide, was 

 strewn with fragments and masses of dead corals of all shapes and 

 sizes, much like the condition of things we have seen on the surface of 

 the Turtle Eeef flat. They seemed even somewhat more abundant, 

 and the scattered masses and fragments were frequently cemented so as 

 to form a breccia of Madrepores, Astrseans, Porites, and the like, incrusted 

 with Nullipores and Alga?. Only here and there on this flat of from 500 

 to 1,000 feet wide could a few living corals be seen. 



A strong current was flowing westward over the reef flat, the result of 

 the back pressure from the mass of water poured over the outer edge of 

 the reef flat by the incessant succession of the huge breakers. 



There is no reason for considering the bulk of the dead corals on these 

 outer reef flats or strips as other than the remnants of the disintegrated 

 elevated reef which once covered these strips and the adjoining patches. 

 The process of erosion and disintegration which took place here must have 

 been more rapid, and has of course continued for a greater length of 

 time than in patches more to the westward, — patches which were pro- 

 tected to a great extent by the outer reefs, and did not become exposed 

 to the action of the sea till the outer reef strips had been reduced to 

 nearly the condition in which we find them now. There is every reason 

 to believe that the outer strips of flats now worn to below the level of 

 the sea were at no very distant time (geologically) covered by a reef, 

 which was elevated from ten to twelve feet above the highest level at 

 which corals are now growing. This reef was gradually cut into by 

 the work of the breakers, long water lanes must have crossed the 

 reef flats, and little by little the elevated reef was changed to a series 

 of walls and pinnacles, and as these became eaten away they fell to the 

 ground and were reduced to the smaller fragments of dead corals which 

 now are thickly strewn over the outer reef flats. Where the process 

 has not gone on quite so long a time, as in interior reef flats, we find the 

 pinnacles of coral heads attesting the former greater extent and elevation 

 of the,reef; while in stretches still closer to the mainland (as on Middle 

 Island and elsewhere) we still find large patches of the elevated reef 

 which have not been completely eroded or changed into negro heads. 



The coral living on the sides of the patches or on the top of the huge 

 heads clearly indicates that this part of the reef when elevated was 



