AGASSIZ: THE GREAT BARRIER REEF OF AUSTRALIA. 191 
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 Reef flat. They seemed even somewhat more abundant, 
and the scattered masses and fragments were frequently cemented so as 
to form a breccia of Madrepores, Astrecans, Porites, and the like, incrusted 
with Nullipores and Alge. 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 te, as in interior reef flats, we find the 
pinnacles of coral heads attesting the former greater extent and elevation 
of the roef ; 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 
