170 GEOLOG\ OF QUEENSLAND COAST, 



the outcropping peaks, islands, reefs, or reef flats are covered 

 with the telluric detritus derived from the decomposition of the 

 rocks forming those islands, or obtained from the slopes of the 

 mainland. Such a section would have shown the layer of corals 

 to be comparativel}^ thin, of not more than twelve to fifteen 

 fathoms, and it would have shown the great probability that the 

 outer line of reefs, even built upon similar bases, once connected 

 with the mainland, had not attained a much greater thickness. 

 8ee sections across the Great Barrier Reef (Plates xxxvii. toxli.). 

 That Jukes himself felt his imaginary section and his explanation 

 of it not to be of universal application can be proved from his 

 own words."^ He says : — 



" ' The most remarkable^deviations from this condition are in 

 the spaces between Cape Melville and Lizard Island, and at the 

 back of Wreck Bay and Raine's Islet. Now in each of these 

 cases there are islands of granite or other rocks advanced from 

 the mainland, and thus causing an original irregularity in the 

 depth of water, as it would be independent of the coral reef. 

 This is very remarkable in the space between 12° 20' and 11° 30', 

 where we have Cape Grenville, Cockburn Islands, and 8ir C. 

 Hardy's Islands, projecting towards Raine's Islet opening, and 

 Fair Cape and Cape Weymouth, Avith Forbes Island and Quoin 

 Island projecting towards Wreck Bay. Near Sir C. Hardy's 

 Islands there is also a remarkable narrow channel of deep water, 

 between them and the large Cockburn Reef, in which there is a 

 depth of thirty fathoms, while on each side of it is either a reef 

 nearly dry at low water, or a depth not exceeding ten fathoms. 

 This channel is about twenty miles long, rarely more than two 

 miles broad, and it runs in the same direction as the islands lie 

 off Cape Grenville, or about east-north-east, and points in a 

 straight line for Raine's Islet opening.' 



" It seems tome tljat Jukes has here struck the correct explana- 

 tion of the structure of the Great Barrier Reef. But having 

 examined only the two extremes, he did not perhaps realise that the 



Jukes, Voyage of H.M.S. " Fly," Vol. i., p. 883. 



