AGASS1Z: THE GKEAT BARRIER REEF OF AUSTRALIA. 



137 



diagram he gives of an imaginary section of the Great Barrier Reef, and 

 which I here reproduce. 1 



Jukes's Section across the Great Barrier Reef. 



a. Sea outside the barrier, generally unfathomable. 



b. The actual barrier. 



c. Clear channel inside the barrier, generally about 15 or 20 fathoms deep. 



d. The inner reef. 



e. Shoal channel between the inner reef and the shore. 



F. The great buttress of calcareous rock, formed of coral and the detritus of 



corals and shells. 



G. The mainland, formed of granites and other similar rocks. 



It seems strange that Jukes should have given a section across the 

 Great Barrier Reef, and have left out the islands which crop out nearly 

 all along the coast of Queensland between the mainland and the outer 

 or inner line of reefs. This would have given his section an entirely 

 different aspect, as he would have had, cropping up and connected with 

 the line of the mainland, a series of peaks rising from ten to thirty 

 fathoms, round which alone, or round the flat bases of islands and peaks 

 which had disappeai'ed from erosion or other atmospheric causes, corals 

 had grown. Such a section is not an imaginary one, for the channels 

 between 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 compara- 

 tively 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. See sections across the Great Barrier Reef, 

 Plates XXXVII. to XLI. That Jukes himself felt his imaginary sec- 

 tion and his explanation of it not to be of universal application can be 

 proved from his own words. 2 He says : — 



1 Voyage of the " Fly," Vol. I. p. 333. 



2 Ibid. 



