A. Agassiz — Great Barrier Beef of Australia. 241 



slope of tlie continent outside of the reef. They show that it 

 varies greatly, being quite steep to the south of Break Sea Spit, 

 beyond the range of the coral reefs, while off the barrier reef 

 it shelves more gradually as we go northward and approach 

 the extensive shallow submarine plateau uniting northeastern 

 Australia with ]^ew Guinea. The depth of the inner naviga- 

 ble channel varies from six to seventy fathoms at its southern 

 funnel-like opening, to a general average of from ten to sixteen 

 fathoms northward as far as Cape York. The passages between 

 the inner reefs and the reef patches leading from the inside 

 channel to the inner edge of the very outer reefs vary in depth 

 from ten to twenty-five or even thirty fathoms. 



The many islands with which the coast of Queensland is 

 studded are either detached single islands or small groups of 

 islands, or even extended archipelagos, and with few exceptions 

 they are at no great distance from the mainland. The aspect 

 of the deeply eroded flanks of the coast mountains, the exist- 

 ence of extensive high table levels, characteristic of the adja- 

 cent islands also, plainly indicate, when taken together, that the 

 coast of Queensland has for a long period been subjected to 

 very extensive denudation and erosion, and that the islands 

 occurring along its eastern face were once a part of the main- 

 land. This supposition is fully confirmed by what is known of 

 the geology and botany of the mainland and of the adjacent 

 islands. 



As a general rule, the islands farthest removed from the 

 Queensland coast have been longest subjected to the agencies 

 which have separated them from the mainland. Many of the 

 more distant remnants of the mainland are now mere islets 

 flanked by extensive flats, or they are steep isolated rocks or 

 clusters of rocks with rounded and worn surfaces devoid of 

 vegetation, or they are simply flats eaten away to below low 

 water mark. 



The conclusion seems inevitable that all the flats and reefs 

 lying between the outer line of reefs and the mainland are but 

 the remnants of former islands extending to the eastern edge 

 of the continental plateau ; islands which once formed a part 

 of the eastern coast of Queensland, and have by erosion and by 

 denudation gradually been separated from the mainland and 

 reduced to the flats forming the outer reef flats of the Great 

 Barrier Reef. This process has, according to the Queensland 

 colonial geologists, probably been going on since the end of the 

 Cretaceous period, and is even now going on at many points of 

 the Queensland coast. 



After the formation of the islands and islets and after their 

 separation from the mainland, corals began to grow upon the 

 eroded surfaces and flanks of the flats and banks, changing 



