242 A. Agassiz — Great Barrier Reef of Australia. 



tliem gradually to reef flats and flanking reefs. Comparatively 

 few corals grow upon the reef flats proper ; they are usually 

 covered with dead corals, and it is upon the steep flanks or 

 upper gentle slopes of the reef flats that corals flourish in 

 greatest abundance from a depth of ten fathoms upward. 

 Their principal belt of growth seems to be between seven and 

 three fathoms, and often to low water mark. 



The existence of alluvial deposits, at a depth of nearly 

 one hundred feet, in the vicinity of Townesville, plainly indi- 

 cates that previous to the last elevation of the (Queensland 

 coast, there has been a subsidence of at least that depth along 

 the northern part of the eastern coast of Queensland. This sub- 

 sidence, however, has only played an insignificant part in the 

 formation of the Great Barrier Reef ; it is to the extensive 

 action of erosion and of denudation apparent in all directions 

 that we must look for the main causes which have shaped the 

 submarine plateau off the coast of Queensland and prepared it 

 for the growth of the Great Barrier Reef. At first the denu- 

 dation and erosion of the northeastern part of the Australian 

 continent must have taken place only on the outer edge of the 

 continental plateau and did not extend to that part of the con- 

 tinent which has subsequently been changed by the same 

 causes to the present inner navigable channel. Thus an outer 

 barrier reef alone may then have existed with an inner channel 

 of perhaps fifteen fathoms depth, and a very narrow belt of an 

 inner series of flats and reef flats with islands and islets ofl the 

 then existing coast of Queensland ; much as we now find a 

 broad inner belt of flats and islets, and an inner navigable 

 channel with its islands and archipelagos, lying within the 

 older and outer reef belt in which all islands and islets have 

 long ago been reduced to flats and reef flats, most of which do 

 not rise to the level of the sea. 



The very moderate subsidence which has taken place in 

 comparatively recent times cannot have shaped the outlines of 

 the present Australian continent and of its submarine exten- 

 sion. For this we must look back first to the subsidence which 

 took place in Cretaceous times, next to the subsequent eleva- 

 tion of the Cretaceous beds and finally to the erosion and 

 denudation to which these beds, since their elevation above the 

 level of the sea, have for so long a period been subjected. It 

 is on the upper part of these submarine slopes, dating back to 

 an earlier geological period, but modified by erosion and denu- 

 dation up to recent times, that during the present epoch corals 

 have obtained a footing and built up the Great Barrier Reef 

 of Australia. 



HoF far the Cretaceous subsidence and the subsequent great 

 elevation of these beds in Australia extended to the eastward. 



