AGASSIZ: THE GREAT BARRIER REEF OF AUSTRALIA. 127 



until the growth was stopped or greatly diminished by coming into a 

 region such as the Whitsunday Archipelago, where the amount of silt 

 and mud annually washed down the slopes of the mainland and of the 

 islands prevented their successful spreading. 



The amount of detritus which is brought down by the Queensland 

 rivers is enormous. The channels of the river harbors, such as Bris- 

 bane, Rockhampton, Douglas Harbor, and Cooktown, are constantly 

 being dredged to keep them open for navigation. During the seasons 

 of flood and at other times, this silt is carried a long way off the coast, 

 and materially affects the purity of the water within a considerable dis- 

 tance off shore. Adding to this the wash from the hillsides, it is not 

 astonishing to find comparatively few corals growing close to the 

 shore, and these always more or less affected at certain seasons by the 

 impurities in the water. We may account in this way for the gradual 

 killing of the corals formed in Moreton Bay, and on the northern ex- 

 tremity of the Breaksea Spit, and elsewhere. 



It has been stated by most writers on coral reefs that a barrier reef 

 could only be formed in a region of subsidence. But it seems to me that 

 what I have seen of the Great Barrier Reef of Australia leads to no such 

 conclusion, — and it certainly is not the case in Florida. On the con- 

 trary, the present condition of the Great Barrier Reef cau be satisfac- 

 torily explained by the mere action of erosion and of denudation, which 

 has been going on for so long a period along the coast of Queensland. It 

 undoubtedly is the same erosion and denudation which have separated 

 Northern Queensland from New Guinea, and have left the shallow conti- 

 nental shelf which now unites them (Plate XXIV.). Here and there 

 the islands and islets and reefs which stud the whole of that shallow 

 sea attest sufficiently to this former connection, — a connection which 

 existed at the time when the desert sandstone was raised in post-creta- 

 ceous times from 2,000 feet or more above the sea level, and has since 

 then been exposed to the most extensive erosion and denudation, — the 

 larger and more numerous islands which extend north of Cape York 

 towards New Guinea, holding to it and to Australia much the same 

 relation which the various archipelagos off the east coast of Queensland 

 once held to the Australian continent. 1 A summary of the geological 

 history of Australia has been given by Mr. A. C. Gregory. 2 



1 See Rattray, A., Notes on the Geology of Cape York Peninsula, Quart, .tyur. 

 Geo'. Soc, 1869, Vol. XXV. p. 297. 



2 Proceedings and Transactions of the Royal Geographical Society of Australia, 

 Queensland Branch, Vol. II. Pt. 3, p. 164. Mr. Gregory says : — 



