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 reofs that a barrier reof 
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 can be satisfac- 
torily explained by the meré 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.). Неге and there 
the islands and islets aud 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 Capo York 
towards New Guinea, holding to it and to Australia much the same 
relation whieh the various archipolagos off the east coast of Queensland 
once held to the Australian continent." A summary of the geological 
history of Australia has been given by Mr. A. C. Gregory.” 
1 See Rattray, A., Notes on the Geology of Cape York Peninsula, Quart. Jour. 
Geol. Soc., 1869, Vol. XX V. p. 297. 
2 Proceedings and Transactions of the Royal Geographical Society of Australia, 
Queensland Branch, Vol. II, Pt. 3, p. 104. Mr. Gregory says: — 
