AGASSIZ: THE GREAT BARRIER: REEF OF AUSTRALIA. 113 
form a nearly continuous belt coating the slope of the reef flat ; the 
patehes and heads diminishing again very rapidly in shallower water 
towards the inner edge of the reef flat where at low tide there is from 
two to three feet of water. The eastern Hope islet is low and woody 
with a steep coral sand beach, on which exists a small strip of beach 
rock. Negro heads of beach rock occur on the flat to the northwest of 
the western Hope Island. West Hope Island is like the eastern island, 
only somewhat longer ! (Plate XXXII.). 
Woe skirted along the eastern face of Turtle Reef to see the fine, some- 
what scattering belt of negro heads which we had partially examined 
before. The reef to the east of Turtle Reef is on the north face 
flanked by numerous coral patches with lanes of deep water crossing 
the reef flat. On the east face there is a belt of huge negro heads. 
About ten miles to the northeast of Cape Bedford we come upon the 
Three Isles (Plates VII. to XI.), which form a shallow lagoon (Plate XI.), 
bare at low tide. It is an interesting group, showing how an irregularly 
shaped atoll сап be generated merely by the same action of the sea, 
which under certain circumstances forms an open aro, under others à 
bank just awash, or islands or islets with banks to indicate their former 
greater extension. These islands were included in the general elevation 
which raised the Great Barrier Reef a few feet above high-water mark, 
At several points on the outer shores of the islands there are long stretches 
of coarse beach rock elevated about eight feet above high-water mark 
(Plate X.).? Their former extent was of course much greater, and 
little by little the beach rock has been disintegrated and reduced to 
1 In the Hope Islands and other places ballast is often thrown overboard by 
the béche de mer fishermen, and care should be taken not to mistake this ballast for 
remains of outcrops of rocks on the reef flats. 
2 Kent has given excellent figures on Plate XXXII. of coral breccia, of shell and 
rocky conglomerates and breccia, and has called attention to the rapidity with 
which their cementation takes place, a subject which has been frequently described 
by Dana, Darwin, Agassiz, and the many writers who have paid attention to the 
structure of coral rocks and the phenomena accompanying the formagion of coral 
reefs. 
Kent has very justly called attention again to the great development of some of 
the deep-sea corals, such as Lophohelia, Amphihelia, Dendrophyllia (in from 100 to 
500 fathoms or more), arid to: the possibility of their forming coral rock quite as 
rapidly as some of the Madrepores if brought within the action of the surf. We 
might add to this list Oculina, which occurs so plentifully in the deeper protected 
sinks of the Bermudas, and which off certain parts of the southern coast of the 
United States forms quite extensive banks in waters of moderate depths (to 50 
fathoms). Oculina is also found at the Galapagos under similar conditions. 
