AGASSIZ: THE GEEAT BARRIER REEF OF AUSTRALIA. 113 



form a nearly continuous belt coating the slope of the reef flat ; the 

 patches 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. Xegro 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.). 



We 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 XL), which form a shallow lagoon (Plate XL), 

 bare at low tide. It is an interesting group, showing how an irregularly 

 shaped atoll can be generated merely by the same action of the sea, 

 which under certain circumstances forms an open arc, under others a 

 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.). 2 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 beche 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 accompanj'ing the formation 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), and 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. 



