AGASSIZ: THE GREAT BARRIER REEF OF AUSTRALIA. 107 



reef flat is partly covered with fragments of dead corals, and is undoubt- 

 edly underlain by the exteusion of the rocky platform of Langford 

 Island, part of which crops out as the rocky islets mentioned above, the 

 rest of the foundations having been worn away by atmospheric agencies 

 above the water line, and by the action of the sea below that line. 



On Woody Island, a small island to the east of Langford Island, the 

 corals have similarly grown upon the slopes of the flat which surrounds 

 the island, but the greater part of the island has been washed away, as 

 has been Langford Island. The coral flat is somewhat lower in the 

 middle than the outer edges. 



Thus far, we found the water of the inner channel of the Barrier 

 Reef comparatively muddy, considerable silt being held in suspension, 

 and this greatly interfered with the use of the water glass, as often we 

 could not clearly distinguish objects on the bottom beyond three or 

 three and a half fathoms. This may be due to the constant prevalence 

 of the trades, accompanied with rain, which scours the slopes of the 

 coast lands and of the adjoining islands, sweeping into the inner channel 

 a large quantity of silt. 



At Port Denison we examined the reef off Adelaide Point mentioned 

 by Kent. It is comparatively poor ; a great part of it is dead, silted 

 over by the material brought down by the river. There are a few fine 

 heads in good condition, in about four fathoms, on the outer edge of 

 this shore reef. But the greater part of the reef is covered with sea- 

 weed. Off Stone Island Reef (at the mouth of Port Denison), there 

 are many insolated heads, a number of which are dead. The bottom 

 of the bay is covered with fine mud and bi'oken shells. 



We reached Middle Island, about half way between Gloucester Head 

 and Port Denison, at low water, a long flat shoal extending from the 

 south face of the island. The outer face of this flat is coated with fine 

 heads of corals rising from six to four fathoms, and becoming less prom- 

 inent as they tend towards the shallower edge of the flat extending 

 from a steep coral sand beach to low-water mai'k. 



At the western extremity of this sand beach we come upon a fine 

 exposure of a coarse conglomerate (Plate IV.) similar to that off Cape 

 Upstart observed and described by Jukes. 1 This conglomerate beach 

 rock is elevated fully eight feet above the highest high-water mark. It 

 slopes towards the sea at an angle of from 8° to 10°. The Middle 

 Island conglomerate is made up of fragments and rounded pebbles of 



1 Captain Stokes ( Discoveries in Australia, Vol. I. p. 332, 1846) gives an account 

 of a raised beach near Cape Upstart, twelve feet above high-water mark. 



