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 extension 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 tho 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. Тһе 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 broken 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 mark. 
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 J ukes.! This conglomerate beach 
rock is olevated 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. 832, 1846) gives an account 
of a raised beach near Cape Upstart, twelve feet above high-water mark. 
