HOUTMAN ABKOLHOS ISLANDS. 



167 



is without sea-water at any state of tide) on the island immediately north of 

 it. This latter hollow (PI. 12) has almost been filled up with guano 

 deposits resulting from the hosts of the Lesser Noddy, which breeds only on 

 Wooded Island and the adjacent one. The amount of guano produced by 

 the birds is so great that it is actually causing the death of these mangrove 

 trees on the northerly of the two islets, and quite half the thicket consists 

 of leafless and bleached branches. The nests of the Lesser Noddy are only 

 to be found in the mangrove trees. Since the guano workers meditate 

 an attack on this deposit, and their working months coincide with the 

 breeding season of the birds, it is quite possible that the Lesser Noddy may 

 leave these islands in the future. 



The Lagoon and Submekged Beefs. 

 The usual reef-flat occurs round the islets of the eastern rim. In fact, 

 this is a continuous structure running at least from Easter passage to the 

 passage south of Wooded Islet, and the islets are only elevations upon it. 

 On the seaward side the reef-flat varies in width, but it has the same 

 character throughout. The surface is fairly smooth, not eroded leaving 

 sharp points, as in some places in the Wallaby Group, where it is more 

 exposed to still water and rain than to the constant wash of the sea at high 

 tide. Large hollows occur here and there, and form shore pools at low tide 

 in which animal life abounds. As a matter of fact, this reef-flat was 

 probably our best shore-collecting ground in the whole of the Abrolhos. 

 On one occasion the sea was calm enough for us to examine the outer 

 margin of the reef from the motor dinghy. At the particular place visited 

 the reef-flat was about 100 feet wide. It rose slightly, not more than a foot, 

 at its extreme margin and then descended to a depth of 15-Jr fathoms in 



Text-figure 11. 



ejofeet from edge of reef FlartSe deplfi wo3 1st fat homa 



100 yards. It was not easy to fix our position and make soundings, being 

 shorthanded, and there was just enough swell to make very careful handling 

 of the boat a necessity, especially when near the edge of the reef-flat ; some 

 figures were obtained, hewever, and the diagram indicates the nature of 

 the slope. 



