148 THE NATURALIST IN AUSTRALIA. 



when the tide is up. This infant coral islet, which measures some twenty feet only 

 in length, is the direct produce of the swirling currents that circulate at high 

 water over the surface of the flat, lifeless expanse of platform reef. During some 

 abnormal tide or storm this heap of loose fragments was first swept into its 

 present position and constitutes the nucleus of an islet that will probably hereafter 

 become amalgamated with Gun Island. The living corals which are to be found 

 growing only around the circumference of this platform reef, beneath low water mark, 

 are thus shown to take no immediate part in the building up of the coral island, 

 which is the direct outcome of the winds and currents acting on the previously 

 detached reef debris. 



The power and selective force of the waves and currents in determining the 

 character of the beaches and the subsequent formation of coral islands, such as the 

 Houtman's Abrolhos, is very instructively illustrated by the two photographic views 

 reproduced in Plate XXVI. The lower of these illustrations depicts an area of 

 the open beach of Pelsart Island, a little to the east of Wreck Point, which is 

 composed almost entirely of the eroded crateriform coralke of the Madreporee 

 that originally grew upon and have become detached from the steep escarpments 

 of the outer reef. The slightly raised coral limestone cliffs at Wreck Point, figured 

 in the Chapter heading, are also composed for the most part of aggregated coralla of 

 the same description, the separately embedded coralla being discernible at various 

 points in the original negative. A striking contrast to the picture last described, with 

 its ponderous elementary components, is afforded by the second one, immediately 

 above it. Here, in place of massive coralla, the beach, for a long stretch, is composed 

 almost exclusively of the pearly shells of the univalve Molluscs Turbo margaritaceus 

 and Trochus coerulescens. This wonderfully prolific shell beach also belongs to Pelsart 

 Island, but is situated a little away inside the lagoon, where comparatively quiet 

 water prevails. 



Passing on to the consideration of zoological groups, other than the Madreporaria, 

 which bear testimony to the essentially tropical character of a large portion of the 

 marine fauna of Houtman's Abrolhos, we find that some of the most remarkable 

 evidence is yielded by that group of the Echinodermata, distinguished by the title 

 of the Holothuridse, which comprises the so-called Sea-Cucumbers or Trepang 

 and B6che-de-Mer of commerce. Torres Straits and the Northern moiety of the 

 Queensland Great Barrier Keef represent the regions on the Australian coast which 

 have alone, so far, yielded the most valuable varieties of the last-named marine 



