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 débris. 
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 coralle of the Madrepore 
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, fora 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 MHolothuride, which comprises the so-called Sea-Cucumbers or Trepang 
and Béche-de-Mer of commerce. Torres Straits and the Northern moiety of the 
Queensland Great Barrier Reef represent the regions on the Australian coast which 
have alone, so far, yielded the most valuable varieties of the last-named marine 
