74 THE GREAT BARRIER REEF. 



communication with the sea, such opening being invariably situated on the more sheltered or 

 lee-side. In the largest atolls this passage is of such a depth that large vessels can gain 

 entry by it. The origin and significance of the passage is clearly discernible in the small 

 atolls, where it constitutes a simple gutter-like channel for the escape of the impounded 

 water on the fall of the tide. The embankment of coral-rock on this lee-side is lowest and 

 thus constitutes the area of least resistance. The prevailing wind, moreover, the south-east 

 monsoon, drifts the enclosed water throughout the greater portion of the year towards the 

 natural opening. 



The reef formations referable to the second series in Mr. Darwin's classification are those 

 of the "Barrier" or "Encircling" type. His descriptive diagnosis of this modified variety, is 

 as follows: — "The term 'Barrier' has been generally applied to that vast reef which fronts 

 the north-east shore of Australia, and by most voyagers likewise to that on the western coast 

 of New Caledonia. At one time I thought it convenient thus to restrict the term ; but, as 

 these reefs are similar in structure and in position, relatively to the land, to those, which, like 

 a wall with a deep moat within, encircle many smaller islands, I have classed them together. 

 The reef, also, on the west coast of New Caledonia, circling round the extremities of the 

 island, is an intermediate form between a small encircling reef and the Australian barrier, 

 which stretches for a thousand miles in nearly a straight line." In a concluding summary, 

 Mr. Darwin remarks : " After the details now given, it may be asserted that there is not 

 one point of essential difference between encircling barrier reefs and atolls. The latter enclose 

 a simple sheet of water ; the former encircle an expanse, with one or more islands rising from 

 it." A highly characteristic feature of all barrier reefs — whether flanking an island-continent, as 

 in the case of Australia, or encircling a small islet, as in that of Bolabola, in the Society 

 Group — specially quoted by Darwin — is that the descent into deep water from their outer 

 margin is very abrupt. Along the Australian Great Barrier, soundings of over a hundred 

 fathoms, which rapidly shelve into abyssal profundity, occur in close proximity to the interior 

 margin that has no greater a depth than thirty or fortj' fathoms. While this same peculiarity 

 characterises most of the lagoon-islands, or atolls, of the Pacific and Indian areas, different 

 conditions, as hereafter shown, may be associated with certain of those of the Australian 

 region. 



Fringing, or shore-reefs, which represent the reef structures of the third order recognised, are 

 thus described by Mr. Darwin : — " Fringing-reefs, or, as they have been called by some voyagers, 

 shore-reefs, whether skirting an island or part of a continent, at first appear to differ little from 

 barrier reefs, except that they are generally of less breadth. As far as the superficies of the actual 

 reef is concerned, that is the case ; but the absence of an interior deep-water channel, and the close 

 relation in their horizontal extension with the probable slope of the adjoining land beneath the sea, 

 present essential pomts of difference." In another paragraph, the same authority writes : — " With 



