THE AUSTRALIAN GREAT BARRIER REEF. 129 



The point is now arrived at when, following upon a brief descriptive summary of the 

 physical features of the Great Barrier Reef, attention may be directed to the evidence throw- 

 ing light upon the telluric conditions under which this vast coral rampart was originally 

 fabricated. 



As shown in the accompanying map, and made manifest in the foregoing descriptive 

 narration, the Great Barrier Reef, throughout its extent, follows, more or less closely, all the 

 sinuosities of the intra-tropical north-east coast-line of Australia, and is concurrent in the 

 extreme north with the shallow soundings of Torres Strait. Its most prominent factors 

 have been shown to consist of a long linear chain of reefs, that constitutes its eastern 

 boundary, outside whose limits water over 100 fathoms deep is immediately reached. 

 The distance of the outer margin of the Barrier from the mainland has been found to vary 

 from as little as ten or twelve miles, at certain isolated points, to as much as ninety miles 

 off Cape York, in the extreme north, and to 160 in the vicinity of Swain's Reefs, in the 

 extreme south. Its average distance from the mainland, however, through the greater portion of 

 its length, is from twenty to thirty miles. The area enclosed within the outer boundary of the 

 Barrier includes a navigable lagoon channel, with an average depth of from fifteen to thirty 

 fathoms. The remaining very extensive superficies is for the most part occupied with coral 

 islets, and with shoals and reefs of every conceivable shape and size. Scattered among them 

 are a few islands of higher elevation, and compounded of the same granitic or bed-rock 

 formation, as the strata of the mainland. 



A few reefs and cays, with surrounding depths of over 500 fathoms, occur at more or 

 less remote distances from the Barrier's outer border ; they, for the most part, exhibit an atoll-like 

 plan of construction. Well-defined channels and passages interrupt the continuity of the 

 Barrier at a few irregular intervals. The most conspicuous of these are associated with its 

 southern moiety, and are opposite to, though at very considerable distances from, important river 

 estuaries. 



Although no direct evidence has been adduced to indicate that the Great Barrier 

 Reef of Australia originated under those movements of subsidence, claimed by Mr. Darwin 

 for the formation of barrier reefs in general, it has, up to within a recent date, been tacitly 

 conceded that the structure now under consideration could have come into existence under 

 no other more logically explicable conditions. Mr. Jukes sought diligently, in the earlier 

 days of the Barrier's survey, for any positive evidence of a motion of upheaval. The huge 

 blocks of coral rock lodging near the outer margin of the Barrier a little north of Raine 

 Island, referred to on another page, and the observed phenomenon of pumice-stone cast up on 

 mainland beaches, in various localities, many feet above the normal limits of the tide, are 

 the most substantial testimony that he could bring forward. In the former instance, however, 

 there can be little or no doubt that the rock-masses he described, are of the same storm- 



