90 



THE GREAT BARRIER REEF. 



" It is observed that there is a reasonable and significant correspondence between these two sections, which 

 as the plan indicates, arc taken nearly ha!f-a-niile apart. In each, the surface of the fringing-reef, after shelving 

 very gently downwards to a dejith of about three or four fathoms, is bounded Ijy a submarine cliff This in 

 one section (No. I.) continues almost unbroken to a depth of about 500 feet, except that a kind of edge or 

 terrace is clearly indicated at a depth of rather less than 100 feet. In the other section (No. II.) the foot of 

 a great submarine cliff is found at about 500 feet, but in this case the cliff is distinctly divided into 

 two precipices by a shelving bank of coral and sand, which begins at a depth of about 140 feet and reaches 

 the l)ro\v of the lower precipice at about 260 feet. This l)ank is covered by ' sand and coral.' At this depth 



No. I. 



-!^ 



"'y ^-cfj 



Island 



in each section the island is, as it were, defended by a deep and narrow ditch, the edge of its steep glacis being 

 formed by a sharp arrte of coral which in one case rises into soundings of about 250 feet. From this the former 

 section shows a second rapid fall tlown to another ditch, the bottom of which lies more than 1,200 feel below 

 sea level. This in section resembles the other one, and the height of its counterscarp is more than 300 feet. 

 From the edge of this, the glacis for a short distance is nearly level, and then descends at an angle of 

 some thirty degrees. In the lower diagram we find no indication of this second ditch, but a long slope begins 

 at the foot of the submarine cliff at a depth of about 850 feet, which is very nearly identical witli that of 

 the flat part of the glacis in tlie former section. 



" It will be observed that the upper ditch (that common to both sections) has its bottom at a depth of 

 full 500 feet, or about CS5 fathoms — that is, at more than three times the average depth at which reef-building 

 corals cease to live, while tlie least depth of the final submarine slope is S50 feet or more than 140 fathoms 

 These ditches seem irreconcilable with any idea of an outward-spreading growth of the reef, and must, I think, 

 be indicative of a subsidence wliirh isolated the outward and more flourishing edge of a shore reef, and progressed 



