FORMATION OF CORAL REEFS. 511 



sometimes other ridges, often at a considerable distance from the 

 shore, but still preserving a direction parallel to it. These are 

 termed barrier reefs ; and the most remarkable of them is the 

 one, which stretches along the north-eastern coast of New Hol- 

 land. The total length of this is above 1000 miles ; along 350 

 miles there is no break or passage whatever ; and for 700 miles 

 northwards towards New Guinea, there are no intervals exceed- 

 ing thirty miles in length. When we endeavour to account for 

 such a structure upon the commonly received opinions alone, we 

 are met by the same difficulty as that, which opposes itself to 

 the supposition of a rocky base to all the coral islands, at a depth 

 of less than 120 feet. There is no mountain range, extending to 

 anything like the length of this reef, of which the summits are so 

 nearly equal in height ; still less any one presenting a continu- 

 ous ridge, such as would be required for the base of the uninter- 

 rupted portion of the reef. We have, therefore, no right to 

 suppose that such a ridge should arise from the depths of the 

 ocean ; and yet without it, our knowledge of the habits of the 

 Coral Polypes does not enable us to account for the remarkable 

 structure under consideration. 



1079. This difficulty, however, appears to be satisfactorily 

 solved by Mr. Darwin's hypothesis. He has adduced several 

 reasons for the belief, that the bottom of the whole Pacific Ocean is 

 changing its level, in some parts slowly subsiding, whilst in 

 others it is undergoing gradual elevation. Now if we imagine 

 the whole of New Holland to have been at a former time con- 

 siderably more elevated than at present, its area would of course 

 have been greater, and it might have extended to the line of the 

 present reef. This reef might have then been formed as a 

 skirting reef in the usual manner ; stretching at the distance of 

 a few hundred yards along the whole coast in shallow water. If 

 a slow subsidence then took place, the coral would be kept up to 

 the surface of the water, by the labours of the Polypes, in an 

 almost unbroken ridge ; whilst the water would gradually gain 



or 500 yards off-shore, with a deep channel, into which ships can enter hy nume- 

 rous passages. These passages are generally opposite the mouths of fresh-water 

 rivulets. 



M M2 



