STRUCTURE OF ISLANDS. 



153 



and rocky, and many of them as much as 500 or 

 600 feet in height. The rocks of the mainland 

 and the islands immediately adjacent, are all por- 

 phyritic, like those described as composing Mount 

 Adolphus. Porphyry, sienite,* and siliceous schists, 

 or compact feldspar, compose all the other islands 

 which I visited, or of which I was able to pro- 

 cure specimens. These islands are in fact merely 

 the submarine prolongation of the great mountain 

 chain of the eastern coast of Australia. This 

 chain runs from Van Dicmen's Land, through 

 Bass's Strait into the colony of New South Wales, 

 which it traverses throughout, at a mean dis- 

 tance of 70 or 80 miles from the sea. It extends 

 along the whole of the North-east coast, where its 

 loftiest and most massive portion is between Cape 

 Upstart and Cape Melville, From that point its 

 mean height gradually decreases to Cape York, 

 where the hills are only 400 or 500 feet in height, 

 and then sinking below the sea, its highest pinnacles 

 only are seen, forming the islands of Torres Strait, 

 from Cape York Island to Mount Cornwallis, on 

 the coast of New Guinea. 



In this central north and south band of Torres 

 Strait there are no independent coral reefs. The 

 coral only occurs in small fringes round the islands. 

 The bottom is either mud or fine sand, and there is 

 a remarkable uniformity of depth, which scarcely 



* Cap Island has been erroneously supposed to be volcanic, 

 it is a bare mass of sienite. 



