328 Voyages of Discovery and Survey. 



part ground down within its influence into sand, the general mass 

 increases. 



The reason why the great barrier reef is interrupted off the 

 southern coast of New Guinea, — for the coral conditions, and with 

 them the coral reefs, again obtain on its western shores, — would ap- 

 pear to be made apparent by the voyage of the " Fly." A bottom 

 of mud, the sediment from some great river or rivers flowing into 

 the sea out of this part of New Guinea, extends over the ground 

 which we can scarcely doubt would be occupied by coral reefs, if the 

 waters were clear. These muddy waters, every gale of wind stir- 

 ring up the bottom for a long distance out, eflfLctually keep off the 

 coral growth ; and clays, by the accumulation of the mud where it 

 can find sufficient repose, represent in geological time the calcareous 

 accumulations on each side, any difference which such conditions 

 make being impressed upon the animal life, the remains of which are 

 being entombed in the different parts of the general sea-bottom. 



Mr Beete Jukes shews that at Erroob and the Murray Islands, 

 there has probably been igneous action during at least the formation 

 of a part of the Eastern Australian and Torres' Straits' coral accu- 

 mulations. Volcanic substances are mingled with white limestone, 

 some of the pieces of the latter and of lava even shewing that they 

 have been rounded. In Erroob, rocks that have been in a molten 

 state are seen to cover the sandstones and conglomerates. These 

 volcanic vents at the northern part of the great barrier reef are sup- 

 posed to form part of a great belt of volcanic operations, ranging at 

 no sreat distance to the northward and eastward along the north 

 coast of New Guinea into the Solomon Islands, New Hebrides, and 

 New Zealand. 



As connected with the formation of the present reefs off Eastern 

 Australia by slow depression of the land, thus causing the corals to 

 raise the reefs in proportion as the general mass of land and the 

 neighbouring shore sank down, in the manner by which Mr Darwin 

 accounts for the atolls and barrier reefs round many islands, our 

 author points out a fact of nmch importance. There are flats of 

 coral conglomerate, half a mile wide, frequent on the north-eastern 

 coast of Australia, in and upon which, and upon other flats of the 

 same coast, pumice pebbles are abundantly scattered, about ten feet 

 above the present high-water mark. Indeed these pumice pebbles 

 are found at the same elevation for nearly 2000 miles along the 

 eastern coast. Hence Mr Beete Jukes infers the coast to have been 

 equally affected as regards elevation or depression since these pebbles 

 were accumulated, and that it has been slightly elevated, or at least 

 has not suffered any depression during a long period of time. He 

 allows for the piling action of breakers, which no doubt during the 

 heaviest gales of wind throw pebbles on shore far beyond the usual 

 average of high water, and would readily force before them such 

 light bodies as pumice pebbles ; and during such times, no doubt. 



