VOLCANIC ORIGIN. 207 



when it gradually recovered its compact, amorphous 

 character. 



As previously mentioned, the summit of Erroob 

 has a small hollow w 7 hich might be taken for a nearly- 

 obliterated crater, but there is only the slight evi- 

 dence of form in favour of such a supposition. 

 Thirty miles north of Erroob, another patch of ig- 

 neous rock shews itself in the centre of a small 

 reef, called by us Bramble Key, but the native name 

 of which is Caedha. This is a mass of rock twenty 

 or thirty feet high, and about twenty yards across, 

 in the centre of a coral reef, which has a sand key 

 on one end of it. Another small patch of the same 

 rock is seen three-quarters of a mile distant, to the 

 S.E. dry only at low water. This rock has a singu- 

 lar appearance, being a dark red cellular lava, the 

 cells of which are filled by a white earthy mineral 

 in a pulverulent state. 



It appears, then, that just at the northern end of 

 the Great Barrier reefs, volcanic vents have burst 

 their way through them at three separate points, 

 ground up the limestone rocks they met in their 

 passage, and ejected a quantity of molten matter 

 into and through them. But there is evidence of 

 this having happened, not once only, but probably 

 through a long interval of time. The sandstone 

 and conglomerates, containing pebbles of lava and 

 limestone, must have been formed horizontally be- 

 neath the sea. This is evident from their laminated 

 and stratified character, from the perfect sifting and 



