PROCEEDINGS OF SECTION C. 



403 



accepted by the eye as being of far larger bulk than the reality. On 

 near approach they resolve into masses of dead coral 5ft. or 6ft. in 

 height and of nearly equal breadth. 



The massive corals, Pontes, Astrea, &c., of which they are com- 

 posed, grow irregularly and circumferentially. It is impossible to 

 orient them by any axis of growth or to tell by inspection of the polyp 

 cells whether they are upright or upset. Once landed, a stranded 

 block might be welded by chemical action to the floor on which it stood. 

 No sign of coral breccia or superposed coral was noticed in connection 

 with the Cairns negroheads. 



No blocks occur on the central or leeward portion. . The whole 

 ■crop are confined to a zone 300yds. or 400yds. from the surf. Often 

 the grit-armed surf has hollowed a pool around the boulder and under- 

 <3ut the base, leaving a stout stalk by which it is attached to the reef. 

 (See fig. 2, plate I.). Above the block may be fantastically carved 

 into pinnacles and hollows. All are obviously melting away under 

 rapid erosion and possibly solution. 



A time measure is afiorded by encrusting oysters. These cluster 

 thickly on the exterior of the negroheads, and may even completely 

 sheath in armor a square foot of the surface. Single aged oysters 

 project like spurs, their point of attachment being not the umbo but the 

 ventral margin. 



Oyster on Denuded Coral Block, showing how the Original Base under the Umbo has 



vanished. 



Their history clearly is that the oyster was first fastened as usual 

 by the umbo, and that as soon as a fresh grip was gained in front so 

 the earlier support behind was removed. During the brief span of 

 the bivalve's life (say four or five years) a layer as much as 2in. thick 

 of the rock crust may have vanished. At this rate of erosion no great 

 antiquity can be ascribed to the negroheads. 



Both Kent and Agassiz have illustrated and described the Queens- 

 land negroheads, but offer opposing explanations of their origin. The 

 former regards them as jetsam flung up by hurricanes ; the latter con- 

 siders them as a residue of elevated reefs cut down by erosion to present 



