128 GEOLOGICAL STRUCTURE. 



varieties of oolite in texture and appearance. Ii 

 contained larger fragments of corals and shells, and 

 some pebbles of pumice, and it yielded occasionally 

 a fine sand that was not calcareous, and which was 

 probably derived from the pumice. Some parts of 

 it made a very fair building stone, but it got softer 

 below, till it passed downwards into a coarser coral 

 sand, unconsolidated and falling to pieces on being 

 touched. In the quarries that were opened next 

 year for the beacon, many recent shells, more or less 

 perfect, were found compacted in the stone, and 

 one or two nests of turtle eggs were discovered, of 

 which, in some cases, only the internal cast had been 

 preserved, but in others the shell remained in the 

 form of white carb. lime. Some drusy cavities 

 also were found in the stone, containing crystals of 

 gypsum, or sulphate of lime. The presence of this 

 mineral seems very odd, as I do not see whence the 

 sulphuric acid could proceed. It is evident from 

 the fossil turtle eggs, that the consolidation of the 

 stone had taken place after it was raised above the 

 sea. It was due, probably, to the infiltration of the 

 rain water percolating through the calcareous sand, 

 that had been gradually piled above high water 

 mark by the combined action of the winds and waves. 

 The thickness of the vegetable soil in its centre 

 shews that it has been above water for a great 

 length of time. 



The whole surface of the island was covered with 

 old and young birds. These consisted of frigate- 



