REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 



91 



have now been driven elsewhere by indiscriminate slaughter. The fact that the caves 

 are situated a little above sea-level, proves that the island has been recently elevated. 

 A raised beach on the top of the cliffs confirms this supposition. 1 



The stratum of volcanic conglomerate at the base of the shore cliffs is, so far as can 

 be determined from the specimens examined, a phonolitic tufa. The bluish grey rock 

 is speckled with white kaolinised patches ; the ground-mass is waxy and considerably 

 altered, and is impregnated with limonite in some places. Under the microscope, the 



Nightingale Island, from the South. 



mass is composed of minute sections of nepheline, usually as grains, but frequently in 

 the form of parallelograms or hexagons. The nature of this mineral is also proved by 

 the microchemical reaction of sodium. These crystals are arranged in line, and like the 

 other mineral constituents show well-marked fluidal structure. Microliths of augite are 

 associated with the nepheline ; these are brownish, show no evident pleochroism, and 

 extinguish at angles large enough to prevent confusion with hornblende. Small sections 

 of sanidine are also present. Several minerals give the rock a microporphyritic appear- 

 ance, plagioclase being the most important, The felspar crystals are often twinned 



1 The above is a summary of Mr. Buchanan's geological observations at Nightingale (loc. cit., pp. 614, 615) 



