130 LORD HOWE ISLAND. 



The felspars seem to belong to two generations. 



The lath-shaped microcrystals with ragged ends in the base were evidently 

 formed during the cooling of the magma, but the porphyritic crystals, to 

 judge from their broken and corroded appearance, were probably formed 

 previously. The latter are colourless and clear. 



The augite occurs in minute granules, as a constituent of the base, and 

 also in large well-formed eight-sided prisms bounded by sharp edges. Some 

 of the augite crystals are corroded at the edges and fractured. 



The large crystals of augite are not penetrated by the felspar, but com- 

 pletely invest or are moulded on to the grains of olivine. 



The large crystals are of a pale-straw colour nearly colourless, with 

 yellowish-brown streaks along the cracks. 



The olivine is in rounded grains, partly serpentinized along the cracks into 

 a bottle-green fibrous mineral, rather strongly pleochroic. 



The olivine was evidently formed before the consolidation of the base, as 

 proved by its enclosure in the large perfect crystals of augite, which must 

 have formed contemporaneously with the base. In one part of the slide a 

 little secondary actinolite (?) is visible. 



Specimen No. 1 [Slide No. 3], from Observatory Point. — This is a dense 

 basalt of a dark greenish-brown to black colour and flakey fracture ; it is rich 

 in olivine, the olivine grains bring from i inch to \ inch in diameter. 



The specific gravity is 3'05. 



In microscopic sections the rock is seen to consist of a micro-crystalline 

 ground-mass of triclinic felspar, granular augite, and dendritic aggregates of 

 magnetic iron, and grains of olivine and crystals of augite and triclinic 

 felspar occurring porphyritically. 



The grand-mass appears to be wholly devitrified, with the exception of 

 one oval patch in part of the section, which contains a little glass. 



The less-decomposed parts of the rock are gray, while those more affected 

 by decomposition are of a reddish-brown colour. 



The triclinic felspar in the ground-mass occurs in minute lath-shaped striated 

 crystals. 



The larger porphyritic fragments of felspar are much broken and corroded. 

 One of them is moulded on to the side of a large grain of olivine, and in one 

 case some crystals of triclinic felspar are completely invested by a large 

 crystal of augite. 



The augite occurs in the ground-mass in microscopic granules, and also 

 porphyritically. The large augite crystals have been much eaten away, like 

 the large felspars. Magnetic iron occurs in the base in black crystalline 

 aggregates, and also in single crystals, showing as opaque black squares and 

 triangles. 



Ilmenite is also present, in well-marked rhombohedral sections. 



The olivine in this basalt is quite clear and transparent, and remarkably 

 free from decomposition, excepting along the cracks, and around the margins 

 where it has decomposed into a reddish brown mineral, which has lent a 

 similar colour to the rock, when viewed in thin sections by transmitted 

 light. 



This reddish-brown secondary product is probably formed partly from the 

 magnetic iron, but its presence is most marked close to the olivine grains. 



A little greenish decomposition mineral is also observable in the gray parts 

 of the base. This probably results from the alteration of the granular augite 

 into chlorite. 



Specimen 35, from north-east point of Ned's Beach, is a cellular basalt 

 slightly porphyritic by augite. The gas-pores are lined or completely filled 



