132 LORD HOWE ISLAND. 



The specific gravity of this rock, taken in water at 80° Pah., is 2822 for 

 average specimens, and 2*838 for the more pyritous. 



Under the microscope the rock is seen to be holocrystalline and com- 

 posed of a microcrystalline ground-mass doleritic in places with micropor- 

 phyritic crystals of augite and amygdules of chalcedony and calcite. The 

 ground-mass is formed of lath-shaped triclinic felspar, granular augite, a 

 grass-green secondary mineral, chiefly epidote, magnetic iron, and secondary 

 iron pyrites. The structure of the felspar and granular augite is subophitic, 

 and decidedly ophitic in the case of the microporphyritic crystals of augite. 



The felspars, with few exceptions, are sufficiently free from decomposition 

 to show distinct multiple-twinning. They penetrate the augites deeply with- 

 out interfering with the orientation of the crystal. In one part of the slide 

 they exhibit a zonal tangential arrangement around a microporphyritic 

 crystal of colourless multiple-twinned felspar. 



A few microporphyritic crystals of felspar are observable, several of 

 which have been fractured in situ, and the broken pieces considerably dis- 

 placed with regard to one another. The augite in the ground-mass is of a 

 pale yellowish-gray tinge, inclining in places to pale purplish-brown. Towards 

 the margins most of the augites are decomposed into a grassy-green epidote. 

 The microporphyritic augites are intefcrystallized with the lath-shaped 

 felspars. They show no signs of decomposition, and polarise brilliantly. 

 One large crystal, however, is in striking contrast to the others, being almost 

 wholly decomposed into a greenish mineral. The green secondary minerals 

 probably result from the decomposition of the augite, as shown by the 

 gradual merging of the latter mineral into the former, and the ophitic struc- 

 ture of this green mineral with the felspars. Tested with the single nicol 

 most of the green mineral shows pleochroism, but not so strongly as horn- 

 blende. In a few instances this green mineral is somewhat fibrous, and shows 

 a strong parallel cleavage in one direction. Acicular crystals of actinolite (?) 

 traverse some of the green patches, and sometimes show a radiate arrange- 

 ment. Chlorite is not definitely recognizable, and there is no evidence of the 

 existence of primary hornblende. 



Magnetite is present in the base and in the microporphyritic crystals of 

 felspar and augite. A few large grains of ilmenite are recognizable from 

 the fact that they are surrounded by rings of cloudy pale yellowish-gray 

 leucoxene. 



Opacite is plentiful, occurring in elongated shapes. 



Summary. 

 The chief conclusions to be derived from the preceding observations would 

 appear to be the following : — 



(1.) All the igneous rocks of Lord Howe Island (so far as represented 



by the collection examined) belong to the Basalt Group. 

 (2.) A vast period of time must have elapsed between the eruption of 

 the diabasic basalt and that of the comparatively recent olivine 

 basalt. 

 (3.) All the basalts, with the exception of the diabasic types, are pro- 

 bably not earlier than Tertiary, and some may be Post-Tertiary. 

 (4.) The diabasic basalt is probably Pre-Tertiary, and may be Palaeozoic. 



T. W. EDGEWOETH DAVID. 



[Plates.] 



Sydney : Charles Potter, Government Printer —1889. 



