PETROLOGY. 



131 



with a white transparent zeolite. The weathered surfaces of the roch show 

 that it is composed of a greenish-hrown ground-mass speckled with rust, 

 (formed by decomposition of magnetite) and containing black crystals of 

 augite from J inch to i inch in length, a few crystals of striated triclinic 

 felspar -I inch long, and reddish-brown, partly decomposed, grains of olivine. 



The zeolite has a hardness of about 4, and a specific gravity of about 2'16. 



Before the blow-pipe it whitens, crumbles, and swells up considerably 

 into white semi-transparent beads. 



It probably belongs to the Chabazite Group, and resembles phacolite. 



Type 2 : Basalt toithmit olivine. /SpecmeM 25 [SlideNo. 4fi!.],from North 

 Point, Ned's Beach.^ — This rock is a much decomposed lateritic amygdaloidal 

 basalt, of a reddish-purple colour. The rock is soft enough to be cut with 

 a knife without much difFiculty. The amygdules consist chiefly of a pale 

 greenish-gray aragonitc. 



The rock, examined in thin sections under the microscope, is seen to 

 consist of a microcrystalline ground-mass of lath-shaped felspar too cloudy 

 to polarise, a feebly translucent brown mineral, a pyro.\enic decomposition 

 product filling in the sj^aces between the microscopic felspars, and crystalline 

 aggregates or individual crystals of haematite pseudomorphous after magnetic 

 iron. Crystals of a mineral resembling nosean (though possibly a variety of 

 pyroxene), It, inch to i inch in diameter, are plentifully distributed through- 

 out the ground-mass. 



The sections afforded by this mineral are generally six-sided, and occa- 

 sionally quadratic. 



Most of the crystals are bounded by an opaque zone of hematite, and the 

 greater part of the space so enclosed is reticulated with pscudomorphs of 

 haematite after magnetite. One crystal in particular shows the characteristic 

 zonal grouping of the enclosures remarkably well. 



Between the opaque hsematito grains in the nosean (?) crystals is a semi- 

 translucent yellowish oily mineral, which has evidently resulted from the 

 alteration of the nosean (?). 



Small and large amygdules also occur of a mineral somewhat similar in 

 appearance, of a pale greenish-yellow colour. The large amygdules have a 

 hardness of about 3f , are infusible before the blow-pipe, and effervesce briskly 

 in hydrochloric acid. They are evidently aragonite. 



The advanced state of decomposition of the only small specimen of this 

 rock available for examination precludes the possibility of the certain deter- 

 mination of the nosean-Hke mineral. 



Specimen No. 10 (34), from the west side of Thompson's Beach, is a 

 purplish-red scoria, perhaps a scoriaceous representative of the preceding 

 rock. 



Type 3: Diahasic lasalt. Specimen 24, [Slide No. 5 a], from beach 

 below Bobbins' House. — This is a hard, dense, dark greenish-gray rock, 

 irregularly jointed. The weathered surfaces are pitted with small 

 hollows, and in places small beads of chalcedony i",- inch in diameter 

 form minute excrescences. White and dark-green spots are visible on 

 freshly-broken surfaces. Some of these white spots are very soft, and 

 efli'ervesce strongly in hydrochloric acid ; others are hard, and cannot be 

 scratched with a steel penknife. The dark-green spots are aggregates 

 of iron pyrites and epidote. Iron pyrites is present in crystals and 

 crystalline aggregates disseminated through the mass of the rock, and 

 also lining the irregular rock -joints, in films k to k of an inch thick. At 

 times calcite takes the place of this interstitial pyrites in the joints, as 

 pointed out to me by Mr. P. Katte, Mineralogist to the AustralianMuseum. 



