118 



Tranmctions. 



In chemical composition the Logan's Point rock is similar to a tinguaite 

 from Edda Gyorgis, Abyssinia,* the analysis of which is quoted for com- 

 parison : — 



The analysis is also quoted of a rock from British East Africa, called 

 by Prior the Losuguta- type. J This rock contains cossyrite, and its ground- 

 mass in particular appears closely to resemble the Logan's Point rock. 

 Eocks with abundant cossyrite, which occurs in a manner analogous to its 

 occurrence in the Logan's Point rock, are described by H. L Jensen 

 from Queensland. § The Logan's Point phonolite was first described by 

 Captain Hutton]j as an augite-andesite. ITe considered the groundmass to 

 be chiefl}' glass, and mentions chlorite and a purple pleochroic augite. 

 What he describes as chlorite is probably a>girine-augite, and the purple 

 augite is not to be fotnid. He evidently also mistook the cossyrite for 

 magnetite. 



Trachytoid Phonolite {Signal Hill Type). 



This rock has been so named by Dr. Marshall.^ It is certainly a trachy- 

 toid phonolite, with only a sinall amount of nepheline. but it contains amphi- 

 bole as its chief ferro-magnesian constituent. Corresponding to the large 

 amount of ferro-magnesian minerals and iron-ores, the content of both ferric 

 and ferrous oxides is unusually high, as shown by the chemical analysis, 

 given later. The rock is tlius an exceptional one. 



Macroscopic Characters. — The colour is greyish-green, and the texture 

 rather coarse. Numerous cleavage-faces of feldspar are to be seen, and also 

 crystals of amphibole of vai'ious dimensions, some being as much as 1 cm. 

 in length and breadth. Sometimes a horizontal platy structure is notice- 

 able, but the weathering is often spheroidal, and isolated boulders have the 

 appearance of basalt. 



Microscopic Characters. — The structure is porphyritic and holocrystalline, 

 and the feldspars of the groundmass in places show flow structure. 



* Prior. Min. Mag., xii. p. 269, 1900. 

 t Contains also IVInO, 0-43 ; P.,0.,. 0-08. 



+ Prior, Min. I\Iag. xiii, 61, Feb., I!i03, p. 238, and plate v, fig. 2. 

 § Jensen, " Geology of East Moreton and Wide Bay'Districts, Queensland," Proc. 

 Linn. Soc. N.S.W., 1906,' pt. i. 



il Hutton, Proc. Royal Soc. N.S.W., 1889, p. 134. 



^1 Marshall, " Geology of Dunedin."' Quart. Journ. Geol. Soc, Ixii, 1906. 



