218 



PETROLOGY OF ROCK COLLECTIONS 



The chemical composition as determined by A. B. Walkom, B.Sc, is as follows 



COMPOSITION OF THE NOEM 



100-02 



99-99 



Chemico-mineralogieal Classification: Class 1, Order 4, Rang 2, Sub -rang 4 (Lassenose). 



Several nearly spherical masses, only a few inches in diameter, of this type of rock 

 were observed embedded in the coarse crystallised granite of the main outcrop at Cape 

 Irizar. In this again the hornblende is abundant and some of distinctly alkaline type. 

 Allanite and pyrites are also frequent. 



In another specimen from Cape Irizar the hornblendes are sometimes surrounded 

 by an outer zone of an irregular greenish -blue colour. In this case the porphyritic 

 crystals are all plagioclase. 



A further specimen of the grey rock occurring as an erratic at Cape Irizar yields 

 additional information. In this the hornblende needles are more distinct. Porphyritic 

 idiomorphic plagioclase crystals are also a feature of the rock. 



Under the microscope (Fig. 7, Plate III) the plagioclase phenocrysts are noted to 

 be well zoned, ranging from an acid labradorite within to a basic oligoclase without. 

 They are much changed to dusty doubly refracting aggregates, so that it is difficult to 

 distinguish exactly the relative proportions of plagioclase and orthoclase. Quartz is 

 abundant but present only as small particles amongst the orthoclase felspars of the 

 base. The hornblendes are idiomorphic and zoned. The usual pleochroism is red-brown, 

 greenish-yellow, light yellow. The extinction angle is nearly straight. The periphery 

 of the hornblendes appear to be undergoing a chloritic change. There are grains of 

 magnetite frequently with a little leucoxene attached. The base is composed of a fine- 

 grained mass of even-sized idiomorphic felspars with some quartz. Several cracks 

 traversing the specimen are occupied by yellowish epidote. 



There are a number of specimens collected as erratics similar to these, but of a 

 general pink tint. A typical example is the following from Cape Irizar. The hand- 

 specimen is of a pink colour and shows abundant porphyritic felspars and some finer 

 hornblende. In the section it is apparent that the porphyritic felspars are 

 almost all plagioclase which are changing with the production of faintly coloured 

 epidote. There are some orthoclase crystals twinned after the Carlsbad law. The 

 hornblende is pleochroic from light yellow to bluish-green to red-brown. In the 

 case of one crystal a bluish-green border was observed on a red-brown basal 

 section. Grains of magnetite and sphene are obvious. The base is principally quartz 

 and orthoclase in eutectic, largely graphic, arrangement ; there is here also a little acid 

 plagioclase. 



