216 



PETROLOGY OF ROCK COLLECTIONS 



freely in the base. The base presents a very even appearance under the microscope. 

 It is composed of uniform grains -025 mm. diameter. This fine granular material is 

 observed to consist of clear quartz and slightly dusty felspar, the latter apparently 

 orthoclase as its refractive index is below that of Canada balsam. This is not the usual 

 devitrified base typical of the quartz porphyries, but approaches the character of 

 granite aplite. The analysis further corroborates this observation. It is clear that 

 this porphyry is a separation from the granite in which it is found. 



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



99-16 



99-82 



Chemico-mineralogical Classification 

 Sub-Rang 3 (Toscanose). 



Class 1, Sub-class 1, Order 4, Rang 2, 



Felspar Porphyry. Black porphyry from a rocky cape about 8 miles south of 

 Cape Irizar. In one specimen the porphyry is conjoined with a grey biotite granite 

 which it has evidently intruded (Fig. 4, Plate II). 



This granite is identical in character to that described as occurring in situ south- 

 east of Mount Larsen. The porphyry, though strikingly different in outward 

 appearance, is, therefore, closely related to that just described. In the hand-specimen 

 small white and pinkish felspars appear distributed through a black cryptocrystalline 

 base. 



Under the microscope the felspar is noted to be orthoclase twinned after the Carlsbad 

 law, and rarely acid plagioclase. A few grains of faintly coloured epidote are present 

 in close connection with the felspars, obviously an alteration product. Biotite is 

 present as occasional flakes, yellowish to greenish-yellow in colour. More rarely grains 

 of ilmenite are met with. The base is a devitrified glass, partly doubly refracting but 

 unidentifiable. 



Orthoclase Porphyry. A rock (Fig. 1, Plate II) from the Stranded Moraines, East 

 Fork, Ferrar Glacier, is best catalogued here. It differs considerably from other 

 specimens, but the character of the orthoclase phenocrysts suggests that it, also, belongs 

 to the granite following. It consists of large white orthoclase crystals, about 1-5 cm. 

 diameter, twinned after the Carlsbad law ; these are scattered through a fine dark- 

 coloured base. The base contains acid plagioclase, quartz, a little hornblende and 

 unresolvable devitrified greyish interstitial matter. 



Red Felspar Porphyry. An erratic specimen from Cape Irizar. 

 the jasperoid base are occasional white felspars. 



Studded through 



