PROCEEDINGS OF SECTION C. 393 



andesine and oligoclase of various composition, while the margin is 

 albite-oligoclase. The range of extinction angle and of composition in 

 these zoned crystals is unusually large. A subordinate amount of clear 

 untwinned alkali felspar occurs as allotriomorphic crystals in the 

 ground mass. 



Uralite. — Some of the hornblende is undoubtedly secondary after 

 augite. This is well shown in one crystal of uralite, octagonal in section, 

 having the crystal boundaries of augite and the cleavage and optical 

 properties of hornblende. The paramorphic change has not been quite 

 complete in the central part of the crystal, and is not quite truly para- 

 morphic, as small quantities of secondary quartz and white mica have 

 been produced. 



Quartz. — Very little quartz is present in the rock. A little is in- 

 cluded in the porphyritic felspars, but generally the quartz was the 

 last mineral to crystallise, and is clearly moulded on the felspars. A 

 slight tendency to a granophyric structure is observed in parts of the 

 ground mass. The rock is clearly more basic than the granite, and may, 

 perhaps, be best described as a hornblende porphyrite. 



SAME DYKE AS J, BUT FROM UPPER PART, SECTION X. 



Megascopic Examination. — In the hand specimen it differs from 

 J mainly in being more definitely porphyritic, many of the phenocrysts 

 being quartz. 



Microscopic Examination. — The same minerals occur as in J (with 

 the exception of uralite), but three points of difference are noticeable. 

 First, the greater abundance of quartz, not only as phenocrysts, but 

 also in the ground mass ; second, the character of the plagioclaso 

 crystals showing lamellae give extinction angles of only 2° or 3°, in- 

 dicating an oligoclase, while sections parallel to 010, showing zoning, 

 give extinction angles ranging from + 7° (acid oligoclase) to — 8°, 

 (andesine). The plagioclases are, therefore, less basic than in J. Third, 

 the structure of the ground mass, which is fine-grained and granular, 

 and consists mainly of quartz and an alkali felspar. All the constituents 

 occur in two generations, and since quartz is abundant the rock is per- 

 haps best described as a granite-porphyry. 



ACID DYKE IN COARSE GRANITE IN SADDLE BETWEEN THE TWO 

 RIDGES OF STATION PEAK, SECTION 3S0. 



Megascopic Examination. — It is an almost white eranular rock, 

 containing quartz, felspar, and a little biotite. 



Microscopic Examination. — The structure is coarse-grained and 

 hypidiomorphic. The most abundant mineral is quartz, showing slight 

 undulose extinction. The quartz is frequently included in felspar, and 

 includes biotite. Of the felspars, microcline, showing characteristic 

 " grating" structure in polarised light and extinction angles of about 

 15° from lamellae, 's the most abundant. The specimen was not quite 

 fresh, so the felspars are kaolinised. Some untwinned felspar — probably 

 orthoclase — occurs, and a little plagioclase, showing albite-lamellse, with 

 extinction angles of 10°. Biotite occurs sparingly in elongated irregular 

 crystals, with inclusions of apatite. The rock is an acid granite. 



