416 Transactions. 



Art. XXXIV. — Additional Facts concerning the Distribution of Igneous 



Rocks in Neiv Zealand : No. 2. 



By J. A. Bartrum, Auckland University College. 



[Read before the Auckland Institute, 22nd December, 1919 ; received by Editor, 31st 

 December, 1919 ; issued separately, 16th .July, 1920.] 



Plate XXVIII. 



In these notes I wish, to record a few identifications of rocks from various 

 places, some possessing considerable interest ; others are quite ordinary 

 types, but none the less it may be useful to have their occurrences placed 

 upon record. 



QUARTZ-NORITE, 



Cleddau-Hollyford Saddle, South-west Otago. 



This rock was gathered, with others, by Mr. R. W. Holmes, Engineer- 

 in-Chief. New Zealand Public Works Department, from the Cleddau- 

 Hollyford Saddle in 1890. In section it appeared to contain an unusual 

 pyroxene, and after some study was sent to Dr. J. A. Thomson, Director 

 of the Dominion Museum, who was so good as to give the rock consider- 

 able attention, and who very kindly has handed over to me the results of 

 his work. His observations corroborated and considerably amplified my 

 own, but unfortunately sufficient information for exact determination is 

 not available from the section, and Mr. Holmes was unable to find the 

 remainder of the rock from which the chip for sectioning was originally taken. 

 I am also greatly indebted to Dr. Thomson for other help, particularly 

 for pointing out that a mineral in this same rock, carelessly identified by 

 me without full investigation as apatite, is probably zoisite, a fact made 

 almost certain by its high index of refraction, straight extinction, low 

 polarization tints, distinct biaxial character with high optic axial angle, 

 and, so far as I can judge from rather unsatisfactory tests, positive optical 

 character. 



The rock itself is a fairly coarse-grained dioritic type showing no out- 

 standing characteristics in hand-specimen. Under the microscope it is 

 seen to be a moderately typical norite but for two considerations : first, 

 the plagioclase, which slightly exceeds the ferro-magnesian minerals in 

 amount and with these latter comprises practically the whole of the rock, 

 is somewhat acid, being in the main andesine-labradorite ; secondly, there 

 is distinct acidity, signalized by the presence of a little interstitial quartz. 

 The chemical analysis given below, for which I am indebted to the Dominion 

 Analyst, through the courtesy of Mr. P. G. Morgan, Director of the New 

 Zealand Geological Survey, fully exempUfies these characteristics, and 

 shows that chemically the rock is an intermediate type. Mineralogically, 

 however, it is very closely allied to the norites, and for this reason I have 

 classed it with them. 



The structure is coarse and even-grained, the ferro-magnesians occasion- 

 ally being recognizably ophitic to the plagioclase. Of the former group of 

 minerals, one that would ordinarily be identified unhesitatingly as hyper- 

 sthene is probably the most prominent, whilst pale-green augite and deep 



