Bell and Clarke. — Geological Reconnaissance of N orthernmost N .Z. 623 



The dominant types of crystalline rock encountered in the North Cape 

 promoutovy are norites and gabbros, passing at places into hartzbergites 

 and Iherzoiites. 



In hand-s>pecimens the gabbros and norites vary in texture from coarse 

 to rather fine. The most coarsely grained rock is a norite obtained near 

 Kerr Point, made up of very conspicuous crystals of enstatite, | in. and 

 more in length, apparently imbedded in a crystalline mass of feldspar, 

 A somewhat finer and more even grained rock is the gabbro from North 

 Cape, hand-specimeris of which show dark-green augites and white and 

 occasionally pinkish feldspars fairly evenly distributed. A much finer 

 gabbro occurring in the clifis east of Tom Bowling's Bay, in hand-specimens 

 can only be described as a brownish rock consisting of a fine-grained mixture 

 of rather clear feldspar and somewhat decomposed ferro-magnesian minerals. 



Under the microscope the plutonics are seen to be holocrystalline. The 

 feldspars range from labradorite to anorthite, and are usually fresh. In 

 the gabbros the ferro-magnesian mineral is a light-brown pyroxene, seldom 

 showing the diallage structure to any marked extent, and often partially 

 altered to bluish-green hornblende or to uralite. Very much altered 

 ohvine is occasionally found. In the norites the hypersthene occasionally 

 is seen to be partially altered into brown or bluish-green hornblende 

 (Plate LVI, fig. 3). In the case of the very coarse rock from Kerr Point, 

 bastite has partly taken the place of enstatite. 



The hartzbergite which is found near Kerr Point is in hand-specimens 

 a pale olive-green rock much flecked with white. It is crossed by a number 

 of fine cracks, along which alteration-products have accumulated. The 

 phenocrysts are large pyroxenes, the greater part of each being dull and 

 earthy, but the cores having a metallic lustre. Microscopic sections show 

 that the rock is made up mainly of serpentinised olivine, which surrounds 

 numerous crystals of enstatite now nearly completely altered to bastite. 

 A considerable quantity of chromite and magnetite occurs in the serpentin- 

 ised olivine. This rock shows a general resemblance to the hartzbergite 

 from Milford Soimd, of which Professor Marshall has kindly lent us sections. 

 The North Cape rock is, however, much more decomposed than that from 

 Milford Sound. Lherzolite has been recognised in bands traversing the 

 gabbro at North Cape. Megascopically the rock is coal-black, finely 

 crystaUine, and traversed by irregular streaks of white, which are probably 

 decomposition-products. Under the microscope the lherzolite proves to be 

 composed mainly of olivine, much of which is converted into serpentine. 

 Some enstatite, diallage, and brown hornblende also occur. This rock, on 

 the whole, shows a closer resemblance to the hartzbergite from Milford 

 Sound than to the lherzolite from Cow Saddle, to the north-east of Milford 

 Sound.* Nevertheless, the presence of diallage seems to place it amongst 

 the Iherzoiites (Plate LVII, fig. 1). Cutting the Iherzohte at North Cape are 

 several clearly defined bands, 1 ft. or more in thickness, of a dull green 

 homogeneous-looking rock, with a hardness of about 6. Microscopic 

 examination shows this rock to be a very fine grained aggregate of diopside 

 and sphene, with a little feldspar (Plate LVII, fig. 2). 



The occurrence of ultra-basic rocks closely allied to those found near 

 Milford Sound and elsewhere in the South Island is of great interest. The 

 foregoing notes, unaccompanied as they are by chemical analyses, must be 

 regarded as of a purely preliminary character. 



* Marshall, Trans. N.Z. Inst., 1904, p. 483. (See Marshall, Trans. N.Z. Inst., 1905, 

 p. 564.) 



