410 PEOF. P. MAESHALL ON THE [Aug. 1906, 



The large number of rounded grains in the basanites at once 

 distinguishes hand-specimens of the rock from those of other basic 

 rocks of a similar black colour. On the other hand, the phonolitic 

 rocks that contain large crystals of nepheline have a dark-green tint. 



There is no large area covered with outcrops of the rock. It is 

 confined to the eastern slopes of Flagstaff, the slopes of Mihiwaka, 

 and to North Otago Head. In the first-named locality it underlies 

 a Papanui type of dolerite, which is covered by a basalt, and finally 

 by a trachytoid phonolite. Much the same arrangement is found 

 at the JSTorth Head and on the northern slopes of Mihiwaka. At 

 Waikouaiti, boulders of the rock occur in a conglomerate that is 

 covered by a basaltic lava-flow. 



16. Dolerites. — The word dolerite is here used as a term 

 covering all the types of coarse basic rocks, irrespective of age, if 

 they are of effusive character. 



The Mount-Charles type contains large conspicuous phenocrysts 

 of olivine, augite, and felspar. In slices, the big olivines are found 

 to be rounded and in parts serpentinized. The augites have a 

 pale-green core (diopside), but a brown margin. The core is 

 usually of irregular shape, but the mantle corrects the irregularities, 

 and gives a completely-idiomorphic form to the crystals. The 

 felspars are but little twinned ; the extinction-angle, however, is 

 high, and shows that the mineral is labradorite. The groundmass 

 is crystalline, and consists of felspar, augite, and magnetite. 



The Papanui type is an even-grained rock, except that the 

 olivines are rather larger than the other constituents. In slices, 

 the rock is quite coarsely crystalline, and often shows an ophitic 

 structure. The augite was the last constituent to crystallize, and 

 is markedly allotriomorphic. The felspar is in highly-twinned 

 elongated crystals : the species is labradorite or bytownite. There 

 is much magnetite present. The rock is always coarse-grained and 

 distinct, but in one example (Harbour Cone) the place of some of 

 the augite is taken by a brownish glass, with, branched growths of 

 magnetite. This example should properly be called a basalt; but 

 it is so closely allied to this dolerite, and so different from the 

 ordinary basalts of the region, that I have always considered it as 

 a member of this group. 



Two specimens of the Papanui type of dolerite, collected from 

 widely-different localities, were analysed ; and one specimen of the 

 Mount-Charles type was taken for comparison : — 



A. B. C. 



Per cent. Per cent. Per cent. 



Si0 2 45-39 44-84 48-40 



A1 3 9-61 11-92 13-05 



Fe;0 3 9-27 9-12 9-02 



FeO 8-48 8-54 7-33 



CaO 9-25 923 8-30 



MgO 10-69 10-34 7-26 



Na 2 3-14 1-43 3'50 



K 0-55 0-68 0-57 



H;o 2-81 3-04 2-84 



Totals 99-19 99-14 100-27 



A & B= Papanui Dolerite ; C = Mount-Charles Dolerite. 



