312 Transactions. 



In the identification of the species of plagioclase the methods advocated 

 by Flett and Dewey* have been used for the determination of aibite, and 

 extended for the determination of bytownite. These writers point out 

 that the use of extinction angles is not always satisfactory, but that a feldspar 

 with indices less than that of balsam, and with optically positive sign, must 

 be aibite or oligoclase-albite. Similarly a feldspar showing symmetrical 

 extinction angles as high as 34° in aibite lamellae (and therefore as basic 

 as labradorite), if it is also optically negative, must be as basic as bytownite. 

 The optical sign is determined by the Becke method (observation of the 

 effect obtained by introducing a gypsum plate at 45° to the nicols when 

 studying the interference figure). 



The use of the Becke effect (displacement of the halo into the medium 

 of lesser refractive index when focussing downwards) is considerably mini- 

 mized by the method of preparation commonly used in New Zealand — 

 viz., the use of an adhesive made of a mixture of Canada balsam and shellac, 

 and of liquid Canada balsam for the cover-glass. The drying-off of the 

 covering balsam is seldom carried so far as is the custom in slides 

 made purely with Canada balsam, and consequently the usual data for 

 the refractive index of the dried balsam cannot be used in the few cases 

 M^here, owing to breaks in the cement, the balsam touches the minerals 

 at the side as well as above. The index of the cement is equally unknown, 

 and must be variable, unless the proportions of shellac and balsam and the 

 period of heating are constant. The advantages of the cement seem thus 

 to be outweighed by its disadvantages, for the use of pure balsam furnishes 

 one of the most delicate methods of distinguishing certain species of plagio- 

 clase. The chief advantage of the cement is the ease of mounting, or, 

 rather, the obviation of the necessity of learning by repeated failures the 

 exact amount of cooking necessary to produce the best result with balsam. 

 A good lapidary, however, will work just as quickly with the pure balsam 

 as with a cement. 



In the slides used in writing this paper the index of the cement was 

 always appreciably greater than that of the (partially baked) balsam, and 

 less, in all cases where it could be observed, than those of quartz. 



The secondary minerals, other than those mentioned above, call for little 

 mention. Epidote forms conspicuous nests in some of the hand-specimens 

 of the lamprophyric rocks, but is not markedly abundant in the slides. 

 Chlorite and carbonates are both widespread. 



KocK Types. 

 Certain clearly marked rock types may be distinguished, but there are 

 no sharp lines separating the basic and intermediate biotitic and hornblendic 

 rocks. Any classification must therefore be more or less arbitrary, and this 

 would be probably still more the case if a larger series of specimens were 

 studied. The so-called dolerites are coarse-grained rocks, but nevertheless 

 they must be regarded as hypabyssal and not plutonic. 



Olivine-hiotite-dolerites. 

 These are the most basic rocks collected, and are very abundant in the 

 gravels. They are dark-coloured coarse rocks, sometimes resembling gabbro- 

 pegmatites. Olivine, augite, and a basic plagioclase (labradorite or by- 

 townite) are the main constituents ; biotite, iron-ore, and occasionally 

 brown hornblende occur in subsidiary amount ; while apatite is the most 

 common accessory. The structure is simple, both olivine and augite showing 



* Geol. Mag., dec. 5, vol. 8, pp. 203-4; 1911. 



