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Transactions. — Geology. 



are rare or absent. Moreover, as already mentioned, the 

 pyroxene in these rocks is frequently of a pale violet or 

 wine colour, in which case it exhibits distinct dichroism, 

 changing from pale yellowish-brown or yellow-violet to 

 brownish-violet. This character is not, I think, met with in 

 any of the pyroxenes except augite, none of the others 

 showing any approach to a violet or purple colour. On the 

 whole, therefore, the evidence, though not conclusive, seems 

 to show that augite is the only kind of pyroxene present. 



The feldspar of these basalts is in too minute a form to be 

 isolated and its nature determined by Szabo's method. I 

 have therefore employed the method of determination by 

 measurement of the angle of extinction referred to the length 

 of the microliths ; and, where the feldspars are twinned, by 

 measurement of the angle between the directions of extinc- 

 tion in two adjacent lamellae. These angles in nearly every 

 case are high — sometimes very high — the average observed 

 in the case of the twin crystals ■■' being 33° ; and in the 

 slides examined by me the crystals in which this angle 

 was lower than 20° were in the proportion of only 2 in 45. 

 Further, in many of the slides the angles given by laths from 

 all parts of the section are very close, indicating a uniformity 

 in the composition of the feldspar throughout. This mineral, 

 therefore, must belong to the labradorite-anorthite group. In 

 most cases it probably consists entirely of anorthite. 



Before leaving the microscopic structure of this basalt we 

 may notice a slight variation which occurs in the blocks con- 

 taining the large porphyritic groups of olivine at Northcote. 

 The groundmass is seen under the microscope to contain a 

 large quantity of augite and a little olivine, in the form of 

 almost circular grains distinctly separated from one another 

 by the rest of the groundmass, and not, as is usually the 

 case, intercrystallized irregularly with it. I have noticed 

 above the similarity between the mode of occurrence of the 

 olivine in this rock and in some of the lava at Lake Takapuna ; 

 and, on examination with the microscope, a further resem- 



