262 FINLAY 



minerals are seen to be orthoclase plagioclase and nephelite. 

 Augite is rare and no hornblende was observed. Magnetite, 

 titanite, apatite and epidote are the accessories. Measurements 

 were made by using the micrometer eye-piece to determine the 

 relative proportions in which these minerals are present. Some- 

 what over one hundred times the grain was recorded and the 

 assignments to orthoclase and nephelite were constantly checked 

 by interference figures. The result is given below. 



Orthoclase '. 60 



Plagiocla se 5 



Nephelite 25 



Augite I 



Magnetite 8 



Titanite 7 



Average grain 36 mm. 



The large amount of feldspar and nephelite in this rock with 

 not more than 10 per cent, of ferromagnesian minerals is very 

 striking. The large amount of magnetite present is also note- 

 worthy. Hornblende was not observed. 



The orthoclase occurs as a rule in simple crystals, less often 

 in Carlsbad twins, kaolinized and earthy looking. It presents at 

 times rectangular boundaries. Cleavage cracks, most numer- 

 ous parallel to the base, are unusually well developed, with a 

 second set of cleavage cracks parallel to (010). The angle 

 between the two cleavages when they appear on sections ap- 

 proximately parallel to (010) is 65°, or nearly that of the angle 

 /9 for orthoclase. 



The plagioclase, which marks by its presence the passage to 

 the variety of nephelite syenite known as litchfieldite, is albite, 

 in automorphic crystals, twinned in excessively fine lamellae. 

 Like the orthoclase it is often much altered. 



The nephelite is older than the orthoclase as being, fre- 

 quently, entirely surrounded by it. The crystals are i mm. in 

 length and show a strong tendency toward an automorphic 

 habit. Their substance is fresh, so that they may usually be 

 separated from the feldspars by this character. The cleavage 

 cracks, extinctions and figure on the basal section are the usual 

 ones. 



