372 Transactions. — Geology. 



(ii.) Determination of Specific Gravity. 



The specific gravity was determined by crushing up frag- 

 ments of the felspar and weighing them in a specific-gravity 

 bottle. The result obtained on several trials was 2-719. This 

 is slightly too high for a typical labradorite, but the inclusions 

 of limonite may contribute to this. 



(iii.) By Optical Methods. 



a. By determination of the angles of extinction of adjacent 

 twin lamella? when they extinguish symmetrically about the 

 brachypinacoid. The following is a series of determinations: — 



1. 173°— 156° — 121° 



17° + 35° = 52° 



2. 47°— 79° — 109° 



32° -f 30° = 62° 



3. 135° — 118°— 108° 



17° + 10° = 27° 



4. 51°— 71°— 98° 



20° + 27° = 47° 



5. 50°— 77° — 103° 



27° + 26° = 53° 



6. 61°— 96° — 124° 



35° + 28° = 63° 



7. 53°— 69°— 83° 



16° + 14° = 30° 



8. 25°— 48°— 72° 



23° + 24° = 47° 



9. 159° — 145° — 130° 



14° + ]5° = 29° 



10. ' 135° — 114°— 84° 



21° + 30° = 51° 



11. 43°— 61°— 76° 



18° + 15° = 33° 



12. 103° — 135° — 163° 



32° + 28° = 60° 



The results given show that the angle of extinction ex- 

 tends from 0° up to 63°. This corresponds almost exactly 

 with what is required for labradorite. There may be other 

 species of felspar present, but the great number that are above 

 37°, which is the maximum angle for oligoclase, shows that 

 the greater proportion must be labradorite. Anorthite may be 

 present, but the other determinations show that it is scarcely 

 possible. 



