250 PROCEEDINGS OF THE ACADEMY OF [1880. 



Vermiculite. 



1. E. ISTottingham , Chestei* Co., Pa. Hallite. In 



green crystals : uniaxial. 0°. 



2. Cecil Co., McL, Magnesia Quarry. Hallite. Con- 



tains enclosed arrow-shaped crystals like Hal- 

 lite : uniaxial. 0°. 



3. Chester Co., Pa., Brown's Quany. Uniaxial. 



(T.D.Rand.) .0°. 



4. Macon Co., N. C. Maeonite. In brown scales ; 



uniaxial or with a divergence of 1°±. (F. 



A. Genth.) 1°±. 



0. MineralHill,Del. Co., Pa. Pale green. (A.N. S.) 19°. 



6. Lenni, Del. Co., Pa. Brown and green ; some- 

 times a very small optic angle occurs. 19°-20°. 



*7. Culsagee, N. C. Culsageeite. Yellowish-brown: 

 variable angle. Sometimes the angle varies 

 as different portions of the same piece are 

 moved into the field. One piece gave 9°, 

 and another was nearly uniaxial. The angle 

 given is the most constant one. 20° dz. 



8. West Chester, Pa. Jefferisite. Variable angle : 



a specimen gave at one part 16°30', and at 

 another 25°, the latter being the most distinct ; 

 a very tliin piece gave 11°30', and a thicker 

 piece 2t°20'. Apparentl}^ the optic-angle in- 

 creases with the thickness of the plate. Some 

 good specimens gave 22°, 25°, and 28°; mean 

 angle probabl}^ 26°±:. 



9. Lafayette Soapstone Quarry, Montgomery Co., 



Pa. Brown scales in chlorite slate : constant 



angle 32°-36°30' ; mean, 34° ±. 



10. Germantown, Phila. Brown plates in hornblende 

 rock. Optic-angle constant w^ithin 31°20'- 

 39°30' ; the most constant angle is 37°±:. 



It is very probable that, as suggested by Prof. Cooke, the varia- 

 tion in the optic-angle of the Yermiculites is caused by twinning 



