180 W. E. Ford— Optical Study of the Amphiboles. 



orientated plates being used in each case. In a few instances, 

 where the character of the material made the use of the total 

 refractometer impossible, the mean index was determined 

 under the microscope by the immersion of small fragments of 

 the mineral in liquids having known indices of refraction. The 

 extinction angle on b (010) was determined on carefully orien- 

 tated plates, the measurements being made in sodium light 

 and repeated a large number of times in order to give a cor- 

 rect average. In the majority of cases these plates were made 

 from well-developed crystals, so that the direction of the 

 inclination of the clino-axis was known. In all these cases the 

 extinction direction c was found to lie, as is usual with amphi- 

 boles, in the obtuse angle /3. In the case of the pleochroic 

 varieties a second plate was prepared parallel to #(100) in order 

 to give the vibration direction b. Below are summarized the 

 results obtained, the corresponding analysis being given in the 

 table on p. 181. 



1. Tremolite from Richville, near Gouverneur, New York. 



a = 1-5992, ft = 1-6132, y = 1 6246. Mean index = 1-612. 

 y — a = -0254. 

 2V = 83° 23'. Ca«= 20° l'. 



2. Tremolite from Lee, Massachusetts. 



a = 1-6022, /3 = 1-6192, y = 1-6347. Mean index = 1-618. 

 y — a= -0325. 

 2V = 86° 29'. C ^ c = 16° 38'. 



3. Actinolite from Russell, St. Lawrence Co., New York. 



a = 1-6162, (3 = 1-6304, y = 1-6412. Mean index = 1-629. 

 y — a = -0250. 

 2V = 81° 30'. C A C = 14° 47'. 



4. Actinolite from Greiner in Tyrol. 



a = 1-6173, /3 — 1-6330, y = 1-6412. Mean index = 1-631. 

 y — a = -0277. 

 2V = 81°38'. c /^ 6 = 14° 59'. 



5. Actinolite from Pierrepont, St. Lawrence Co., New York. 



a = 1-6237, (3 = 1-6382, y = 1-6503. Mean index = 1-637. 



y — a = -0236. 



2V = 84° 8'. CaC impossible to measure because of wavy 

 character of extinction. 

 Pleochroism, a = almost colorless, b = faint yellow-green, 



C = faint bluish green. 

 Absorption, c > b > a. 



6. Actinolite from the mines of Kragero, Norway. 



a =1-6280, £ = 1-6442, y = 1-6547. Mean index = 1*641. 

 y — a = -0267. 

 2Y --= 76° 58'. c ^ c = 13° 35'. 

 Pleochroism, a and b = pale brown, c ^ light green. 

 Absorption, c > b = a. 



