186 W. _£. Ford — Optical Study of the Amphiboles. 



figures are based and that there will be many exceptions found, 

 yet, it is thought that by their use the composition of a normal 

 and typical amphibole can be predicted within reasonable 

 limits when its mean index of refraction is known. 



It will be of interest to next consider two other amphiboles 

 analyzed by Stanley, together with several described by other 

 authors, in order to see how far they agree with those given 

 above. As already stated, the literature shows surprisingly 

 few cases of recent analyses with which a complete description 

 of the optical properties are given. Unfortunately also for use 

 in the present discussion, such amphiboles, as have been 

 described, are for the most part exceptional in their nature 



Figs. 8, 9. 



since the interest in them, which led to their description, was 

 naturally caused by their unusual character. Therefore, many 

 of these analyses cannot be taken as belonging to amphiboles 

 that could be considered normal and typical in character. 

 Nevertheless it will be of value to consider them and endeavor 

 to account for their divergence from the series given above. 



Analysis 11 is of a tremolite from Switzerland. Its optical 

 characters are given by Kreutz as follows : a = 1-6000, /3 = 

 1-6155, 7 = 1-6272. Mean index = 1-6136. 7 - a, — -0272. 

 2V = 85° 30' (calculated by present writer). This analysis will 

 be found to answer closely to what would be expected, from the 

 above study, to be the composition of an amphibole having its 

 mean index of refraction. 



Analysis 12 is of the unusual hornblende from Grenville town- 

 ship, Quebec, Canada, analyzed bv Stanley. Its optical proper- 

 ties are as follows : a = 1-6*128, /3= 1-6180, 7 = 1-6328. Mean 

 index = 1-623. 7 -a = 0-020. 2V = 56° 8'. c^c = 29°44'. 

 For a hornblende with its mean index of refraction, it is char- 

 acterized by a very low percentage of silica, by a high amount of 

 alumina, by low percentages of the oxides of iron and by high 



