258 



PROCEEDINGS OF THE AMERICAN ACADEMY 



Aside from the fact that it is very rare to find a crystal in these 

 rocks large enough to manage in cutting : i. e., not less than J^ inch 

 square, there was the possibility that these might be different from the 

 dominant feldspar of the rock. The results obtained by this method 

 are given in the table. 



It occurred to me that this method might be modified by determining 

 the range of this angle in the zone : it, and measuring the angle 

 between extinctions in several individuals in a section, always taking 

 care that the angles should be equal or nearly so in both bauds. 



The following is a proximate determination of the range of the angle 

 included between the principal sections of the alternatkig bands in the 

 zone 0: it for the different feldspars. These values are based on the 

 position of the optic-axial plane and of the bisectrix, as established by 

 Des Cloiseaux. 



Orthoclase 



Albite 



Oligoclase and Andesine . 

 Labradorite 



Anortliite 



Sections cut 

 parallel to O. 



7° 30' — 10° 



0° — 4° 



IQo _ 140 



Sections in i I. 

 20° — 26° 



Maximum in 

 the zone O it. 



0° 



32° — 34° 



36° 



62° 



Sections in O. 



40° — 80° 



Between line 

 of extinction 

 and edge 0\il 

 sections par- 

 allel i {. 



4° — 5° 



15° — 20° 



0° — 7° 



20° — 27° 



330 4go 



From this table, the angle in question appears to stand in an inverse 

 relation to the acidity. By measuring a considerable number of twins 

 in a thin section, taking care to select only such as form equal angles 

 on the opposite sides of the cross hair, it is often possible to distinguish 

 anorthite and labradorite, but it is scarcely practicable to discriminate 

 between albite and oligoclase by this method alone. 



In giving the result of these measurements in the general table, I have 

 given the amount of the double angle, and have put down only those 

 measurements in which the angles in the two members of a twin were 

 even, or very nearly so. Where the angle was ^ or ^ greater in one 

 member than in the other, the result is given in italics. 



This is the only method that could be used in these rocks, which 

 are generally too much altered to admit of a calculation of the mineral 

 constitution from chemical analyses ; and the feldspars are apt to be so 



