and their Contact Zones. 25 



parallel to the basis ; or in the latter 33° to 46° when it is 

 parallel to the brachypinacoid. He states further that 

 in the triclinic potass felspar microcline the same optical 

 angle is 15° SO', while in orthoclase the direction of 

 greatest obscuration in basal sections is parallel to the 

 edge formed by the basis and the clinopinacoid. 



In order to test these statements I have prepared, and 

 also procured from Germany, a number of slices taken in 

 the direction of the basis and of the lateral pinacoids 

 from both the mono'clinic and triclinic felspars. I have, 

 besides making observations upon these type specimens, 

 also measured the required optical characters of all those 

 sections of felspars, which appeared suitable in several 

 hundred thin slices in my possession. The result has been 

 to assure me that these characters may be provisionally 

 used for the classification of the felspars, and to hope that 

 further investigations will give to this method of examina- 

 tion still greater precision and reliability. Following out 

 the indications given by Des Cloizeaux, the following is the 

 standard of comparison which I have used : — 



1. Albite and oligoclase : inclination of the optical section 

 in zone o — ii to the edge o — ii, 0° to 20°. 



2. Labradorite and anorthite : inclination of the optical 

 section in zone o — ii to the edge o — ii, 6° to 30° and 20° to 

 45° respectively. 



3. Microcline: inclination of the optical section in the 

 zone o — ii to the edge o— ii, 10° to 16°.* 



4. Orthoclase : inclination of the optical section in the 

 zone o — ii to the edge o — ii, 0°. 



In endeavouring to apply practically these indications, 

 it is evidently necessary to measure these sections only 

 which are perfectly, or nearly, in the zone o — ii, if triclinic, 

 or o — ii, if monoclinic. In those cases where there was not 

 a perfect accord, the discordance not being greater than 3°, 

 I have taken the mean. By these observations I have found 

 myself enabled to class the felspars I have examined under 

 four heads, as above. I now proceed to consider these 

 classes more particularly. 



* The angle 10° in slices parallel to ii is approximate only. 



