MINERALOGY, 89 



from this position six degrees in the other direction, the other set will 

 be dark and the first set bright colored. Hence this is oligoclase, since 

 the elasticity and brachy-diagonal axes make an angle of 3° with one 

 another in a basal section. Now since these angles are small, and the 

 necessity of very close measurements is evident, the mere alternations 

 of light and darkness furnish a rather crude method of measurement. 

 It is plain that the calcite plate put between the ocular and upper Nicol 

 would be of no assistance since the alternations of crystals in different 

 positions would distort the interference cross of calcite, no matter how 

 the crystal might be placed ; hence the quartz plate which Mr. Rosen- 

 busch places over the objective in the tube of the microscope, is here of 

 the greatest value. This quartz plate produces circular polarization, and 

 rotates the planes of vibration of the different colors to different degrees ; 

 hence, by turning the upper Nicol we can intercept any given color that 

 we desire. The violet color is thought by most workers to be the most 

 dehcate. If now we have the lower Nicol in its primary position with 

 the plane of the light corresponding with one of the hair lines in the 

 ocular, and the upper Nicol so placed as that the field of the microscope 

 is a delicate violet, on interposing a section of a triclinic feldspar, one set 

 of the bands of the feldspar will be of this violet color, when an elas- 

 ticity axis in them corresponds with the plane of the vibration of the 

 light, while the very slightest variation in the position will modify this 

 color. Hence this microscopic method of measurement is very accurate, 

 and gives a means with small basal cleavage splinters to determine the 

 species of the feldspar. This method has been extensively used in the 

 study on the composition of our rocks. 



It has been advanced as a theory that orthoclase, albite, and anorthite 

 are the three well defined feldspars, and that all others may be derived 

 from them by supposing them to be composed of a mixture of a certain 

 definite number of molecules of these admitted species. These interme- 

 diate species, — labradorite, andesite, and oligoclase, — have been classed 

 together under the common term plagioclase, and the individual members 

 of the group considered as subspecies. This is the theory advanced by 

 Hunt and Tschermak, and which Des Cloizeaux's determinations are 

 regarded as disproving. It seems to be that careful measurements can 

 with certainty determine to which species a feldspar, of which a basal 

 VOL. IV. 12 



