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