THE FELSPARS. 
while augite sinks. By careful dilution quartz and felspars can be 
then separated, and even Orthoclase, from the Triclinic ones. This is 
also proposed by Goldschmidt* as a very convenient and very accurate 
means of obtaining the sp. gr. of the constituent minerals of a rock 
even when only one or two grains can be detached, for if the dilution 
be carried so far that the fragment remains suspended anywhere 
indifferently in the fluid the sp. gr. of the solid and that of fluid must 
be equal, and may be easily determined in the case of the latter by 
means of the sp. gr. bottle. 
The optical method of determining what is the particular felspar 
in a rock is founded on the fact that the position of the optic axes 
with respect to those of form varies in the different species. Of 
course Orthoclase is distinguished from the Plagioclases by its mono¬ 
clinic symmetry— i.e., in a section of a rock some crystal sections will 
probably be found in which “extinction” happens when the com¬ 
position line of the twin structure is parallel to the principal plane of 
one of the Nicols. It is, however, the discrimination of the Triclinic 
specifics which is so difficult, and in many cases quite impossible. We 
have to pick out in the section those cr 3 'stal sections which extinguish 
at equal angles on both sides the Nicol plane, and then measure this 
angle in as many cases as we can find. Now symmetrical extinctions 
only show that the plane of section has accidentally passed through the 
crystal perpendicular to the twinning plane, and therefore the 
extinction angle may vary within wide limits, and it is only by noticing 
the maximum angle that we can form any conclusion whatever ; f e.g., 
in trying to determine a felspar a short time ago I found an extinction 
of 16° on one side and 17|° on the other— i.e., 33|° from one to the other. 
Now if I had only been able to find this one tolerably symmetrical 
extinction I could not have told which felspar it was. It would 
probably not have been Albite, seeing that the angle in that case 
should not have exceeded 31^°, but I could have gone no further. 
When, however, I found further angles of 44°, 48°, 53J°, 54°, 56°, 58°, 
66°, 71°, 73°, there was enough to assure me of the presence of a very 
basic Lime felspar, and the observation of one grain showing the zonal 
structure to be presently mentioned with an extinction angle of 48° in 
some parts and of 82° in others, made the presence of bothLabradorite and 
Anorthite almost a certainty. In this case a large number of observa¬ 
tions could he made, and therefore such a degree of probability 
produced that I was not at all surprised to find the observations quite 
confirmed by Szabo’s flame reactions, some of the grains which I 
was able to detach being much more fusible than others, and all 
* Neues Jahrbuch I., 1831. Beilage Band, p. 179. 
+ The following table of the angle between the extinction positions of two 
Twin Lamellae in the various Felspars may be of service. The section is 
supposed perpendicular to the plane of twin composition, and the angle given 
is the maximum observable for each species:—Orthoclase, 0°; Microcline, 36°; 
Albite, 31^°; Oligoclase, 37°; Labradorite, 621°; Anorthite, 741° and upwards. 
In sections parallel to the Basal cleavage:—Microcline, 30°—32°: Albite, 7“—8° 
Oligoclase, 2°—3°; Labradorite, 10°—141°; Anorthite, 57°—-74°. 
