SPHER ULITIC CRYSTALLIZATION. 
457 
approximately parallel to their length, is sometimes less and 
sometimes greater than that at right angles to this direction. 
The prisms have a small extinction angle of variable size, 
and it is observed that the negative rays exhibit less double 
refraction than the positive ones and have a lower extinction 
angle. From these characters it is evident that the rays are 
prisms of orthoclase elongated parallel to the vertical axis, c, 
and that the plane of the optic axes is normal to the plane 
of symmetry. In spherulites of this sort the positive and 
negative rays are generally uniformly mixed throughout the 
whole. Such a spherulite sometimes has an outer zone or 
border of compact, finely fibrous structure, which is negative 
and is the same growth as the small granophyric spherulites. 
Tridymite is scattered through these spherulites, besides 
small grains of magnetite, and sometimes a few grains of 
fayalite and a little mica. 
In other cases the branching rays are all positive. This 
indicates that the feldspar prisms have the same develop¬ 
ment—that is, parallel to the axis c —but that the plane of 
the optic axes is in the plane of symmetry, which is fre¬ 
quently observed to be the case in prisms of orthoclase which 
have crystallized independently in the tridymite in other 
parts of the rock. 
The distinctly arborescent growths of feldspar in which 
the long slender rays branch off from a stouter stalk is shown 
in the figure on Plate 8. The prisms become thinner and 
more crowded together as they grow outward, and terminate 
in broad fronds like leaves. The stems are usually twinned 
throughout their length, as are also the fronds, which are 
sometimes found as isolated growths. These are twinned 
in the same manner, the composition plane dividing the 
crystal in two in the direction of its length. These twinned 
prisms of orthoclase are always negative, and the inclination 
to the twinning plane of the axis of greatest elasticity is 
about 7° or 8°. These characteristics could only be found 
in orthoclase prisms elongated parallel to the clinoaxis, a , 
and twinned according to the Manebach law, which is the 
