458 
IDDINGS. 
orientation already given for this form of feldspar in the 
article on Obsidian Cliff (p. 278). 
In the large porous spherulites several forms of feldspar 
growths occur together. In one instance the center consists 
of an aggregation of partial spherulites of small size and 
positive character. This is surrounded by a narrow zone 
of cloudy material, but slightly doubly refracting, and with 
a positive character. Outside of this zone the porous portion 
of the spherulite begins. It is made up of nearly straight 
radiating fibers of feldspar, with weak double refraction, which 
are all positive. From points at various distances from the 
center of the whole spherulite, within these positive fibers, 
there start stouter fibers of feldspar with stronger double re¬ 
fraction and negative character. These branch out into 
radiating bunches, which unite to form the outer zone of the 
spherulite, where the fibers are partly negative and partly 
positive. In this outer zone it is observed, on closer inspec¬ 
tion, that the negative feldspars form stems from which 
thinner positive feldspar fibers branch like the needles of a 
pine twig. These needles curve to a position parallel to the 
stem and to the radii of the sphere. All of the porous por¬ 
tion of the spherulite is thickly spotted with pellets of 
tridymite. The structure is very crudely represented by 
figs. 6 and 7, Plate 7, the actual structure being ex¬ 
tremely complex, formed as it is by innumerable delicate 
crystals. The first porous zone of weakly refracting rays of 
feldspar, all of which are positive, must be composed of 
prisms elongated in the direction of the vertical axis, c, with 
the plane of the optic axes in the plane of symmetry. The 
branching groups of strongly refracting feldspars, which are 
all negative, must he prisms parallel to the clinoaxis, a; 
they are twinned according to the Manebach law. In the 
outer zone these latter prisms send out thinner ones in 
the direction of the vertical axis, c, which are positive. 
These thinner needles branch forward from both sides of 
the twinned stem; consequently the crystallization of the 
twinned prism must have advanced out from the angle 
2 P made by the c-axes of the twinned halves of the crystal. 
