SPHERULITIC CRYSTALLIZATION. 
453 
influence the interference phenomena appreciably. The 
latter is most probably the case, for in a large granophyric 
group with the same structure, which was studied for com¬ 
parison, it was observed that the quartz, though appearing 
to be present in considerable amount, was not sufficient to 
change the character of the double refraction of the feldspar, 
which was the predominant mineral. It modified it, how¬ 
ever, to a variable extent; and in places where quartz was 
more abundant its optical character was predominant. 
The small spherulites of this rock are unquestionably com¬ 
posed of orthoclase prisms or needles elongated in the direc¬ 
tion of the clinoaxis, which radiate from a center and are 
intergrown with quartz after the manner of granophyre or 
micropegmatite; and it is this microscopic intergrowth which 
gives them the granulated or feather-like structure. 
In the case of the spherulites with projecting rays of pure 
feldspar it is evident that the free silica ceased to crystallize 
as quartz in intimate connection with the orthoclase and 
allowed the latter to continue alone and project into a highly 
siliceous residual paste, which finally crystallized as tridy- 
mite in most instances. 
In certain cases the zone of clear feldspar does not occur on 
the margin of the spherulite,but forms a crescent-shaped trans¬ 
parent belt within it, as shown in fig. 4, Plate 7. In ordi¬ 
nary light this belt appears to be a gaping circular crack, 
though its definition is lost at one end. Between crossed nicols 
it is found to consist of pure feldspar, oriented in accord with 
the radiating prisms, and producing no disturbance of the 
dark cross which passes regularly through it. At the lower 
end, fig. 4, it is seen to be part of the same crystallization as the 
purer feldspar rays of that part of the spherulite; and in the 
upper part it differs from the rest of the spherulite simply by 
being free from the granulated or micropegmatitic structure. 
There can be no doubt that it is a part of the original crys¬ 
tallization of the spherulite, and that from some cause the 
free silica ceased to crystallize for a short space and then con¬ 
tinued in other portions of the spherulite. This is in har- 
58—Ball. Phil. Soc., Wash., Vol. 11. 
