SPHERULITIC CRYSTALLIZATION. 
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present in variable amounts, including orthoclase, and mag¬ 
netite, with less regularly tourmaline, mica, and fayalite. 
The fayalite occurs in relatively large irregular individuals, 
with an opaque border, which at times entirely replaces the 
original mineral. 
The tourmaline and mica occur in minute crystals about 
.025 mm long and .Ql mm thick. They are abundant in places 
and lie scattered through the tridymite or quartz and also 
in the margin of the bordering spherulites. They occur 
sometimes together, but usually one is present to the exclu¬ 
sion of the other. The tourmaline is recognized by its de¬ 
cided pleochroism, the strong absorption being across the 
prisms, 0 > E. Its color is brownish green; colorless parallel 
to E. The double refraction is strong and negative. Trans¬ 
verse sections exhibit a uniaxial cross and are bounded by 
six sides, alternately three short and three long. It also 
occurs in the tridymite coating the walls of the hollow 
cavities in some cases. 
The mica is green and also yellowish brown to reddish. 
It forms stout tablets with six sides, and exhibits strong 
absorption, from colorless to almost opaque, which is of 
course in the opposite direction from that of the tourmaline, 
the axes of elasticity also in the long and narrow sections 
being reversed in the two minerals. The dark-green mica 
may be easily mistaken for the tourmaline. 
The tourmaline and mica are idiomorphic and must have 
crystallized just before the outer portion of the small spher¬ 
ulites and the tridymite and quartz in which they lie. They 
are confined to the region of these interspherulitic spaces, 
and are not found scattered indiscriminately through the 
compactly spherulitic portion of the rock. Their period of 
crystallization is therefore later than that in which the small 
spherulites began to crystallize and earlier than the final 
crystallization of the residual magma or paste. Their sep¬ 
aration from the magma was preceded by that of quartz 
and orthoclase, and was also followed by the same. Their 
crystallization in such a siliceous lava is abnormal, for they 
