M1CE0GEAPHIC SPHERULITES. 



413 



and of the spherulite, since there appears to be but one variety of feldspar 

 present, indicates that the orthoclase is rich in soda, the molecular ratio of 

 the potash to the soda in the rock being 1 to 1. 



The projecting prisms are free from the granulation which is at the 

 center of the spherulites. In some instances this assumes a radiating, 

 feather-like structure which suggests the micrographic intergrowth of 

 feldspar and quartz. This is undoubtedly its true character, as shown else- 

 where; so the small spherulites are unquestionably composed of orthoclase 

 prisms elongated in the direction of the clinoaxis, which radiate from a 

 center, and are intergrown with quartz in a micrographic maimer. When 

 the margin of pure feldspar was being crystallized, the free silica remained 





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mr^t 







Fig. 4.— Sections of spherulites with projecting prisms of orthoclase and a crescent-shaped belt free from granulation. 



uncrystallized until subsequently, when it filled the interspherulitic spaces 

 with tridymite or quartz, according to circumstances. 



There are instances in which the clear zone of feldspar does not occur 

 on the margin of the spherulite, but forms a crescent-shaped transparent 

 belt within it (c of fig. 4 above, and fig. 2 of PL LIV). The optical behavior 

 of such spherulites shows that the feldspar fibers or prisms are continuous 

 from center to circumference, and that it is simply the granulated or feather- 

 like intergrowth that was interrupted. This is in accord with other signs of 

 a lack of uniformity of microstructure in these highly siliceous rocks. 



Still larger forms of this kind of spherulites are the small megascopic, 

 blue-gray spherulites, which are from less than 1 mm. to 5 mm. and rarely 

 10 mm. in diameter. In thin section they have much the same characters as 

 the microscopic ones. They are distinctly fibrous in sections, the fibers 

 not all radiating from a single point. They often have a micrographic 



