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University of California. 



[Vol. 3. 



structure, and spherical cracks, often lined with minute sparkling 

 crystals, are generally present. The rock shows a distinct 

 banded structure, marked by the different size of the spherulites 

 in the different layers. 



Under the microscope, a few small phenocrysts of quartz and 

 feldspar become visible, the latter including sanidine and an acid 

 plagioclase. The spherulites, often formed about a phenocryst 

 as a nucleus, appear by transmitted light of the usual cloudy brown 

 tint. The fibres always have a small extinction angle, and their 

 elongation is invariably positive when the double refraction is 

 perceptible. A considerable proportion, however, seem practi- 

 cally isotropic. In most described spherulitic rhyolites the fibres 

 are either all negative or partly negative and partly positive. 

 The facts observed in this rock may perhaps be explained by 

 supposing them to be of sanidine and anorthoclase elongated in 

 the direction of the vertical axis, which would be nearly parallel 

 to the axis of elasticity b. Both feldspars have a small optic 

 axial angle, which means that the difference of elasticity in the 

 directions b and C is small. Now if such fibres were viewed in 

 the direction of the axis 3 their effect on polarized light, owing 

 to the nearly equal elasticities parallel to b and C, would be 

 small, and, if the fibres were very slender, imperceptible. If, 

 on the other hand, they were viewed in the direction of C, they 

 would have an apparent double refraction neai'ly equal to the 

 maximum and the direction of elongation would be optically 

 positive. 



Numerous small granophyric intergrowths of quartz and 

 feldspar are included in the spherulites, from which they are dis- 

 tinguished by being quite transparent. The material lining the 

 erescentic cracks is found to be mainly quartz and opal, with 

 occasional crystals of a zeolite which has weak double refraction 

 and a perfect pinaeoidal cleavage. The groundmass shows well 

 marked perlitic structure, and is eviently an altered glass, but it 

 is much devitrified, and speckled with minute, weakly birefring- 

 ent crystal grains. 



The large spherulites gathered from the disintegrated rock 

 generally show considerable irregularity of form. They are, 

 first, often formed by the coalescence of several individual spher- 



