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University of California . 



[Vol. 3. 



brushes lying' like the arms of the dark cross of a spherulite, 

 parallel to the cross-hairs of the microscope. If a gypsum plate 

 giving the sensitive red of the first order is introduced in the 

 usual manner, it is always found that the bright sectors, lying 

 between the dark brushes, which are bisected by the 8 axis of 

 the plate, raise its polarization color to purple, while the other 

 sectors throw that color down to orange. The effect is the same 

 as if we were dealing with imperfect spherulites of positive fibres. 

 But the most careful scrutiny, with high-power lenses, fails to 

 reveal any fibrous structure. It therefore seems probable that 

 the phenomenon described is due to molecular strains in the glass 

 produced in the process of cooling and contraction, rather than 

 by spherulitic crystallization. 



The position of this rock in the scheme of classification, even 

 in the absence of chemical analysis, is pretty clearly indicated by 

 the petrographical characters. The predominance among the 

 feldspars of anorthoclase, the uniformly vitreous structure of the 

 groundmass, and the scarcity of ferromagnesian constituents, 

 indicate that it is a rhyolite. It is presumably not very different 

 in chemical composition from the Antelope rock and the flow 

 immediately underlying the gray rhyolite itself. 



The contrast between the glassy groundmass of this rock and 

 the fibrous groundmass of the flow just beneath it is striking and 

 not easy of explanation. Two hypotheses seem worthy of con- 

 sideration. First, the lithoidal rhyolite may be much older .than 

 the glassy rhyolite, and its devitrification may be due. to second- 

 ary agencies acting for a long interval between the extravasation 

 between the two flows. There are two objections to this hypo- 

 thesis. There is no field evidence of any considerable difference 

 of age. The fibrous structure, independent of any system of 

 cracks or joints, and developed to a comparatively uniform 

 degree throughout the rock, has no appearance of being second- 

 ary. The alternative hypothesis assumes that this fibrous 

 structure is original, and due to some conditions of eruption not 

 active in the case of the glassy flow. In this connection, an idea is 

 suggested by the fact that the numerous small cavities are present 

 in the lithoidal and absent from the glassy lava. It is submitted 

 that watery vapor may have been abundantly occluded in the 



