10 



PROCEEDINGS OF THE NATIONAL MUSEUM 



VOL. OS 



to distinguish except through high magnification. The larger 

 groups are white and have a waxy luster, while the smaller ones are 

 somewhat more translucent. The groups are irregularly rounded 

 and show no suggestion of crystal form. 



Under the microscope the pellets of cristobalite are seen to be made 

 up of radiating laths with low birefringence. The surface is com- 

 posed of minute octahedral or larger dodecahedral crystals, the 

 latter being usually flattened and very much resembling the hex- 

 agonal plates of tridymite, from which they can be distinguished 

 only by their higher indices of refraction. The mean refractive in- 

 dex of cristobalite is 1.486 with a birefringence too low to give a 

 recognizable interference figure, and in thin sections is so feeble that 

 close inspection is necessary to differentiate it from glass. 



Fig. 3. Crystal habit of fayalitb from thb lithophysae of Obsidian Cliff 



The optical properties and general physical appearance of the cris- 

 tobalite are near those of the tridymite, and the similarity is height- 

 ened by the presence of the cristobalite as flattened dodecahedral 

 crystals showing angles of 120 degrees. This striking similarity 

 raised a doubt as to the actual presence of two distinct minerals in the 

 rocks. A sample of the white pellets was submitted to E. W. G. 

 AVyckoff of the geophysical laboratory, Carnegie Institution of 

 Washington, for X-ray examination. His comparison of its pattern 

 with that of the synthetic form, definitely determined the mineral to 

 be cristobalite. 



Fayalite. — Fayalite is present in all parts of the flow, but never 

 in abundance. Fresh, unaltered crj^stals, a millimeter or less in 

 length and of a fine, honey-yellow color, are found in the upper 

 portions of the flows in association with cristobalite and fibrous 



