STUDY OF STRUC TURK OF FULG URI TES 689 



form, structure, and optical character, to the artificial microlitic 

 bundles obtained in the experiments of J. S. Diller {^loc. cit.) 

 by long fusion of amorphous fulgurite-glass in crucibles. 



The difference of the above results from those hitherto 

 reported from study of the fulgurite of this peak may be 

 attributable to variation in the structure of the fulgurite, particu- 

 larly in regard to devitrification, in different parts of the surface. 



IV. Fulgurite from summit of Central Butte, Little Missouri 

 Buttes, Wyomi7ig; specimen collected by Mr. John D. Irving. 

 This is a small fragment of phonolite, apparently from the edge 

 of intersection between two joints, down which corner the fulgu- 

 rite runs in a shining, cream-colored, slightly brownish crust, 

 about 15™™ wide and 3""™ thick. On a fractured cross-section it 

 shows distinct lamination parallel to the surface of contact, as if 

 the latent structure had been developed by weathering. 



The rock consists mainly of orthoclase-phenocrysts, whose 

 idiomorphic character is obscured by fractures, imbedded in a 

 smaller volume of granular, somewhat ochreous holocrystalline 

 groundmass, through which needles of hornblende, plagioclase, 

 apatite, and granules of magnetite are dispersed. Between the 

 crossed nicols, the transparent minerals, in a thin section, gener- 

 ally display a decided undulatory extinction, in evidence of con- 

 dition of strain. 



This fulgurite exhibits under the microscope the unique char- 

 acter of a wholly devitrified mass, light brownish-gray, delicately 

 laminated in cross-section {a, Fig. 3) by a very irregular flow- 

 structure,' with interruptions and intersections of laminae sug- 

 gesting those observed in the cross-stratification of certain sand 

 deposits. Some laminae, especially near the contact-line {^e. g., 

 just below c'), are bent and faulted, with ends slipped past each 

 other. The thicker laminae consist of very minute, colorless 



' Compare the thin underl3dng coat, in which fusion is less complete, with dark 

 fluidal banding, and which envelops numerous crystal remnants, described by Diller 

 in the "mixed zone" of fulgurite-glass in immediate contact with basalt at Mt. 

 Thielson Uoc. cit.). Of this he states, it is difficult to conceive how it has been pro- 

 duced, "unless it is due to the repulsion of the particles among themselves." 



