WRIGHT: LITHOPHYSAE IN A SPECIMEN OF OBSIDIAN 367 



PETROLOGY. — Note on the lithophysae in a specimen of obsidian 

 from California. F. E. Wright, Geophysical Laboratory. 



In a specimen of obsidian 1 from Little Lake, about 40 miles south 

 of Owen's Lake, Inyo County, California lithophysae occur which 

 resemble the lithophysae of the obsidian from Hrafnthmuh- 

 ryggur, Iceland 2 and are of interest because of their bearing 

 on the general theory of the formation of hollow spherulites. 

 Two hypotheses have been proposed to explain the develop- 

 ment of lithophysae : (a) the total effect is ascribed to hydro- 

 static tension or uniform pull resulting from the contraction of 

 the magma during cooling; (b) emphasis is placed on the out- 

 ward pressure of the gases set free during the crystallization of 

 the spherulites. These two hypotheses are not mutually exclu- 

 sive and probably both factors, shrinkage of the cooling magma 

 and pressure of gas liberated on crystallization, play an impor- 

 tant role in the development of most lithophysae. In the case 

 of the Icelandic lithophysae it has been shown that gas pressure 

 was probably the more important of the two factors ; similarly 

 in the present specimen from California the evidence presented 

 below indicates that gas pressure rather than hydrostatic tension 

 was the effective agent. 



The obsidian. The specimen as a whole is jet black in color 

 and of the characteristic vitreous aspect and conchoidal fracture 

 of obsidian; thin splinters are relatively clear and transparent. 

 The refractive index of the glass is low (n = 1.484) and indi- 

 cates high silica content. Small microlites of a lath shaped 

 mineral showing approximately parallel extinction and negative, 

 rarely positive, elongation are scattered through the glass. The 

 mineral is probably soda-potash feldspar. Minute particles of 

 magnetite are abundant. The total amount of crystallized mate- 

 rial is less than 2 per cent. The specific gravity is 2.353. 



1 Collected by Mr. Chas. R. Fletcher of Los Angeles, Cal., and sent by him 

 for examination to Mr. F. L. Ransome, U. S. Geological Survey, who, in turn, 

 presented it to the writer for study. The specimen is now deposited in the 

 U. S. National Museum. Spec. No. 88922. 



2 F. E. Wright. Obsidian from Hrafntinnuhyrggur, Iceland: Its lithophysae 

 and surface markings. Bull. Geol. Soc. America, 26: 255-286. 1915. 



