THE MINEKALS OF OBSIDIAN CLIFF, YELLOWSTONE 

 NATIONAL PARK, AND THEIR ORIGIN 



By William F. Foshag 



Assistant Curator, Divisio-n of Mineralogy and Petrology, United States 



National Museum 



INTRODUCTION 



This study was undertaken with two objects in view; first, to 

 determine and describe the minerals that are found in the cavities 

 in extrusive volcanic rocks, and, second, to obtain, if possible, in- 

 formation on the character and action of solutions given off by mag- 

 mas. The results of my study of the minerals of Obsidian Cliff, 

 Yellowstone National Park, are here given, and it is expected that 

 other investigations will follow as material becomes available. 



No field examination of the occurence was possible, and this 

 paper is based upon a study of the extensive suite of specimens in 

 the United States National Museum, including the material collected 

 and described by the late Dr. J. P. Iddings.^ A number of specimens 

 from similar occurrences in California, Utah, Mexico, and Lipari 

 Islands were examined for comparison. 



The minerals in the cavities of volcanic rocks may be considered 

 as the final products of chemical reactions, and may be expected 

 to furnish some evidence as to the original nature of the solutions. 

 In the case of flow rocks, the physical environment can be determined 

 with a fair degree of certainty. The physical chemistry of solutions 

 at high temperatures and pressures has been determined to some 

 extent by Smits.- Centnerswer,^ Niggli,* and Morey,^ and will not 

 be discussed at this time. 



^ U. S. Geol. Surv., Mod. 32, pt. 2, 1899. 



2 A. Smits, Zeits. f. Phys. Cliem., vol. 51, 190.^, p. 193; vol. 52, 1905, p. 498; vol. 54, 

 190G, pp. 498, 512 ; vol. 67, 1909, p. 464 ; vol. 76, 1911, p. 445. 



3M. Centnerswer, Zeits. f. Phys. Chem., vol. 46, 1903, p. 427; vol. 61, 1908, p 356. 



*P. Niggli, Neues. .Tahrb. f. Min., 1914, p. 69; Zeits. f. Anorg. Clieiii., vol. 76, 1912, 

 p. 161 ; vol. 77, 1912, p. 321 ; .ilso Niggli and Morey, Journ. Amor. Chem. Soc, vol. 35, 

 1913, p. 1086. 



6G. W. Morey, Journ. Amer. Chem. Soc, vol. 39, 1917, p. 1174. 



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