ai:t. 17 



MINERALS OF OBSIDIAN CLIFF — FOSHAG 



ance, and usually free of phenocrvsts, though those found elsewhere 

 in the park show well-formed crystals of feldspar scattered throui^h 

 a glassy groundniass. The rock presents all gradations from that 

 purely hyaline to a lithoidite, with but a minor amount of glass. 

 One of the features to first attract attention is the abundance of 

 stony spherulites scattered through the glass, sometimes singly, 

 sometimes aggregated into bunches, and often coalesced to form well- 

 defined layers parallel to the planes of flow. In some specimens the 

 glass carries hollow spherulites lined with a wliite coating of crys- 

 talline material, either sparsely disseminated or so abundant as to 

 make up the bulk of the rock. 



The distribution of the spherulites in zones parallel to the flow 

 structure at once suggests that the glass varies somewhat in its 

 physical or chemical properties. The boundary between the laminae 

 rich in these bodies and those free from them is quite sharp in some 

 specimens and in others gradational. The larger spherulites occupy- 

 ing a position in these bands of stony matter are often semihoUow 

 and seem to be the result of spherulitic crystallization that has been 

 carried to a more advanced stage. The luster of the larger ones 

 is dull and the structure more earthy than that of the small, com- 

 pact bodies, and their entire mass is porous owing to minute, closely- 

 spaced cavities. In these spherulitic bands the small spherulites, 

 joined together to form a line, usually pass through the center of 

 the larger, open bodies, frequently dividing them into halves, and 

 at their intersection, the material of the smaller ones, usually 

 rather glassy, changes to a more granular texture. 



Tenne " first brought out the fact that spherulites are but special 

 cases of the crystallization of obsidian glass. A comparison of the 

 following analyses given by Iddings - will show the chemical simi- 

 larity of the glass and the spherulites. 



Awili/ses of ohsididn and sphcniliirx from Obsidian Cliff 



Silica (Si02) 



Alumina (AI2O3). 



Terric oxide (FejOs) 



Ferrous oxide (FeO) 



Iron disulphide (FeS:)... 

 Manfjanous oxide (MnO) 



Lime (CaO).. 



Mapiiesia (MgO) 



SodaCNaaO) 



Potash (K2O) 



Ignition 



Obsidian 



Per cent 



74.70 



13. 72 



1.01 



.62 



.40 



Trace. 



.78 



. 14 



3.90 



4.02 



.62 



99.91 



Spherulites 



Per cent 

 76. 70 

 11. OH 

 1.45 



.39 



3.89 

 4.73 

 .66 



99.80 



■^C. A. Tenne, Zeits. d. Deutsch. Oeol. -Oesell., ]S8.'>, p. 010. 

 8 J. P. Iddings, U. S. Geol. Surv., Mon. 32, pt. 2, 180!), p. 42l>. 



