4 PEOCEEDINGS OF THE NATIONAL MUSEUM vol. C8 



LITHOIDITE 



Through an increase in the proportion of stony structure, there is 

 a complete gradation of the glassy obsidian into a rock entirely 

 crystalline, though usually microcrystalline — the lithoidite. In this 

 rock the groundmass is partly minutely crystalline, partly aphanitic, 

 and through it are sometimes scattered phenocrysts of feldspar or 

 quartz. The aphanitic facies are colored some shade of brown or 

 dark gray, while the crystalline i^ortions are usually light gray, so 

 that this contrast of colors gives the rock a distinctively mottled 

 appearance. Scattered throughout the mass are irregular cavities 

 lined with minute crystals, causing it to appear rough and harsh to 

 the touch. As the aphanitic j^ortions become more visibly granular, 

 the entire rock becomes more uniformly gray in color, and the irregu- 

 lar cavities give way to those more symmetrical. These cavities are 

 often of extreme delicacy and beauty, especially in those facies of the 

 lithoidite that are decidedly laminated. 



Although this rock is usually placed among the rhyolites, it differs 

 from the true rhyolites in its mineralogical composition. It may be 

 chemically classed as a soda-rhyolite^ but none of the calculated 

 normative minerals of rhyolite are found in its mode. This is 

 brg^uglit out by the following analysis by J. E. Whitfield,^ and the 

 calculated norm and mode of the rock. 



Analysis of Lithoidite, Obsidian Cliff 



Per cent 



Silica (SiOi) 75. 50 



Titanium oxide (TiOi) uone 



Alumina ( AI2O3) 13. 25 



Ferric oxide (Fe203) 1.02 



Ferrous oxide (FeO) .91 



Manganese oxide (MnO) uone 



Lime (CaO) . 90 



Magnesia (MgO) . 07 



Lithia (LisO) . 00 



Soda (NasO) 4. 76 



Potasli (K2O) 2. 85 



Phospliorus pentoxide (P2O5) uone 



Sulphur trioxide (SO3) .32 



Water (H2O) . 41 



100. 05 

 Norm and mode of Lithoidite, Obsidian Cliff 



Norm 



Per cent 

 Quartz 34. 21 



Orthoclase 17. 2 



Albite 40. 3 



Anorthite 4. 4 



Corundum . 6 



Hypcrsthene 1. 1 



Magnetite , , 1. 4 



» U. S. Geol. Surv., Bull. 150, 1898, p. ICO. 



Mode 



Per cent 



Trid.vmite 34.5 



Natrosanidine 1 61. 6 



Fayalito 2. 7 



Hematite 1. 



Hornblende . 8 



