454 EEPORT UNITED STATES GEOLOGICAL SURVEY. 



This rock lias the appearance and qualities of a very compact or 'baked, clay. It 

 adheres strongly to the tongue, when breathed upon has a distinct pipe-clay odor, 

 and the powder yields a plastic mass with water. Not being familiar with this region, 

 I am unable to give a full explanation of the character of this rock, but I incline to 

 the view that it is a clay metamorphosed by proximity to a dike or lava stream. 

 The specimens examined may have been taken at some distance from the source of 

 heat, as it seems difficult *to suppose that a clay would retain its plasticity after 

 having been heated to the degree that immediate contact withi the melted rock would 

 cause. 



No. 4. Trachyte from Junction Valley. Collected hy W. H. Holmes. 



The pieces are greenisli blue, interspersed with white and dark spots, and particles 

 of free quartz. Slightly porous. Fracture, uneven. Hardness, 4. 5 ; specific gravity, 

 2.84. Pyrology: Fusibility, 5. 5 ; with borax gives reaction for iron; moistened with co- 

 halt nitrate and heated, the white part turns blue and the green part brown. 



ANALYSIS. 



Per cent. 



SiO.2 69.90 



AL2O3 17.58 



FezOa 2.41 



CaO Trace. 



MgO Trace. 



K2O 4.16 



NajO 2.41 



H2O (by ignition) ^,..... 3.65 



100. 11 

 Note. — In those analyses in which the alumina and ferric oxide are included in the 

 same determination, the quantity of ferric oxide was quite smalU 



