270 



MINERALOGY 



FIG. 384. Dendritic Psilomelane. Leadville, Colorado. 



Geodes. Are cavities in clays or other formations, which have 

 been encrusted with a wall of quartz or other mineral, and which 



FIG. 385. Platinum Nugget. Nischi- 

 Tagilsk, Siberia. 



FIG. 386. Nodular Pyrite. 



separate as a hollow mass, the interior walls of which are usually 

 studded with crystals, Fig. 387. Small almond-shaped steam cavi- 

 ties occurring in lavas are often filled by minerals, deposited from 



