214 GEOLOGY OF THE OOMSTOCK LODE. 



rents are greatly obstructed, and move in labyrinthine paths of least resist- 

 ance. Of this the lithologist is constantly reminded by meeting wholly fresh 

 crystals and entirely decomposed ones of the same mineral close together. 

 One tiny current percolating through the rock may meet with comparatively 

 lai'ge quantities of carbonates and become saturated, while another in the 

 same neighborhood remains well charged with carbonic acid and oxygen. 

 If the suggestion made is correct, the former coming in contact with chlo- 

 rite would convert it into a mass of carbonates, quartz, and ferric oxide; 

 while the latter, which would be a solvent for carbonates, would convert 

 chlorite into epidote. 



Nature of the decomposition of the bisiiicates. — Qualified by all tlie doubts whicli liave 

 been expressed, the observations considered in connection with the chemical 

 possibilities lead to the following as the most probable statement of the 

 deconi):)Osition of the bisiiicates of the Washoe rocks. Waters charged 

 with hydrosulphuric and carbonic acids, but containing no free oxygen, at 

 temperatures probably very near the boiling-point, acted upon the fresh 

 augite and hornblende ('or mica), producing from them pyrite, chlorite, 

 quartz, and carbonates of the alkaline earths sinuiltaneousl)' Of these a 

 large portion of the carbonates passed into solution. At a later period sur- 

 face waters at lower temperatures, containing carbonic acid and free oxygen 

 in solution, produced a further alteration of a portion of the chlorite in the 

 rocks near the surface, or peculiarly accessible from it. Where carbonic 

 acid was present in excess epidote resulted ; where, through saturation with 

 carbonates, the carbonic acid was deficient, the chlorite was altered to car- 

 bonates, quartz, and metallic oxides, no doubt with admixtures of less impor- 

 tant compounds. 



Magnetite. — No phicc luis bcen given to magnetite among the decomposi- 

 tion products of the bisiiicates. As all the rocks contain large quantities 

 of this nnneral constantly associated with the bisiiicates, and often so thickly 

 distributed in perfectly fresh crystals (particularly of hornblende) as to leave 

 but little of the host visible, it is difficult to distinguish sharply between 

 the primitive and the secondary occurrences of the iron ore. In fact, I have 

 not been able to make absolutely sure of more than one or two instances 

 of secondary n)agnetite, though such an origin seems probable enough in 



