ROCK ALTERATION. 

 ALTERATION OF EARLIER ANDESITE FROM NEAR MIZPAH HILL. 



Rock No. 4 may be compared with the average type thus: 



Comparison of Tonopah rock No. 4 with average type. 



223 



In this rock, as in rock No. 3, the removal of magnesia, iron, and soda 

 has gone on till only trifling quantities remain. In this rock also, the iron, which 

 was relatively free from attack in the first three specimens, has been partly dis- 

 solved, so that over half has been removed. Even the difficultly soluble alumina 

 has apparently lost a little, though this is doubtful. Of the metallic bases, iron, 

 lime, magnesia, and alumina, about 40" per cent has been removed, and of the 

 same, excluding alumina, about 73 per cent. On the other hand, while soda has 

 diminished, the amount of potash has increased, the increase of one nearly com- 

 pensating for the loss of the other. The silica also has increased largely. 



In this case, then, the waters which altered the rock were charged with an 

 excess of silica and potash, which they deposited, attacking and dissolving all 

 the other components of the rock, the relative order of attack, dependent on 

 their solubility in the attacking waters, being lime, magnesia, soda, iron, and 

 alumina. 



a In this case, as in many of the similar cases in the following pages, the percentages given are in terms of each 

 constituent. The reader will notice, however, that the percentages are elsewhere given in terms of the entire rock, 

 where such presentation has best lent itself to statement. This is the case with all of the figures on the preceding pages 

 and some in those which follow. The writer believes there will be no confusion brought about by the use of these two 

 methods of presentation; if any such should arise, a glance at the compared analyses will suffice for an explanation. 



