32 Rocks of Noyang. 



The alterations which have taken place in this rock make 

 it somewhat difficult to calculate satisfactorily the different 

 mineral percentages. All that can be hoped for, in the 

 absence of more precise knowledge of the composition of the 

 alteration products, is to arrive at a fair approximation. The 

 analysis of the mica affords some light; but I found that on 

 analysis the hornblende would not give reliable results 

 unless that mineral were separated by methods which I had 

 not at command. In the following calculation I have disre- 

 garded the small amount of chloritic alteration, and also the 

 minute plates of some micaceous mineral, one of the altera- 

 tion products of the felspars: — 



