ALTERATION OF THE EARLIER ANDESITE. 217 



DIFFERENCES OF PHASES EXPRESSED BY DIAGRAMS. 



The changes in the proportions of the various elements in the rocks can be 

 illustrated by diagrams in such a way as to be clearer than discussion. In tig. 73 the 

 proportions are represented by straight lines. As is usual and more accurate, the 

 proportions plotted are the quotient figures obtained by dividing the weight per- 

 centages by the molecular weights. The scale is 0.01 in the quotient figure = one- 

 fortieth inch (fig. 73). 



The diagrammatic lines representing the different elements may be grouped 

 together for each analysis, and be arranged as radii of a circle, with lines connecting 

 the ends of the radii to form an irregular, polygon, forming a diagram slightly 

 modified from that used by Brogger" (PI. XXIV). 



STUDY OF ALTERATIONS INDICATED BY ANALYSES. 

 ALTERATION OF EARLIER ANDESITE FROM LOWER PART OF SIEBERT SHAFT. 



The proportions of the different constituents represented by the diagrams of 

 rock No. 1 (PI. XXIV) are practically identical with those in a fresh andesite. That 

 this is so is shown by the diagram (), prepared in a similar way to those referred 

 to above, by Prof. W. H. Hobbs, to illustrate the typical composition of andesite. 6 

 The analysis upon which this diagram was based was obtained by averaging seven 

 analyses of mica and hornblende andesites from the Eureka district, Nevada; Ouster 

 County, Colo.; Cartagena, Spain; the Siebengebirge on the Rhine; Panama; and 

 Colombia. The scale of the diagram has been adjusted by the writer to correspond 

 with the scale of his own. From this general correspondence it becomes apparent 

 that the profound alteration which rock No. 1 has undergone has resulted in 

 decomposing the original minerals and changing the constituent elements to new 

 minerals more stable under the new conditions that is, in the presence of the 

 permeating waters. 



a Hobbs, W. H., Jour. Geol., vol. 8, pp. 1-31. 6 Op. cit., p. 23. 



