ALTERATION OF THE LATER ANDESITE. 



243 



carbonic acid, and sulphur in the form of iron sulphide the writer has constructed 

 diagrams altered from the preceding, so that these may also be represented (fig. 

 76). Ten radii instead of eight are taken, representing the different elements as 



KEY 



Scale: .01 (quotient f igure) = A inch 

 Silican and water.OI *, inch 



FIG. 76. Diagram showing changes in composition during alteration of the later andesite. 



shown in the key. The arrangement of the elements differs from that in the 

 preceding diagram, the water, carbonic acid, and iron sulphide beiog grouped 

 together, as well as lime, magnesia and iron, and soda and potash. Silica is 

 assigned one radius instead of two, as in the preceding diagrams, and since its 



