170 THE JOURNAL OF GEOLOGY. 



It is not to be expected that all of the volcanoes of the 

 Andes are in exactly the same phase of differentiation, which 

 they undoubtedly are not, but in a general way they have not 

 progressed beyond the limits of olivine-bearing pyroxine-andes- 

 ite and dacite, and may be considered as having the basalt and 

 rhyolite phases before them. They thus show themselves as 

 comparatively young, or perhaps middle-aged. It will be 

 observed, however, that rhyolite occurs in small amount in 

 Ecuador, as shown by analyses 14 and 15 in Table II. 



The chemical similarity of the magmas of the Andes with 

 those of general occurrence in western North America, which 

 are fairly represented by the volcanic rocks of the Yellowstone 

 National Park, is seen upon constructing a diagram of the molec- 

 ular variation of the essential constituents in a manner employed 

 by the writer'^ and also by Dakyns and Teall^ in discussing the 

 differentiation of molten magmas. 



In Table I are given the chemical analyses of perfectly fresh 

 rocks from Colombia, published by Dr. Kiich. The general 

 molecular character of the magmas is shown by diagram I. In 

 the lowest part of the diagram the molecular variation of all of 

 the essential constituents is represented, that of silica being 

 given by the abscissas, the zero point being some distance to the 

 left. In the middle part of the diagram alumina, soda and pot- 

 ash are separated from the lime, magnesia and iron-oxide, which 

 are given by themselves in the uppermost part of the diagram, in 

 order to avoid the confusion of lines in the lowest part. The 

 iron is represented as ferrous oxide. The character of this dia- 

 gram is quite similar to that of the diagrams for the rocks from 

 the Yellowstone National Park in the paper on the origin of 



biddings (J. P.). The mineral composition and geological occurrence of certain 

 igneous rocks in the Yellowstone National Park. Bull. Phil. Soc, Washington, 8vo. 

 Washington, 1890. Vol. 11, pp. 191-220. 



. The origin of igneous rocks. Bull. Phil. Soc, Washington. 8vo, Washing- 

 ton, 1892. Vol. 12, pp. 89-214, PI. 2. 



=> Dakyns (J. R.) and Teall (J.J. H.). On the plutonic rocks of Garabal Hill and 

 Meall Braec. Quart. Journ. Geol. Soc. 8vo. London, 1892, May 2, Vol. 48, part 2, 

 No. 190, pp. 104-120. 



