136 



GEOLOGY OF THE YELLOWSTONE JNATIONAL PARK. 



and Nos. 521 and 523 are dacites from the ridge south of Cache Lake. The 

 structure of 521 is shown in PI. XXI, fig. 2. 



The range of variation in the percentage of silica is about the same as 

 that of the rocks at Electric Peak. The character of the variations of the 

 other oxides in these rocks is shown by the accompanying diagram, fig. 3, 

 which represents the variations in the molecular proportions of the essential 

 oxides, and has been plotted in the manner already described. 



A glance at this diagram shows that it has the same form as that of 

 the group of analyses of the rocks from Electric Peak. The variations in 



Fig. 3. — Molecular variation of the rocks of Sepulchre Mountain. 



the oxides other than silica are quite irregular for a gradual change in the 

 silica. The alumina varies rapidly in places and retains a high position in 

 the diagram. The alkalies gradually increase with the silica, the soda mole- 

 cules being twice as numerous as those of potash, and their variations being 

 alike, with one exception. Magnesia varies most widely, and in striking 

 contrast to the alumina; in each instance they vary in opposite directions. 

 The lime is nearly as irregular as the magnesia, both decreasing rapidly 

 from the less siliceous to the more siliceous end of the series. The two 

 oxides of iron are strikingly reciprocal in their variations, the significance 

 of which has been pointed out in discussing the diagram for Electric Peak. 



