136 
« 
IDDINGS. 
and partly with other neighboring centers where similar 
series of rocks have been erupted. For purposes of discus¬ 
sion they have been grouped by themselves as “ Exceptional 
dike rocks, and lava flows in the Absaroka range.” 
Perhaps the chemical relationship between different varie¬ 
ties of rocks is at first more .apparent than the mineralogical 
relationship, because they all contain the same chemical 
elements in only slightly different proportions, whereas the 
minerals are often quite different. When the chemical varia¬ 
bility is considered in connection with the occurrence of the 
rocks in mass and it is realized how considerable a range of 
variation may occur in one body of rock, and when it is 
remembered that a number of bodies of rock may constitute 
overlapping series of variations, an original connection of 
the whole group of rocks becomes apparent. 
The chemical composition of various rocks of the Electric 
peak and Sepulchre mountain group is shown by the analy¬ 
ses in Table I, to which have been added six. analyses 
of rhyolitic lavas and obsidian from the Yellowstone Park 
plateau, which will be referred to subsequently. The former 
group range from 55.83 to 69.24 per cent, of silica and have 
been discussed in detail in the article on this district in the 
Twelfth Annual'Report of the U. S. Geological Survey. A 
list of the rocks analyzed is given in connection with the 
tables. v 
The molecular variation of the principal oxides in this 
group of analyses is expressed by diagram 1, Plate 2, those of 
silica being taken as abscissas and the others as ordinates. 
The lines are drawn to connect the successive ordinates of 
each oxide in the group and indicate the direction of the 
variation. Diagram la shows the alumina and the alkalies 
by themselves, and diagram lb the lime and magnesia, with 
the iron reckoned as ferrous oxide. The vertical and hori¬ 
zontal scales are the same in all cases and the lateral space 
allowed each diagram represents the same range of silica 
molecules—that is, from .765 at the left-hand end to 1.265 
at the right-hand end, which corresponds to a range of from 
