192 
IDDINGS. 
rocks in the field and their petrographical examination ad¬ 
vance hand in hand. 
The relation, however, between the geological occurrence 
and the mineral composition of igneous rocks is less under¬ 
stood, and seems not to have entered sufficiently into the 
consideration of these rocks as crystallized magmas. This 
relation is a matter of fundamental importance for the estab¬ 
lishment of the proper connection between different varieties 
of eruptive rocks, which necessarily underlies a natural sys¬ 
tem of their classification. 
The study of the igneous rocks at Electric Peak and Se¬ 
pulchre Mountain—incidental to the investigation of the 
Yellowstone National Park under the charge of Mr. Arnold 
Hague—offers a valuable contribution to our knowledge of 
these relations. In order to present some of the results of this 
study it will be necessary to sketch the geology of the region 
and describe the eruptive ro#ks occurring in it. After which 
the variations in their mineral composition and crystalline 
structure will be discussed. 
Geological Sketch of the Region. 
Electric Peak is the highest point of that portion of the 
Gallatin Mountains lying within the Yellowstone National 
Park, and reaches an altitude of 11,100 feet. It has been 
carved out of Cretaceous shales and sandstones, which, with 
the underlying strata of this range of mountains dip grad¬ 
ually toward the northeast. Occupying a synclinal trough 
between two bodies of Archaean gneiss, these strata have 
undergone more or less folding and displacement, which 
has been accompanied by the eruption of igneous magmas. 
The earlier of these eruptions forced sheets and laccolites 
of intrusive rocks between the strata wherever the nature of 
the beds offered planes of least resistance to the dynamical 
forces engaged in their plication. Hence the sheets are 
more abundant and smaller where the strata were very 
fissile, and abound in the shales and thinly bedded sand¬ 
stones of the Cretaceous. The mass of Electric Peak is 
