BYSMALITHS 707, 
they should be horizontal in the body of a volcanic plug, and 
only vertical in the central part of its summit. In the case of 
Inyan Kara, mentioned by Russell, the dip of the limestone in 
the encircling wall, taken in connection with the diameter of the 
circle and the elevation of the igneous rock, is just what it might 
have been, had the igneous mass been a laccolith. The occur- 
rences mentioned by Russell cannot be considered as illustrations 
of volcanic plugs without further evidence of their relations to 
the surrounding rocks. 
In the Yellowstone National Park* at the southern end of the 
Gallatin Mountains, a great body of dacite-porphyry, three miles 
long and two miles wide, forms Mount Holmes and a group of 
mountains at the head of Indian Creek. In vertical extent the 
exposure of this mass is in all 2300 feet and throughout the 
exposure the character of the rock is so uniform as to indicate 
its being one mass solidified at one time. 
Three quarters of the circumference of this igneous mass is 
in contact with stratified rocks, whose general position is nearly 
horizontal, but which in the immediate vicinity of the intruded 
body are bent abruptly upward, dipping away from it at steep 
angles. In several places the character of the contact plane is 
well shown, especially on the south side of the Dome, where a 
nearly vertical contact can be traced for almost a thousand feet. 
In each case the contact plane is almost vertical, inclining away 
from the intruded mass. From this mass small veins of igneous 
rock have penetrated the adjacent stratified rocks. The latter 
contained a large intruded body of andesite-porphyry in the 
form of a laccolith when the magma of the dacite-porphyry was 
intruded. The western boundary of the Holmes mass lies against 
gneiss and along a fault plane. An opening on this fault plane 
was probably the conduit through which the molten magma rose, 
for similar rock occurs along this fault line three miles to the 
north. While we have at present no knowledge of the configu- 
“Geologic Atlas of the United States, Yellowstone National Park Folio, Areal 
Geology, Gallatin Sheet, Washington, 1896. See also the forthcoming monograph 
32 of the U.S. Geological Survey on the Geology of the Yellowstone National Park 
chap. i. 
