
128 B. N. A. BOUNDARY COMMISSION. 
and from analogy with other circumstances, elsewhere more fully 
discussed, must have taken place to a great extent before the glacial era. 
They have, indeed, the appearance of having been subjected to a very 
long course of erosion in later Tertiary times, and may probably date 
from that period of disturbance closing the ‘Lignite Tertiary period, 
of which very extensive traces are found elsewhere in this western 
region. 
314. The quaquaversal dip of the sedimentary rocks round the 
central igneous masses of the Buttes, suggests a resemblance to Von 
Buch’s craters of elevation. This, though rather an exceptional mode 
of mountain formation elsewhere, would appear from the observations 
of Dr. Hayden and others, to be common in the west. Bear Peak, near 
the Black Hills, may be taken as an instance; and there the features 
represented in the Buttes appear to be almost exactly paralleled. Dr. — 
Hayden thus writes of the locality: ‘This peak is an isolated protrusion 
of basaltic rock, very compact, rising to a height of 1,500 feet above the 
prairie around, and separated from the Black Hills by an intervening 
space of seven or eight miles. All round the peak the disturbed beds 
form annular ridges, receding from the central point like the waves of 
the sea.” ** The central mass is elsewhere described as consisting of 
“porphyritic trappean rock.” Near Fort Benton,—which is only about 
eighty miles south of the Buttes,—eruptive rocks of post-cretaceous age 
appear to be frequently met with, and their contemporaniety with such 
masses as those of these mountains seems not unlikely. In the Report 
ust quoted, Dr. Hayden says: “ We have near the Arrow Creek a bed of 
erupted material thrust between Cretaceous rocks, which presents a 
vertical wall of 150 to 200 feet at one point, and then suddenly ceases. 
These small centres of effusion of melted rock seem to cover the whole 
region.” 
315. In the very valuable coloured map accompanying the same 
report, the geology of the country surrounding the Buttes, or Sweet 
Grass Hills, is conjecturally indicated, and reproduced without question 
in Hitchcock and Blake’s map, published in 1872. The annular structure 
there extended to these mountains from analogy, proves to be justified 
by the fact; but whereas in conformity with the structure of similar 
mountain masses to the south, rocks of Carboniferous, Triassic, and 
Jurassic ages, are represented as forming wide zones round the central 
protrusions, in a country based on the overlying Cretaceous rocks, I 
* Geological Report Yellowstone and Missouri Expedition., 1869, p, 42. t Ibid., p. 93, 
