

CRETACEOUS AND TERTIARY—THREE BUTTES. 123 
them, numerous more or less extensive exposures of the rocks of the 
Lignite Tertiary, occur. As might be anticipated from the nature of the 
deposit, as it has been already noticed on the Souris River and elsewhere, 
few of the beds are found to have any great horizontal constancy; but, 
from their general similarity, any very detailed account of the various 
exposures would be but repetition. Clays, and arenaceous clays, with 
intercalated sands and sandstones, and in some places nodules and thin 
sheets of ironstone; in various modifications of thickness and arrange- 
ment, build up the formation. 
303. The isolated mountains, called by the MHalf-breed hunters, 
Montagnes du Foin de Senteur, are known to traders of the Missouri 
Region, as the “Sweet Grass Hills.” They are roughly indicated on 
most good maps of the west, and are there found under the geographical 
appellation of the ‘Three Buttes.” As indicated by the latter name, 
there are three distinct mountain masses. A line passing from the peak 
of the eastern, through the central mass of the western Butte, would 
have a direction of about N. 80° W., the Central Butte lying between 
them, but some miles to the south. The highest summits are those of 
the eastern and western Buttes, which are about twenty miles apart, 
and rise nearly 3,000 feet above the level of the plains at their base. The 
height of the summit of the East Butte, as ascertained by the aneroid 
barometer, and taking the mean of two readings separated by about an 
hour, is 6,200 feet. That of the West Butte was found to be 2,746 feet 
above the Depét Camp at its base, by comparison with nearly simultan- 
eous readings there; and taking the height of the latter locality as 3,787 
feet, the height of the West Butte above the sea, would be 6,483 feet. 
304. The central masses of the Buttes, are composed of eruptive 
trappean rock, and around them, the previously horizontal beds of the 
plains have been tilted up, those immediately surrounding the igneous 
masses, resting at very high angles. The western Butte is the most 
important, and forms quite a little mountain region, having numerous 
peaks and ridges, with round or blunted tops, and deep, almost precipit- 
ous vallies. The eastern Butte is next in importance, and consists of 
four main peaks, arranged nearly as the angles of a square. The 
north-western of these is the most prominent and conical; the south- 
western is round-topped and connected with the north-western by a ridge, 
and not far below it in height. The north-eastern and south-eastern 
summits are nearly equal in elevation, but considerably less than the others. 
The central Butte was not ascended, but appears to be considerably 
