400 WEED AND PIRSSON 
The only geological observer who has heretofore visited this 
region is Professor E. S. Dana, who in 1875 made a visit to the 
southern slopes, east of the debouchure of Rock Creek, noted 
the occurrences of tilted Carboniferous and Cretaceous rocks, 
FIG. I 
and described the peculiar porphyry of the laccolithic butte at this 
point. * | 
In general view, the mountains rise above the plain as a broad 
and low, dark mountain mass, presenting an undulating crest 
line with no sharp peaks. A few points do stand apart from the 
general mass, but when seen from a distance the uplift isa single 
t Reconnaissance from Carroll, Mont., to the Yellowstone National Park, by Col. 
William Ludlow, War Dept., Washington, 1876, pp. 127-130. 
