The coal-bearing rocks of Montana belong to three geological horizons, all of Mesozoic age, and over three-fourths of the 145,000 square miles comprised within the boundaries of the state is underlain by these rocks. The larger part of the great plains, that, monotonous expanse of arid, treeless country forming the eastern two-thirds of the state, is underlain by seams of lignite, which occurs in great quantity and is often of exceptional purity. The banks of the smaller streams and the bluffs of the rivers very frequently show the outcrops of these lignite seams, and their dark lines can be traced continuously for many miles in the buttes of the so-called “ bad lands.”
Approaching the mountains, the low relief of the plains is broken by outliers of the Rocky Mountains, and as we near the eastern slopes of these ranges the younger strata of . . .