GEOGRAPHICAL DESCRIPTION OF ROCKY MOUNTAINS REGION. 
103 
Trap rock, in its different forms, is the principal geological formation from the divide separating 
the waters of Snake river from those of the three forks of the Missouri to Fort Hall, and lime¬ 
stone, occasionally intermingled with trap, thence northward to the limits of the survey. Granite 
discovers itself in the southern portion of the survey, and elsewhere. A very superior limestone, 
suitable as a building-stone and for the manufacture of lime, exists in the mountain range east 
and north from the British trading post already referred to, and undoubtedly exists elsewhere 
in large quantities. 
A belt on the eastern slope of the mountains, including the valleys of the streams, possesses 
much the same characteristics of soil as already noticed on the western slopes, but has less 
advantages for lumbering, and has a colder climate. This fertile strip gradually passes into 
the Grand Prairie country, and, on leaving the vicinity of the mountains, the soil gradually 
becomes more thin, except in the numerous broad river valleys, as those ot the High Wood, the 
Judith, the Muscle Shell, &c., &c. The pines end with the mountains, and the only trees are 
found in the growth of cotton-wood lining the streams. Immediately under the mountains is a 
region capable of profitable tillage, and with unlimited pasturage, delightful in summer, and 
though colder than the western valleys, is still milder than the climate of the plains still farther to 
the east. 
I estimate that in the valleys on the western slopes of the Rocky mountains, and extending 
no farther west than the Bitter Root range of mountains, there may be some 6,000 square miles 
of arable land, open grassed lands with good soils, and already prepared for occupation and 
settlement; and that, in addition to this amount, there are valleys having good soils, and favora¬ 
ble for settlement, which will be cleared in the removal of lumber from them. The faint 
attempts made by the Indians at cultivating the soil have been attended with good success, and 
fair returns might be expected of all such crops as are adapted to the northern States of our 
country. The pasturage grounds are unsurpassed. The extensive bands of horses owned by 
the Flathead Indians occupying St. Mary’s village, on Bitter Root river, thrive well winter 
and summer. One hundred horses belonging to the exploration are wintered in this valley, 
and up to the 9th of March the grass was fine, but little snow had fallen, and the weather was 
mild. The oxen and cows owned here by the half-breeds and Indians obtain good feed and 
are in good condition. 
Probably 4,000 square miles of tillable land is to be found immediately on the eastern slopes, 
and the bottoms of the different streams, retaining their fertility for some distance after leaving 
the mountains, will considerably increase this amount. There is a marked difference of climate 
between the two sides, and the comparison of the meteorological results of the winter posts estab¬ 
lished—one at Fort Benton, on the Missouri, and the other near St. Mary’s village, on opposite 
sides of the mountains—will be of great interest as determining with some definiteness the extent 
of this difference. The question of climate will be considered more fully hereafter. 
To bring out more clearly the character of the mountain region, I will, at the risk of some 
repetition, quote from Lieutenant Mullan’s report of his exploration to Fort Hall: 
“Thus we found ourselves at the main camp after an absence of forty-five days, during which 
time we had crossed the mountains four times, completely turning the eastern portion of the 
Bitter Root range, by a line of seven hundred miles, experiencing a complete change of climate, 
and crossing two sections of country, different in soil, formation, natural features, capability, and 
general character; crossing, therefore, in all their ramifications, the headwaters of the two great 
rivers, Missouri and Columbia. We had now a fine opportunity to compare the climate and 
character of the Bitter Root valley with that of the Hell Gate and others in its vicinity. In the 
latter, snow from four to six inches deep was to be found, while in the former the ground was 
perfectly free from snow. It seemed as if we had entered an entirely different region and 
different climate; the Bitter Root valley thus proving that it well merits the name of the valley 
of perennial spring. The fact of the exceedingly mild winters in this valley has been noticed 
