
THE SYLVA OF MONTANA. 415 
the grass grows well, and therefore trees became very scarce 
when we entered the "Cretaceous formation No. 1," which 
is of a porous character, not retaining moisture in its strata 
like many parts of the Tertiary farther down, though covered 
with a very close growth of grass. As usual throughout 
the route it is the slopes facing the north that have most of 
the woods on them. The species met with were the eastern 
Pinus rigida and Juniperus Virginiana, the boreal J. com- 
munis, the western Pinus monticola and Abies Douglassit. 
If any other occurs it is, probably, Pinus ponderosa, which 
grows in the Black Hills toward the south-east, according to 
Dr. Hayden. 
Along the rivers a different group, the deciduous trees 
found in that situation throughout the plains of the Missouri 
basin, reached a little above Milk river, nearly all, however, 
ceasing at the point where the mountain woods begin.* 
Above here only Populus balsamifera occurs in scattered 
spots with stunted shrubs of Megundo aceroides and Prunus 
Virginiana, so that for several days below Fort Benton, 
one hundred and seventy-five miles by the river, the boats 
could scarcely obtain enough wood for fuel, and there is 
almost none to be seen. Populus angustifolia also begins at 
Maria’s river, and is the prevailing species along thes upper: 
branches of the Missouri. The same destitution of wood 
continues from Fort Benton to the “Gate of the Mountains” 
along the Missouri, though its branches are better supplied 
with the same trees. Thus the influence of the soil belong- 
ing to "Cretaceous No. 1" is the same throughout its limits, 
dii I believe is due to the causes above mentionand rather 
than to its Cretaceous nature, since on the lower Missouri 
it is much more productive of timber than “No. 4” of Dr: 
Hayden’s section, or his “lignite tertiary basin,” probably 
because it there receives more rain. 
*Those seen below only (above Fort Union) were Frazinus A Americana (or sambuci- 
cedo eee U imus Americana, the the former preity common, the latter rare: Quer- 

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