THE NORTHERN PACIFIC ROUTE. 99 
Anceney. The plain continues northwestward almost as far as Logan, 
Manhattan. and throughout that distance the only rocks which 
Elevation 4,255 feet, re exposed are the gravel and sand of the river 
Population 570.* flood plain and the slightly more indurated materials 
St. Paul 1,052 miles. of the old lake beds. 
Near Logan the whole of the Paleozoic succession present in this 
region is exposed across Gallatin River, on the north. The first rocks 
to be seen are those of the Belt series (Algonkian), 
Logan. which are so far away that their character can not be 
Elevation 4,114 feet. determined from the train, but the traveler, in the 
ne ill course of his journey farther west will see this series 
in many places and will have opportunity to study it 
at close range. The rocks across the river near Logan constitute the 
southeastern limb of a great synclinal trough (see fig. 25, p. 116) and 
therefore dip toward the northwest, or away from the observer. The 
next system above the Algonkian is the Cambrian, which is repre- 
sented by two formations. The lower one (Flathead) is composed 
mostly of shale and sandstone, but includes a layer of massive lime- 
stone through which the railway passes in a deep cut near milepost 
164. The Gallatin limestone, overlying the Flathead formation, 
makes a prominent bluff across the river which can be seen just as” 
hs ee 
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bation. 
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Pistine 160. ake ea io oe a ee aie Bok ra iat ukcek 
the train enters the yards at Logan. The section as seen from the 
station is represented in figure 18. The Gallatin limestone, at the 
extreme right of the section, is generally light blue or gray and is 
easily distinguishable from the overlying Jefferson limestone (Devo- 
nian), which is dark and more resistant. The Jefferson limestone is 
one of the most conspicuous members of the section, for it is much 
darker than any of the other limestones, and as it lies directly below a 
fairly soft shale it forms a distinct ridge or prominent spurs. It is 
overlain by the Threeforks shale, which is conspicuous only in that, 
as it is softer than the limestones on either side, it forms valleys or 
ravines. Overlying the shale is the massive bluish-gray Madison lime- 
stone, which forms the bluff opposite the station and extends westward 
on both lines of the railway for a distance of at Jeast half a mile. 
From Logan to Garrison the Northern Pacific has two lines, the 
original line turning to the right and running by way of Helena and 
the other keeping to the left by way of Butte. A description of the 
Helena line begins on page 116. 
