THE NORTHERN PACIFIC ROUTE. 89 
The main outer range which forms the gateway through which the 
train enters the mountains is composed of very old (Paleozoic) rocks, 
chiefly limestone, shale, and quartzite. 
The great mountain mass beyond this outer range is anticlinal in 
structure;, that is, it once formed an immense arch. The top of the 
arch is eroded and the traveler can not see the formations rising on 
one side of the fold, curving over the top, and descending on the 
other side, but they once formed such an arch, and the flanks and 
1 The smooth slopes of the terrace on the. 
not recognizable fr 
posite mile- 
underlie the dark shale of the Colorado. 
These beds constitute one of the most re- 
=- tig 
es 
i 
The next formation to attract attention 
oe £ ee Pee + a es 
makes the mountain front and which is 
ons os the — conspicuous sedimen ntary 
e Rocky Mountain region. 
It will be seen many times on the trip and 
be traveler will doubtless learn to recog- 
ot wherever seen. This thi ck- bedded 
is known as the Madi ison limes It 
makes the front of the sar? on Segoe 
east side of the river as far as the eye can 
see and on the west side for a distance of 
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FIGuRE 13.—C ti the rim of tk tai th of Livingston, Mont. The rocks were 
sistant formations of the plains and on the 
de of the river they make a hogback 
4 thich is a fine example of its kind. 
‘This formation was at one time regal 
asthe Dakota sandstone, but isnow known 
to be the Kootenai, of Lower Cretaceous 
age. At is the same formation as that 
the coal beds at Sand Coulee and 
Belt, southeast of Great Falls, but in the 
icini 
there are shale and 
fossil sea shells of Jurassic age. 
7 or 8 miles. Its base can be seen about 
th old : 
than the wey Peter sandstone, which was 
Paul. 
seen at St. 
