THE NORTHERN PACIFIC ROUTE. 161 
at that time, but unfortunately no one has studied them carefully 
enough to read their history in detail. 
The bedrock on all sides is the basalt, which consists either of hard, 
dense rock that represents the interior of a lava flow or the more 
scoriaceous material of the upper part caused by the expansion of 
steam.' 
Although the flows of lava were very extensive, either the molten 
material did not completely engulf the hills composed of older rocks, 
cr the lava that buried them has since been removed, uncovering the 
schist at the surface over small areas. Such an area can be seen on 
the left (east) just before reaching Marshall (see fig. 34), and there are 
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of old rocks in the midst 
of the flood of lava are 
found only near the mar- 
gins of the lava plain, where the depth of the once molten mate- 
rial was never great or where the underlying surface was particu- 
larly hilly. 
From Marshall a branch of the Northern Pacific runs southward 
to the Palouse country. Two other railways that run parallel with 
the Northern Pacific between Marshall and Spokane 
The nearer line is that of 
Figure 34,—Island of gneiss or schist in the lava (Yakima 
basalt) near Marshall, Wash. 
Marshall. can be seen on the right. 
Elevation 2,137 feet. the Oregon-Washington Railroad & Navigation Co. 
nie tance? geal and the other is the Spokane, Portland & Seattle 
St. Paul 1,514 miles. 
Railway. Just beyond milepost 98 the Northern 
Pacific crosses both these lines and continues on their right to Cheney, 
the next station. 
1 Molten Java, as it issues from the earth, 
is always charged with steam. While the r 
lava remains underground the steam can | vertical, is the most universal and strik- 
not escape, but when the lava reaches the | ing structural feature of the basaltic flow. 
e pressure is relieved and the tly in size and 
steam expands and collects in bubbles, 
which rise to the surface and burst, giving 
off the clouds of vapor that always rise 
from a moving flow of lava. Many bub- 
bles, however, can not escape, and they 
become, as it were, frozen in the upper 
part of the flow, giving it a vesicular or 
honeycombed appearance, as can be seen 
from the train in many places. 
95558°—Bull. 611—15——11 
a tule pentagonal (five-sided) or hexag- 
onal (six-sided), and polygons of four, 
seven, or eight sides are also common. 
Columns, such as those described above, 
may be seen in almost every good ex- 
posure of the hardened lava. 
