130 GUIDEBOOK OF THE WESTERN UNITED STATES. 
the harder rocks above, so that the red sandstone cliffs rise almost 
directly from the water. 
As the traveler enters the canyon below Gypsum he may see that 
the bright-red beds lie in the form of a downfold (syncline)—that is, 
they are higher at the ends of the canyon than in the middle. This 
structure may not be apparent to him at first, but at a point between 
mileposts 338 and 339 he may easily see that the red beds directly 
opposite the train are lower than the same beds are either to the right 
or to the left. This lowest point is called the axis of the syncline; it 
is the line toward which the beds dip from both sides. The layer of 
rock at the extreme top of the hill on the right is dark brown and not 
red like the underlying beds, and it does not lie parallel with the 
other beds but caps the hills without conforming to the dip of the 
beds beneath. The dark rock is so far away that the traveler can 
not distinguish its character, but if it were nearer he would see that 
it is basalt, similar to the sheet of basalt that caps the canyon walls 
below Wolcott. 
In passing down the canyon, before he arrives at the junction of 
Eagle River and Colorado (Grand) River, the traveler has spread 
before him one of the finest examples of a recent lava flow that can 
be found in this country. He can first see this lava flow in the dis- 
tance on the right soon after he passes milepost 340, in a low, dark 
hill in the bottom of the valley. The rock of this hill may not at first 
attract his attention, but on approaching it nearer he can see that it 
is nearly black and presents a striking contrast to the light-colored 
rock of the sides of the valley. This rock can be seen at close range 
at a point about half a mile farther along, where it forms a terrace 
across the river bottom which suggests that the valley was at one 
time filled up to a certain level with this black rock. On closer in- 
spection this black rock is seen to be very rough and broken (see 
Pl. LVI, B), and those who are familiar with lava flows will at once 
recognize its character, though others may have difficulty in realizing 
that this mass of rock was once molten matter that was forced up 
from the interior of the earth through some vent in the solid crust 
and that flowed down into this valley much as thick molasses flows 
in cold weather. This fiery mass could not flow rapidly, for its outer 
part was continually cooling and being “frozen” into solid rock. 
The crust thus formed would hold the lava for a time, but it would 
finally burst and the fiery flood would once more roll along until it 
was again held up by the cooling of the surface. This drawing off of 
the liquid lava produced caverns beneath the solid crust, which in 
time broke and fell in, so that the surface is now very rough. The 
edge of the flow, shown in Plate LVI, C, can be seen from the train 
as it follows the bank of Eagle River on the opposite side. 
