130 GUIDEBOOK OF THE WESTERN UNITED STATES. 



the harder rocks above, so that the red sandstone cUffs 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 Eiver 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 

 PI. LVI, 5), 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. 



