In some places cinder cones were built about the openings by the
fragments of lava which were hurled out. In other places, during
periods of less explosive eruption, molten lava flowed out in vast
quantities. The lava was very hot and almost as liquid as water,
so that it spread in thin sheets over hundreds of square miles
of lowland.

One important series of fissures through which eruptions took place
marked the line where the Cascade Range was to be built. Other
volcanoes appeared over the surface of southern Idaho, central
Washington, Oregon, and northeastern California.

The eruptions were not continuous over the whole field; now in
this place, now in that, there came long periods of quiet. During
such periods the earthquakes ceased, the lava became cold, and
the clouds of volcanic ashes cleared from the air. Frequently the
lava intercepted streams and blocked the valleys so that large
lakes were formed. Whenever the periods of quiet were very long,
plants spread over the surface and animals of many kinds made their
homes about the lakes.

In eastern Oregon the John Day River and its branches have eroded
canons through the later lava and have exposed the sands, clays,
and gravels which collected at the bottom of one of those ancient
lakes. In these beds the skeletons of many strange and interesting
animals have been found. Evidently they had once lived about the
borders of the lake, and the streams had washed their bones into
the water and mingled them with the sediment.

[Illustration: FIG. 8.--BLUE LAKES, IDAHO

Formed by springs issuing from underneath the lava of the plateau]

One of these animals appears to have been an ancestor of the present
horse. It was about the size of a sheep, and had three toes instead
of one. Another, probably a very dangerous animal, was related to
our present hog, but stood nearly seven feet high. Others resembled
the rhinoceros, camel, tapir, or peccary. All but the peccary are
now extinct upon this continent. Of the carnivorous animals there
were wolves and cats of large size.

The eruptions continued, filling the valleys little by little,
until in places the lava reached a thickness of nearly four thousand
feet. The lower mountains were hidden from sight. We know of the
existence of these buried mountains because the wearing away of
the lava in some places has exposed their summits to view.

The lava flood reached farther and farther. In southern Idaho it
formed the Sn