That first age of mountain uplift was ended by the coming on of the age of fire. The granite upheaval of the Blue and the Cascade Mountains was blown apart and cracked asunder by volcanic eruption and seismic force. A vast outflow of basalt and andesite swept westward from the Blue Mountains to meet a similar outflow moving eastward from the Cascades. Thus, throughout the Columbia Basin, the surface is mainly of volcanic rock overlying the shattered fragments of the original earth crust. At many points, however, the primeval granite or sandstone surface was not covered, while at frequent intervals the breaking forth of the fiery floods transformed those original rocks into various forms of gneiss, porphyry, and marble. But the greatest result of the age of volcanic outflow was the elevation of the stupendous isolated snow peaks which now constitute so striking a feature of Columbian landscapes.
With the close of the age of fire, the mountain chains were in place, as they now stand, but the plains and valleys were not yet fashioned. Another series of forces must needs come to elaborate the rude outlines of the land. And so came on the third great age, the age of flood. The upheaval of the mingled granite and volcanic masses of the Cascade and Blue Mountains, while at the same time the Rockies were undergoing the same process, imprisoned a vast sea over the region now known by Westerners as the Inland Empire. In the depths of this sea the sediment from a thousand torrents was deposited to fashion the smooth and level valleys of the Yakima, the Walla Walla, the Spokane, and lesser streams, while a similar process fashioned the valleys of the Willamette and other
