Farther to the east, on the border of the region which we have described, beyond the drainage of the Little Colorado and San Juan and within the drainage of the Rio Grande, there lies an interesting plateau region, which forms a part of the Plateau Province and which is worthy of description. This is the great Tewan Plateau, which carries several groups of mountains. The western edge of this plateau is known as the MESAS AND BUTTES. 55 Nacimiento Mountain, a long north-and-south range of granite, which presents a bold facade to the valley of the Puerco on the west. Ascending to the summit of this granite range, there is presented to the eastward a plateau of vast proportions, which stretches far toward Santa Fe and is terminated by the canyon of the Rio Grande del Norte. The eastern flank of this range as it slowly rose was a gentle slope, but as it came up fissures were formed and volcanoes burst forth and poured out their floods of lava, and now many extinct volcanoes can be seen. The plateau was built by these volcanoes--sheets of lava piled on sheets of lava hundreds and even thousands of feet in thickness. But with the floods of lava came great explosions, like that of Krakatoa, by which the heavens were filled with volcanic dust. These explosions came at different times and at different places, but they were of enormous magnitude, and when the dust fell again from the clouds it piled up in beds scores and hundreds of feet in thickness. So the Tewan Plateau has a foundation of red sandstone; upon this are piled sheets of lava and sheets of dust in many alternating layers. It is estimated that there still remain more than two hundred cubic miles of this dust, now compacted into somewhat coherent rocks and interpolated between sheets of lava. Everywhere this dust-formed rock is exceedingly light. Much of it has a specific gravity so low that it will float on water. Above the sheets of lava and above the beds of volcanic dust great volcanic cones rise, and the whole upper region is covered with forests interspersed with beautiful prairies. The plateau itself is intersected with many deep, narrow canyons, having walls of lava, volcanic dust, or tufa, and red sandstone. It is a beautiful region. The low mesas on every side are almost treeless and are everywhere deserts, but the great Tewan Plateau is booned with abundant rains, and it is thus a region of forests and meadows, divided into blocks by deep, precipitous canyons and crowned with cones that rise to an altitude of from 10,000 to 12,000 feet. For many centuries the Tewan Plateau, with its canyons below and its meadows and forests above, has been the home of tribes of Tewan Indians, who built pueblos, sometimes of the red sandstones in the canyons, but oftener of blocks of tufa, or volcanic dust. This light material can be worked with great ease, and with crude tools of the 56 CANYONS OF THE COLORADO.