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Show position, and rests upon different types of surface from place to place throughout the region. Along the northern margin between Springerville and Snowflake, it consists of only a few relatively recent, flat- lying flows of basalt with small, partly- weathered cinder cones superimposed. Farther south toward Alpine, the volcanic series is much thicker and includes many deposits of soft tuff, but cinder cones are lacking. South of this area, where the scenic Coronado Trail reaches its highest elevations, deep canyons dissect the mountains and show that the landscape is formed of a great number of flat- lying basaltic flows with a total thickness of thousands of feet. Such vast outpourings of lava are unexcelled elsewhere in this region and represent a truly impressive volcanic history. Along the Coronado Trail south of Rose Peak, the volcanic rocks exposed to view are largely silicic or intermediate in composition, in contrast to the basic types farther north, and they are much folded and faulted. Furthermore, these lavas probably are considerably older than the basalts and at one time were covered by a southward extension of the basic lavas. Westward from this area across the great valley of Eagle Creek, the products of volcanism cover the surface for many thousands of square miles. Flat- lying sheets of relatively recent basalt, locally with beds of tuff, cover all of the Natanes Plateau and the area northward. Examination of the volcanic rocks along the western margin of the field, that is, in the San Carlos and Fort Apache Indian Reservations, shows that strata of sedimentary rocks upon which the lavas rest are progressively younger and higher in altitude from south to north. In all of this area the underlying formations remain in a nearly horizontal position, having been uplifted vertically as in the Grand Canyon area. Near San Carlos, however, the basalts rest on a surface of Mississippian limestone. Near Fort Apache and White River the black lavas are on red Supai sandstone, and far to the north they are underlain by Permian limestone and later formations typical of the Grand Canyon and Navajo countries. Geologic features of this volcanic field that have outstanding scenic interest include the deep canyons eroded by the Black and the White Rivers, the narrow gorge of the Blue River, and the black volcanic necks or plugs that rise spectacularly above the red standstone east of Fort Apache and once served as feeders to lava flows long since removed. Uinkaret and Shivwits Plateau volcanic fields.- The two westernmost plateau blocks north of Grand Canyon, known as the Uinkaret and Shivwits, are centers of relatively recent volcanic activity. Although these volcanic fields are small beside the San Franciscan and White Mountain fields to the southeast, they contain many elements of interest, especially in areas where the lavas and craters are in proximity to the rim of Grand Canyon. The outstanding features of these areas are discussed in this report, in the section on " Plateaus on north side of Grand Canyon." BASIN AND RANGE PROVINCE The term " basin and range" is applied to a type of country formed of long, usually parallel, mountain ranges separated by flat alluvial plains which are the result of mud, sand, and gravel filling former intervening valleys of considerable depth. West and south of the great plateau is a vast, low desert region, traversed by the lower part of the Colorado River and drained by its tributaries such as the Gila and Salt Rivers. Geologically speaking, this is but a part of the entire area commonly designated as the basin and range province, which includes the interior drainage of the Great Basin and parts of certain other watersheds. Mountain ranges within the basin and range province are of many types whether classed according to structure or rock character. Most of them trend from northwest to southeast, though there are notable exceptions. Some of them are composed largely of stratified sedimentary rocks, some are granitic or metamorphie masses, and still others are dominantly of recent volcanic rock. A majority contain combinations of these materials. Certain of the ranges owe their present positions to the simple tilting of strata along nearly vertical fault planes, whereas others are the result of folding, overthrust- ing, or complex combinations of these processes. Various parts of the province can be classified and grouped in a rough way according to these characteristics. The treatment in this report follows such natural groups. 50 |