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AMERICA
[north Alps of Alaska, the site of some of the greatest glaciers with elaborately carved walls, from which the most imin the world. The wall of a huge fracture, now elaborately pressive lessons have been learned regarding the origin of carved, constitutes the western slope of the Wahsatch valleys by river action. Upon the plateaus themselves, range, facing the desert basin of Utah. Ranges of a long and ragged cliffs of recession attest an even greater relatively simple arch structure are seen in the Uinta work of erosion than the canons. In all the mountain ranges mountains of Wyoming and Utah. Arched upheavals except those of youngest uplift, valleys have been actively also characterize the front range of the Rocky Mountains eroded, sometimes producing steep peaks as in Mount proper in Colorado and Wyoming and in the Black Hills Assiniboine (11,500 feet) in the Canadian Rockies, rivalof South Dakota, bending up the strata of the adjacent ling the Swiss Matterhorn in sharpness of form; but the plains in the simplest fashion, and producing dome-like greater number of summits have been worn to roughly mountains, now deeply dissected by outflowing consequent pyramidal form between wide-flaring valleys, and the streams. A remarkable change occurs in the structure of mountain flanks have thus come to be extensively covered the Rocky Mountains north of the Missouri river in with rock waste lying on slopes of relatively uniform Montana and northward into Canada, where the front declivity. Some of the ranges are in a second cycle of range is of synclinal or trough structure, with the dissection, having been once worn down to moderate youngest instead of the oldest rocks along the axis, while relief and now being elevated for renewed erosion; the the strata of the plains are bent down and overridden in Sierra Nevada of California is believed to be, in part, of the most abnormal manner. Indeed, mountain structure this history, having at least in its central and northern occurs of so great diversity in various parts of the parts been well reduced and now again enjoying a Cordilleran region as to elude general description. The mountainous character in virtue of a later slanting uplift disturbances extend directly to the western coast line, en bloc, with rapid descent on its eastern fractured face. including not only the coast range of California, but the Other ranges, almost completely worn down, still remain peninsular area of Lower California (belonging to Mexico) low, as in south-eastern California, where they are now and the detached mountainous islands of British Columbia represented by gently sloping rock floors veneered with and Alaska. gravel and retaining only small remnants of their original Volcanoes of commanding form here and there domi- mass still unconsumed; thus the end, as well as the nate the plateaus and mountains. Orizaba, Popocatepetl beginning, of the cycle of erosion, together with many and their neighbours, terminating the Cordilleran system complications of its progress, are illustrated in different in Mexico; Mount San Francisco, bearing snow and Arctic parts of this great and varied mountain system. In the plants above the nearly desert plateau of Arizona; Mount fiorded coast of Alaska, signs of intense glacial erosion are Shasta, with small glaciers in northern California; Mount seen in the discordant junction of the “ hanging ” lateral Rainier, with extensive glaciers surmounting the Cascade valleys and the deep trunk valleys—the floors of the range of Washington; Mount Wrangell in Alaska, and former being cut off on the walls of the latter. farther on the many cones in the curved chain of the Fitting complements of the deeply-eroded mountains Aleutian islands: all these have been heaped up around are found in the great accumulations of mountain waste vents through which their lavas rose from some deep now occupying basins of depression between the various source. Vast lava floods have been poured out at different ranges, as in Mexico, Utah, Nevada, Montana, and elsetimes. Extensive lava beds, barren and rugged, cover where. Erosion and transportation here combine to build large areas in north-eastern California. The basins of up the floors of the basins with the waste of the surroundSnake and Columbia rivers in Idaho and Washington are ing highlands; a result that is peculiarly beneficial in flooded with older and more extensive lava sheets, whose Mexico where the climate of the plateau basin is rendered borders are varied by promontories and islands of the relatively temperate by reason of its altitude, and where “mainland.” Still older lava flows in British Columbia the surface is easily habitable by reason of its smoothness. are now deeply dissected by the branches of Frazer river, In the larger depressions, as along the boundary of the and remain only in disconnected upland areas. High United States and Mexico, isolated ranges frequently rise plateaus in Utah are protected by a heavy lava capping, like islands over the plain of waste that has been built up the result of great eruptions before the plateaus were up- on their flanks. Shallow saline lakes or playas (wet-weather lifted. Here and there rise dome-like mountains, the lakes) without outlets lie on the lowest parts of the wasteresult of the underground intrusion of lavas in cistern-like filled basins; their failure to overflow in rivers discharging spaces, forming “ laccoliths,” and blistering up the over- to the sea being less the result of enclosure by barriers lying strata. Thus, by mountain upheaval or volcanic than of deficiency of rainfall; for it is chiefly in the arid eruption, great altitudes have been gained. Where the region that the waste-floored basins are best developed. uplift has been strong, ranges of truly Alpine form with Indeed, the rainfall is often so scanty that the streams extensive snow-fields and glaciers occur, as in the Selkirk from the mountains—where most of the little precipitarange of Canada (now traversed by the Canadian Pacific tion occurs—often fail even to form lakes, withering away railway), and again in Alaska. Heights of 12,000 and on the waste plains. In all these cases, the wash of rock 14,000 feet are exceeded by numerous summits in the waste from the mountains remains on the continent and central part of the system; but the dominating peaks are builds up the basin plains, instead of being carried away found far in the north-west and in the south. Several from the land to form stratified sediments on the sea floor. mountains in Alaska exceed 18,000 feet (Mount McKinley The habit of gathering mountain waste in interior basins Mouut Lo a g n, 19,500 feet; Mount St Elias’ that characterizes so much of the Cordilleran region to-day 18,0_/4 feet); and the great Mexican volcanoes rise nearly is only the continuation of an earlier practice, for extensive as high (Orizaba, 18,250 feet). Widespread plateaus main- basin deposits of Tertiary date are found in many parts of tain upland altitudes of more than a mile over vast areas. the Cordilleran region; some of them are famous for preAs in all regions of great altitude, the erosion of valleys serving vertebrate fossils, such as those of the many-toed has progressed on a magnificent scale in the Cordilleran ancestors of the horse. region, and the actual form of many of its parts is more Between the loftier Western highlands and the lower the result of sculpturing than of uplifting. The plateaus Eastern highlands (Laurentian and Appalachian) lies a of Arizona are traversed by the deep canons of the great extension of Medial plains, stretching in moderate Colorado river and its branches, at places a mile deep, and altitude from the Arctic Ocean to the Gulf of Mexico,
