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 sometimes styled the Cordilleras of North America (the Rocky Mountains being the eastern members of the system in the United States and Canada), differ from the Laurentian and Appalachian regions in having suffered numerous disorderly movements at dates so recent that the existing relief of the region bears a significant relation to its irregular uplifts; a relation that doubtless once obtained in the older mountain areas of the east, where it has now been obliterated by erosion. It is not, however, only in modern geological periods that mountain-making disturbances have prevailed in the regions of the Western highlands; their geological history is one of repeated and long-continued movement—the ruins of the more ancient upheavals supplying materials for the strata of newer ranges. For example, in Canada an axial belt of ancient rocks is bordered on the east and west by stratified formations of enormous thickness (40,000 to 60,000 ft.), those on the west including a large share of contemporaneous volcanic materials; all three belts having been deformed and upheaved, as well as deeply dissected in the later chapters of geological time. It is, however, important to note that the interval between Palaeozoic and Mesozoic time, in which mountain-making disturbances were so general in western Europe and eastern North America that the older geologists thought them to be of world-wide extent, was here generally passed over in relative quiet, so that continuous sedimentation produced in certain districts a conformable series of deposits from Silurian to Cretaceous time. Furthermore, the Carboniferous period, which gained its name from the extensive coal deposits that were then formed in western Europe and eastern North America, was a marine limestone-making period in the Cordilleran region.

There is here exemplified, as might be expected in a region extending over 3000 m. from Alaska to southern Mexico, and measuring over 1000 m. in breadth at its middle, a great variety of plateau and mountain structures. The broad upheaval of adjacent blocks of earth-crust without significant tilting or disturbance has produced the plateaus of Arizona and Utah. Some of the simplest and youngest mountain ridges in the world are to be found in the broken and tilted lava blocks of southern Oregon. Tilted blocks on a larger scale, much more affected by processes of sculpture, are found in the lofty St Elias Alps of Alaska, the site of some of the greatest glaciers in the world. The wall of a huge fracture, now elaborately carved, constitutes the western slope of the Wahsatch range, facing the desert basin of Utah. Ranges of a relatively simple arch structure are seen in the Uinta mountains of Wyoming and Utah. Arched upheavals also characterize the front range of the Rocky Mountains proper in Colorado and Wyoming and in the Black Hills of South Dakota, bending up the strata of the adjacent plains in the simplest fashion, and producing dome-like mountains, now deeply dissected by outflowing consequent streams. A remarkable change in the structure of the Rocky Mountains occurs north of the Missouri river in Montana and northward into Canada, where the front range is of synclinal or trough structure, with the youngest instead of the oldest rocks along the axis, while the strata of the plains are bent down and overridden in the most abnormal manner. Indeed, mountain structure occurs of so great diversity in various parts of the Cordilleran region as to elude general description. The disturbances extend directly to the western coast line, including not only the coast range of California, but the peninsular area of Lower California (belonging to Mexico) and the detached mountainous islands of British Columbia and Alaska.

Volcanoes of commanding form here and there dominate the plateaus and mountains. Orizaba, Popocatepetl and their neighbours, terminating the Cordilleran system in Mexico; Mount San Francisco, bearing snow and Arctic plants above the nearly desert plateau of Arizona; Mount Shasta, with small glaciers in northern California; Mount Rainier, with extensive glaciers surmounting the Cascade range of Washington; Mount Wrangell in Alaska, and farther on the many cones in the curved chain of the Aleutian islands: all these have been heaped up around vents through which their lavas rose from some deep source. Vast lava floods have been poured out at different times. The southern part of the Mexican plateau is built up in large measure of lava sheets, capped with volcanoes. Extensive lava beds, barren and rugged, cover large areas in north-eastern California. The basins of Snake and Columbia rivers in Idaho and Washington are flooded with older and more extensive lava sheets, whose borders lap around promontories and islands of the “mainland.” Still older lava-flows in British Columbia are now deeply dissected by the branches of Frazer river, and remain only in disconnected upland areas. High plateaus in Utah are protected by a heavy lava capping, the result of great eruptions before the plateaus were uplifted. Here and there rise dome-like mountains, the result of the underground intrusion of lavas in cistern-like spaces, forming “laccoliths,” and blistering up the overlying strata. Thus, by mountain upheaval or volcanic eruption, great altitudes have been gained. Where the uplift has been strong, ranges of truly Alpine form with extensive snow-fields and glaciers occur, as in the Selkirk range of Canada (now traversed by the Canadian Pacific railway), and again in Alaska. Heights of 12,000 and 14,000 ft. are exceeded by numerous summits in the central part of the system; but the dominating peaks are found far in the north-west and in the south. Several mountains in Alaska exceed 18,000 ft. (Mount McKinley, 20,300 ft.; Mount Logan, 19,540 ft.; Mount St Elias, 18,000 ft.); and the great Mexican volcanoes rise nearly as high (Orizaba, 18,250 ft.). Widespread plateaus maintain upland altitudes of more than a mile over vast areas.

As in all regions of great altitude, the erosion of valleys has progressed on a magnificent scale in the Cordilleran region, and the actual form of many of its parts is more the result of sculpturing than of uplifting. The plateaus of Arizona are traversed by the deep cañons of the Colorado river and its branches, at places 1 m. deep, and with elaborately carved walls. Upon the plateaus themselves, long and ragged cliffs of recession attest an even greater work of erosion than the cañons. In all the mountain ranges except those of youngest uplift, valleys have been actively eroded, sometimes producing steep peaks as in Mount Assiniboine (11,500 ft.) in the Canadian Rockies, rivalling the Swiss Matterhorn in sharpness of form; but the greater number of summits have been worn to roughly pyramidal form between wide-flaring valleys, and the mountain flanks have thus come to be extensively covered with rock waste lying on slopes of relatively uniform declivity. Some of the ranges are in a second cycle of dissection, having been once worn down to moderate relief and now being elevated for renewed erosion; the Sierra Nevada of California is believed to be, in part, of this history, having at least in its central and northern parts been well reduced and now again enjoying a mountainous character in virtue of a later slanting uplift en bloc, with rapid descent on its eastern fractured face. Other ranges, almost completely worn down, still remain low, as in south-eastern California, where they are now represented by gently sloping rock floors veneered with gravel and retaining only small remnants of their original mass still unconsumed; thus the end, as well as the beginning, of the cycle of erosion, together with many complications of its progress, are illustrated in different parts of this great and varied mountain system. In the fjorded coast of Alaska, as well as in the higher northern ranges, signs of intense glacial erosion are seen in the cirques at the valley heads and in the discordant junction of the “hanging” lateral valleys and the deep trunk valleys—the floors of the former being cut off on the walls of the latter.

Fitting complements of the deeply-eroded mountains are found in the great accumulations of mountain waste now occupying basins of depression between the various ranges, as in Mexico, Utah, Nevada, Montana and elsewhere. Erosion and transportation here combine to build up the floors of the basins with the waste of the surrounding highlands; a result that is peculiarly beneficial in Mexico where the climate of the plateau basins is rendered relatively temperate by reason of its altitude, and where the surface is easily habitable by reason of its small relief. In the larger depressions, as along the boundary of the United States and Mexico, isolated ranges frequently rise like islands over the plain of waste that has been built up on their flanks. Shallow saline lakes or playas (wet-weather lakes) without outlets lie on the lowest parts of the waste-filled basins; their failure to overflow in rivers discharging to the sea being less the result of enclosure by barriers than of deficiency of rainfall; for it is chiefly in the arid region that the waste-floored basins are best developed. Indeed, the rainfall is often so scanty that the streams from the mountains—where most of the little precipitation occurs—often fail even to form lakes, withering away on the waste plains. In all these cases, the wash of rock waste from the mountains remains on the continent and builds up the basin plains, instead of being carried away from the land to form stratified sediments on the sea floor. The habit of gathering mountain waste in interior basins that characterizes so much of the Cordilleran region to-day is only the continuation of an earlier practice, for extensive basin deposits of Tertiary date are found in many parts of the Cordilleran region; some of them are famous for preserving vertebrate fossils, such as those of the many-toed ancestors of the horse.

Between the loftier western highlands and the lower eastern highlands (Laurentian and Appalachian) lies a great extension of medial plains, stretching in moderate altitude from the Arctic Ocean to the Gulf of Mexico, and having in their middle a breadth of 1500 m. They are composed throughout of nearly horizontal strata and mark a region long exempt from strong disturbance. Although for the most part floored by marine formations, their structure and composition indicate, as has already been said, relatively shallow water. The ancient sea that once occupied the middle belt of the continent therefore had little likeness to the abysmal oceans, but resembled rather the shallow ocean margins that to-day overlap various continental masses—the largest example of this kind now existing being between Asia and Australia. The eastern part of the plains is underlaid by Palaeozoic strata, already mentioned as having been laid down upon the subsiding Archaean continent or folded in the making of the Appalachians; coal beds are here included in the Ohio and middle Mississippi basins. The area of the western plains remained submerged to a later date, preserving a stretch of marine waters to the end of Mesozoic time, and thus resembling the lowland belt of western Asia, which was similarly covered by a broad and a shallow arm of the ocean extending from the Arctic to the European mediterraneans until a late geological date. The surface of the medial plains is not always so even as might be inferred from their name. Both