North America

The Western Highlands.

The great complex of mountains in the Western highlands, sometimes styled the Cordilleras of North America (of which the Rocky mountains are eastern members in the United States and Canada), differs from the Eastern high lands in having suffered strong deformation in late geological periods. On the other hand, the transition from Palaeozoic to Mesozoic times, when mountain-making disturbances were so gen eral in western Europe and eastern North America that the older geologists thought them to be of world-wide extent, was here gen erally accomplished in relative quiet, so that in certain districts a conformable succession of stratified formations was accumu lated in great thickness from Cambrian to late Mesozoic or early Tertiary time. Further, the Carboniferous period was marked by the deposition of marine limestones in the Cordilleran region. In contrast to the long quiescent Atlantic coast, the Pacific coast of North America is bordered for a good part of its length by mountains of late origin.

Volcanoes of commanding height here and there dominate the Western plateaux and mountains. Orizaba (18,240 ft.), Popo catepetl and their neighbours crown the southern portion of the Cordilleran system in Mexico; Mt. San Francisco rises over the desert plateau of Arizona and bears snow and Arctic plants; Shasta has small glaciers in northern California ; Rainier, bearing large glaciers, surmounts the Cascade range in Washington ; Wrangell is a lofty volcanic mass in Alaska. Vast lava floods have been poured out at various periods ; those of the Snake and Co lumbia river basins in Washington, Oregon and Idaho are the most extensive. Similar lava-flows in British Columbia have been broadly uplifted and are now deeply dissected by Frazer river and its branches, leaving only disconnected highland patches.

As in all regions of great altitude, the erosion of valleys has on a magnificent scale in the Cordilleran region. The plateaux of northern Arizona are traversed by the canyons of the Colorado river and its branches, at places over a mile in depth ; yet upon the plateaux themselves, long and ragged cliffs of reces sion attest a vastly greater erosion before the uplift of the pla teaux than is demanded by the canyons after uplift. Along the Pacific coast, as well as in the higher ranges of the northwest inte rior, intense glacial erosion during the Glacial period excavated huge cirques at the valley heads and deep troughs along the valley courses; on the coast these troughs are now occupied by sea water as fjords. Fitting complements to the deep erosion of the mountain masses are found in the great accumulations of moun tain waste in various intermont and piedmont basins, of which the so-called valley of California is the finest example. Similar basin deposits of Tertiary date abound in the Cordilleran region.

Central Plains.

Between the lower Eastern highlands and the higher Western highlands lies a great extension of medial plains, stretching in moderate altitude from the Arctic ocean to the Gulf of Mexico and having at their mid-length a breadth of 1,50o miles. They are composed throughout of nearly horizontal strata and mark a region long exempt from deformation. The eastern plains, best represented in the Ohio valley, are underlain by Palaeozoic strata, already mentioned as having been laid down on the subsiding Archaean continent ; the western plains are com posed of Mesozoic and Tertiary strata. Both east and west large areas of the plains do not owe their present evenness to the preservation of their originally smooth surface, but to the de gradation of that surface from the considerable altitude of its first uplift to low relief. The degraded surface has, however, been

smoothed by veneers of till plains in the Ohio and upper Mis sissippi valleys, and by the addition of extensive piedmont detrital deposits in the central-western plains.

The Cordilleras of North and South America are not only far out of line with each other, but are separated by some 1,200 m. in Central America, where, in association with the West Indies, an Antillean system of late geological deformation with east and west trends is believed to prevail, with abundant volcanic addi tions on the Pacific border and along the curved range of the Lesser Antilles, while in an intermediate space the calcareous low lands of Yucatan resemble those of Florida in being the emerged parts of a larger area, much of which is still below the waters of the Gulf of Mexico. The warm waters that bathe the West Indies permit the growth of coral reefs in the Caribbean ; the Bahamas are the slightly overtopping parts of broad plat forms of calcareous deposits, of which the greater area constitutes extensive shallow banks which descend steeply to great depths.

Rivers.

The successive crustal movements by which North America has been developed have determined the growth of sev eral great river systems. The broad upheavals which developed the medial plains had the effect of engrafting many rivers from the Eastern and Western highlands upon trunks of unusual dimen sions. Thus the Mississippi system, some of whose eastern tribu taries probably date from early Mesozoic times, received great reinforcement by the addition of many long western branches in late Tertiary time, roughly contemporaneous with the uplift of the southern coastal plain, by which the lower trunk of the river was extended from its mid-length into the gulf. The present head waters of that river-trunk to which the name of Mississippi has been rather arbitrarily applied are of very modern date, as they are consequent upon the abundant glacial deposits of northern Minnesota; and relatively modern courses appear to have been taken by the earlier-born Ohio and Missouri around the margin of invading Canadian ice sheets, which displaced them from earlier courses. The evolution of the Mackenzie resembles that of the Mississippi in a general way, but it has presumably been much affected by glacial erosion and deposition, in consequence of which it, like the St. Lawrence, has many large lakes in its course. The regime of this great north-flowing river is strikingly unlike that of its south-flowing analogue on account of its course being from a warmer to a colder climate ; hence while Mississippi floods have a free southward discharge, the floods of the Macken zie have an obstructed northward discharge due to ice dams. In deed, but for the complications that appear to be related to the outspread of Laurentian ice sheets, the areas drained by the Nelson and the St. Lawrence, now flowing to Hudson bay and St. Law rence gulf, would be discharged by the Mackenzie and Mississippi. For a time, during the presence of the ice sheets that simpler sys tem was realized for the Mississippi, when it carried to the Gulf of Mexico much drainage now received by the St. Lawrence and Nelson; the flood plain of its lower trunk was probably given its great breadth at that time. Lake Superior is peculiar in apparently owing its great depth to a somewhat pronounced dis placement of its basin floor, in addition to whatever deepening it gained by glacial erosion.