north]
AMERICA 361 and having in their middle a breadth of 1500 miles. structure and surface form are not Cordilleran; and They are composed throughout of nearly horizontal strata numerous soundings in the adjacent mediterraneans sugand mark a region long exempt from strong dis- gest that the islands are best interpreted as the somewhat ^Mediai tmljance. Although for the most part floored denuded crests of great crustal ridges. The warm waters Plains. by marine formations, their structure and com- that bathe the West Indies come with a high temperature position indicate, as has already been said, from the equatorial Atlantic, and favour the growth of relatively shallow water. The ancient sea that once corals along the shores. Fringing and elevated reefs are occupied the middle belt of the continent therefore had known on many of the islands. The Bahamas are the little likeness to the abysmal oceans, but resembled rather slightly overtopping parts of a broad platform of coral the shallow ocean margins that to-day overlap various and other calcareous marine deposits, of which the greater continental masses—the largest example of this kind now area constitutes extensive shallow banks, which descend existing being between Asia and Australia. The eastern by a steep slope on the north-east to great depths in the part of the plains is underlaid by Palaeozoic strata, already Atlantic. The lowlands- of Yucatan resemble Florida in mentioned as having been laid down upon the subsiding being the emerged part of a much larger mass, of which an Archaean continent or folded in the making of the equal portion is still under water in the shelf around the Appalachians; coal beds are here included in the Ohio Gulf of Mexico. All this region is luxuriantly productive and middle Mississippi basins. The area of the western and is advantageously surrounded by waters which would plains remained submerged to a later date preserving a be barren and desert if replaced by lowlands. The active stretch of marine waters to the end of Mesozoic time, volcanoes on the Pacific slope have built many cones and and thus resembling the lowland belt of western Asia, uplands, some of their historic eruptions having been of which was similarly covered by a broad and a shallow arm terrible violence. Thus Lake Nicaragua, once a bay of of the ocean extending from the Arctic to the European the Pacific, has been cut off by volcanic deposits, leaving mediterraneans until a late geological date. The surface only the Gulf of Fonseca open to the western ocean, of the Medial plains is not always so even as might be in- raising the level of the lake behind the barrier and turnferred from their name. Both the eastern and the western ing its discharge eastward to the Caribbean Sea across areas have been extensively denuded, even to the point what was once the inter-oceanic watershed. of being reduced to lowlands of denudation. Their present The successive crustal movements by which the land altitude is not so much the result of their original uplift area of what we now know as North America has been from the sea as of a later elevatory movement. The increased and connected have determined the R, great river basins, for which North America is famous, growth of several great river systems through have thus been formed between the Eastern and Western which the broader part of the continent is drained. The highlands—the Mississippi receiving the drainage of a movements that resulted in the emergence of the Plains vast area (about 1,240,000 square miles) for discharge to had the effect of engrafting many ancient rivers of moderate the south, while the Saskatchewan and Mackenzie gather size upon trunks of unusual dimensions. The Mississippi their waters from somewhat less extensive areas in the system, some of whose eastern branches probably date from north. Pleistocene glaciation covered the plains of the early Mesozoic time, received great reinforcements by the Ohio, upper Mississippi and Winnipeg districts with addition of many long western branches in Tertiary time, extensive deposits of ice-laid or water-laid drift, furnishing roughly contemporaneous with the uplift of the Gulf a generally smooth surface and a fertile soil: here are the coastal plain by which the lower trunk of the river was extended to the sea. The present headwaters of that true prairies—treeless, but richly grassed. The traditional continuity of the Cordilleras of North stream to which the name of Mississippi is applied, and and South America has been broken by investigations in which for that reason have gained an undue subjective importance, are of relatively modern date, as they are conCentral the isthmian portion of the northern continent. trolled by the abundant glacial deposits of northern The structural peculiarities of the Western highAmerica and the lands of North America may be traced only to Minnesota. The evolution of the Mackenzie resembles West the east and west belt of great volcanoes by that of the Mississippi in a very general way, although Indies. which the plateau of central Mexico is termi- some of its eastern branches may be the descendants of nated on the south. The ranges of the Andes fail to ancestors more ancient than those flowing westward from reach Panama, from which the nearest one is separated the Appalachians; but the regime of the great northern by the valley of the Atrato. The two Cordilleras are out river is strikingly unlike that of its still greater southern of line with each other, and their ends are some 1200 analogue on account of its course being from a warmer to miles apart. Central America, the West Indies, and a colder climate: hence ice-dams, obstructed discharge, and various submarine ridges by which they are connected overflows. The Nelson and the St Lawrence systems, with one another and with the mainland to the west, as draining eastward to Hudson Bay and St Lawrence Gulf, well as certain ranges along the northern margin of South receive drainage from areas that would belong to the America, all belong together in what has been termed the Mackenzie and the Mississippi systems under a simplexAntillean mountain system, in which east and west trends plan of continental growth ; and there is much reason for of late geological date predominate, with abundant volcanic thinking that this simpler plan obtained until the occuradditions on the Pacific border of Central America, and rence of those changes, in association with the Glacial along the eastern end of the system in the Windward period, whereby sea waters gained access to the depressions islands of the Lesser Antilles. The unity of this system that now hold the bays and sounds of the north-eastern has been until recently overlooked partly because the coast. In exemplification of the rule that the larger ocean Antillean ranges are for the most part still under water, receives the drainage of the smaller continental area, the and yet further because the volcanoes which form the rivers that flow into the Pacific rank below those belongstrongest reliefs of the isthmian region are so arranged ing to the Atlantic. The greatest is the Yukon, of farther along the Pacific coast as to suggest the continuity of the Canada and inner Alaska, one of the great rivers of the Cordilleran systems on the north and south; but these world, little known until the recent active exploration of volcanoes are really only superadded to a foundation of its basin for gold fields. The Frazer drains much of the quite another kind. Geological studies on the mainland mountainous area of southern British Columbia, as the and on the islands have shown that both fundamental Columbia drains that of the north-western United States; S. I. — 46
