The Hudson and Champlain Valley played an important part in the history of the colonies a century before the existence of the United States, and its importance as a gateway to eastern Canada is not likely to be lessened. The Mohawk gap was the first practical route to be maintained between the Atlantic seaboard and the food-producing region of the Great Central Plain. It is to-day the most important one. It is so nearly level that the total lift of freight going from Buffalo to tide-water is less than five hundred feet. [Illustration: A PASS--THE ROUTE OF A RAILWAY] =Rivers.=--River-valleys are closely connected with the economic development of a country. Navigable rivers are free and open highways of communication. In newly settled countries the river is always the least expensive means of carriage, and often it is the only one available for the transportation of heavy goods. In late years, since the railway has become the chief means for the transportation of commodities, river transportation has greatly declined. The river-valley, however, has lost none of its importance; in most instances it is a naturally levelled and graded route, highly suitable for the tracks of the railway. As a result, outside of the level lands of the Great Central Plain, not far from eighty per cent. of the railway mileage of the United States is constructed along river-valleys. =Plateaus.=--Plateaus are usually characterized by broken and more or less rugged surface features. As a rule they are deficient in the amount of rainfall necessary to produce an abundance of the grains and similar food-stuffs, although this is by no means the case with all. Most plateaus produce an abundance of grass, and cattle-growing is therefore an important industry in such regions. Thus, the plateaus of the Rocky Mountains are famous for cattle, and the same is true of the Mexican and the South American plateaus. The Iberian plateau, including Spain and Portugal, is noted for the merino sheep, which furnish the finest wool known. The plateau of Iran is also noted for its wool, and the rugs from this region cannot be imitated elsewhere in the world. =Plains.=--Plains are of the highest importance to life and its activities. Not only do they present fewer obstacles to intercommunication than any other topographic features, but almost always they are deeply covered with the fine rock-waste that forms the chief components of soil. Plains, therefore, contain the elements of nutrition, and are capable of supporting life to a greater extent than either mountains or plateaus. About ninety per cent. of the world's population dwell in the lowland plains.