The deep canyons of mountainous regions are quite as difficult to overcome as the high ranges. In modern methods of transportation a range that cannot be surmounted may be tunnelled, and a tunnel five or six miles in length is no uncommon feat of engineering. A canyon, however, cannot be tunnelled, and if too wide for cantilever or suspension bridges, a detour of many miles is necessary. In crossing a deep chasm the route of transportation may aggregate ten or fifteen times the distance spanned by a straight line. Excepting the mining regions, the population of mountainous countries is apt to be found mainly in the intermontane valleys. A reason for this is not hard to find; the valleys are usually filled with rich soil brought from the higher slopes and levelled by the water. The population, therefore, is concentrated in the valley because of the food-producing power of the land. For this reason the Sound, Willamette, and San Joaquin-Sacramento Valleys contain the chief part of the Pacific coast population. The Shenandoah and the Great Valley of Virginia are similar instances. What is true of the larger intermontane valleys is true also of the narrow stream valleys of mountain and plateau regions. Thus, in the New England plateau the chief growth during the past forty years has been in the valley lands. In that time if the uplands have not suffered actual loss, they certainly have made no material gains. Upland farming has not proved a remunerative venture, and many of the farms have either been abandoned or converted to other uses. =Passes.=--Transverse valleys form very important topographic features of mountain-regions. Inasmuch as the ranges themselves are obstacles to communication, it follows that the latter must be concentrated at such cross valleys or gaps as may be traversed. Khaibar Pass, a narrow defile in the Hindu Kush Mountains, between Peshawur and Jelalabad, for many years was the chief gateway between Europe and India. Even now the cost of holding it is an enormous tax upon England. Brenner, St. Gotthard, and the Mont Cenis Passes are about the only land channels of commerce between Italy and transalpine Europe, and most of the communication between northern Italy and the rest of Europe is carried on by means of these passes. Every transcontinental railway of the American continent crosses the various highlands by means of gaps and passes, and some of them would never have been built were it not for the existence of the passes. Fremont, South, and Marshall Passes have been of historic importance for half a century.