Intermarine mountains as a rule offer the easiest passways where they sink to meet the flanking seas.  The Pyrenees are crossed by only two railroads, the Bayonne-Burgos line, along the shore of the Bay of Biscay, and the Narbonne-Barcelona line, overlooking the Mediterranean.  Between these extremities the passes are very high and only two are practicable for carriages, the Col de la Perche (5280 feet or 1610 meters) between the valleys of the Tet and the upper Segre, and the Port de Canfranc (7502 feet or 2288 meters) on the old Roman road from Saragossa to Oloron.  The coastal road around the eastern end of the Cheviot Hills has been the great intermediary between England and Scotland.  It was the avenue for early Teutonic expansion into the Scotch Lowlands, the thoroughfare for all those armies which for centuries made Berwick a chronic battleground.

For purposes of trade these intermarine mountains are less serious barriers, because they can be avoided by an easier and cheaper sea route.  Hence on each side of such ranges grow up active ports, like Narbonne and Barcelona, Bayonne and Bilbao with San Sebastian, on the piedmont seaboard of the Pyrenees; Petrovsk and Baku on the Caspian rim of the Caucasus, balancing the Crimean ports and Poti with Trebizond on the Black Sea.  Analogous is the position of Genoa and Marseilles in relation to the Maritime Alps.  Such ports are inevitably the object of attack in time of hostilities.  In the Peninsular War almost the first act of the French was to seize Barcelona, San Sebastian and Bilbao; and throughout the seven years of the conflict these points were centers of battle, blockade and siege.  If Russia ever tries to wrench the upper Euphrates Valley from Turkey, Trebizond will repeat the history of Barcelona in the Peninsular War.

[Sidenote:  Pass roads between regions of contrasted production]

As the world’s roads are used primarily for commerce, pass routes rank in importance according to the amount of trade which they forward; and this in turn is decided by the contrast in the lands which they unite.  The passes of the Alps and the Pass of Belfort have been busy thoroughfares from the early Middle Ages, because they facilitate exchanges between the tropical Mediterranean and the temperate regions of Central Europe.  Or the contrast may be one of economic and social development.  The Mohawk depression forwards the grain of the agricultural Northwest in return for the manufactured wares of the Atlantic seaboard.  The passes of the Asiatic ranges connect the industrial and agricultural lowlands of India and China with the highland pastures of Mongolia, Tibet, Afghanistan and Russian Turkestan.  Hence they forward the wool, skins, felts, cloth and carpets of the wandering shepherds in exchange for the food stuffs and industrial products of the fertile, crowded lowlands.  Where passes open a highway for inland countries to the sea, their sphere of influence is greatly increased.  San Francisco, New York, Marseilles, Genoa, Venice, Beirut and Bombay are seaports which owe their importance in no small degree to dominant pass routes into their hinterland.