[Sidenote:  Passes in mountain barriers.]

The barrier nature of mountains depends upon their height and structure, whether they are massive, unbroken walls like the Scandinavian Alps and the Great Smoky range; or, like the Welsh Highlands and the Blue Ridge, are studded with low passes.  The Pyrenees, Caucasus and Andes, owing to the scarcity and great height of their passes, have always been serious barriers.  The Pyrenees divide Spain from France more sharply than the Alps divide Italy from France; owing to their rampart character, they form the best and most definite natural boundary in Europe.[1218] Epirus and Aetolia, fenced in by the solid Pindus range, took little part in the common life of ancient Greece; but the intermittent chains of Thessaly offered a passway between Macedon and Hellas.  The Alps have an astonishing number of excellent passes, evenly distributed for the most part.  These, in conjunction with the great longitudinal valleys of the system, offer transit routes from side to side in any direction.  The Appalachian system is some three hundred miles broad and thirteen hundred miles long, but it has many easy gaps among its parallel ranges, so that it offered natural though circuitous highways to the early winners of the West.  The long line (400 miles) of the Hindu Kush range, high as it is, forms no strong natural boundary to India, because it is riddled with passes at altitudes from 12,500 to 19,000 feet.[1219] The easternmost group of these passes lead down to Kashmir, and therefore lend this state peculiar importance as guardian of these northern entrances to India.[1220] The Suleiman Mountains along the Indo-Afghan frontier are an imperfect defence for the same reason.  They are indented by 289 passes capable of being traversed by camels.  The mountain border of Baluchistan contains 75 more, the most important of which focus their roads upon Kandahar.  Hence the importance to British India of Kandahar and Afghanistan.  Across this broken northwest barrier have come almost all the floods of invasion and immigration that have contributed their varied elements to the mixed population of India.  Tradition, epic and history tell of Asiatic highlanders ever sweeping down into the warm valley of the Indus through these passes; Scythians, Aryans, Greeks, Assyrians, Medes, Persians, Turks, Tartars, and Mongols have all traveled these rocky roads, to rest in the enervating valleys of the peninsula.[1221]

[Sidenote:  Breadth of mountain barriers.]

Mountains folded into a succession of parallel ranges are greater obstructions than a single range like the Erz, Black Forest, and Vosges, or a narrow, compact system like the Western Alps, which can be crossed by a single pass.  Owing to this simple structure the Western Alps were traversed by four established routes in the days of the Roman Empire.  These were:  I. The Via Aurelia between the Maritime Alps and the sea, where now runs the Cornice Road.