The Alps, whose ancient Celtic name probably refers to the whiteness of their snowy summits, stretch in an immense curve, more than 600 miles in length, from the shores of the Mediterranean to the plains of the Danube. They consist in reality of more than thirty mountain masses, representing as many geological groups, and joined to each other by elevated passes; but their rocks, whether they be granite, slate, sandstone, or limestone, form one continuous rampart rising above the plains. In former ages the Alps were higher than they are now. This is proved by an examination of their detritus and of the strata disintegrated by natural agencies. But, whatever the extent of detrition, they still rise in hundreds of summits beyond the line of perennial snow, and vast rivers of ice descend from them into every upland valley. Looked at from the plains of Piedmont and Lombardy, these glaciers and snow-fields present the appearance of sparkling diadems encircling the mountain summits. In the eastern portion of the Alpine system—that is to say, between the Mediterranean and Mont Blanc, the culminating point of Europe—the average height of the mountain groups gradually increases from 6,500 to more than 13,000 feet. To the east of Mont Blanc the Alps change in direction, and, beyond the vast citadels represented by Monte Rosa and the Bernese Oberland, they gradually decrease in height. To the east of Switzerland no summit exceeds a height of 13,000 feet, but this loss in elevation is fully made up by increase of breadth. And whilst the general direction of the principal axis of the Alps remains north-easterly, very considerable mountain chains, far exceeding the central mass in breadth, are thrown off towards the north, the east, and the south-east. A line drawn across the true Alps from Vienna has a length of no less than 250 miles.