The biggest Swiss glaciers of to-day, compared to the great glacier of the Rhone Valley, of which they are but the highest tributaries, still surviving unmelted among the mountain-tops, are in size as a mountain freshet is to the great stream of Loch Lomond, or as the Serpentine in Hyde Park to the neighbouring Thames. Vast as was the great glacier of the Rhone Valley, and immense as has been the work done by water and ice in carving the great highway in the mountain-mass of Switzerland, it has all been effected since the date of the formation on the sea-bottom and the subsequent elevation of the strata which we call “the chalk”--a deposit which comes not very far down in the series of strata of the earth’s crust. Only 3,000ft. of deposit exist above it, whilst below it are more than 60,000ft. of water-deposited or “sedimentary” rocks. The huge Alps have risen since the date of the “chalk,” for we find strata containing marine shells of the Tertiary period at a height of 10,000ft. in those mountains. Where those shells now are was the bottom of the sea at a comparatively recent date, probably not more than fifty million years ago! And not only have the Alps been raised since then from the sea level to 15,000ft. (the height of Mont Blanc), but the huge mountain valleys and the great chasm of the Rhone Valley many miles wide, with its floor thousands of feet below the mountain ridges, have been scoured out. Deeper and wider it has gradually become as it has taken shape, whilst the mountain sides have been removed first by water and later by ice--by the great glacier consisting of solid ice, miles wide and a thousand and more feet in thickness. The water no longer fills the valley in solid form, but once again rushes along as an irresistible torrent, tearing and wearing the rock without rest or mercy, carrying it off by thousands of tons day by day, year by year, to the plains of Provence and the deep floor of the Mediterranean Sea. The blue colour of the glacier ice--like that of pure water--is now known to be due to no impurity or admixture of other substances. It does not, as was supposed by Tyndall, owe its blueness to a dust of finest colourless particles as do blue smoke, the blue sky, and as do the blue eyes which have attracted the observation of naturalists (and others) in Ireland and the North of Europe. Water, whether liquid or solid, is blue, just as “blue copperas” is, or as “Prussian blue” is; but light must pass through some ten or twenty feet thickness of it to make the colour evident to our eyes. The green tint is due to an admixture of yellow, the exact cause of which is not quite easy to discover. Probably it is due to minute quantities of earthy matter mixed with the surface snow.