Page:Whymper - Scrambles amongst the Alps.djvu/372

316 § 6. "Each of the lakes lies in an area once covered by a vast glacier." And went on to reason—

§ 7. "If a glacier can round, polish, and cover with striations the rocks over which it passes—if, flowing from its caverns, it can charge rivers thickly with the finest mud, then it can wear away its rocky floor and sides."

§ 8. He assumed that glaciers are competent to produce lake-basins, and that they have done so by scooping out softer parts of the country, leaving hollows surrounded by a framework of harder rocks; "but perhaps more generally they (the rock-basins) were formed by the greater thickness and weight, and consequently proportionally greater grinding pressure of glacier-ice in particular areas," "the situations of which may have been determined by accidental circumstances, the clue to which is lost, from our inability perfectly to reconstruct the original forms of the glaciers." The particular manner in which he supposed the great lake-basins of the Alps were formed was as follows:—

§ 9. "It will be evident that when the general inclination of a valley was comparatively steep, a glacier could have had no opportunity of cutting for itself any special basin-shaped hollows. Its course, with a difference, is like that of a torrent. But in a flat bottomed part of a valley, or in a comparative plain that lies at the base of a mountain range, the case is not the same. For instance, to take an extreme case, if a glacier tumble over a slope of 45°, no one would dream of the ice-flow producing any special effect, except that in the long run, the upper edge of the rock that forms the cataract being worn away, its average angle would be lowered. And so of minor slopes; if the ice flowing fast (for a glacier) rendered the rocky surface underneath unequal, such inequalities could not become great and permanent; for the rapidly-flowing ice