power gradually relaxes, and finally the eroding agent quits the mountains altogether, and the grand effects which it produced in the earlier portion of its course entirely disappear."[1] But does this conclusion agree with the fact that the valleys are usually wider—much wider—at their mouths than elsewhere, and that the beds of the valleys at their mouths are at a lower level than at the upper extremities? If the glaciers had flowed up the valleys, these facts might be explicable; but they are unintelligible if the valleys were excavated by glaciers which flowed down them.
The mouths, the beds, the walls, and the terminations of the valleys, and the slopes of the mountains which bound them, proclaim alike that the present modelling of the Alps has been only slightly modified by glaciers. It would, however, be unreasonable to conclude, because such is the case, that glaciers are incompetent to excavate valleys under any circumstances; and, before taking leave of Professor Tyndall, it is only due to him to examine his opinions upon the subject. He is, like Professor Ramsay, a great believer in soft places. He believes that glaciers not only erode soft rocks more rapidly than hard ones (which is a reasonable belief), but he considers that all the chief inequalities which are now seen in valleys that have been eroded by glaciers are due to the greater or less resistancy of the rocks to the action of the ice. "Were its bed uniform in the first instance, the glacier would, in my opinion, produce the inequalities."[2] Now, I could not differ greatly from Dr. Tyndall, if he were to say that glaciers must erode soft rocks more rapidly than hard ones, and that they might, in consequence, ultimately produce inequalities, if set to work upon a smooth surface containing both hard and soft places. But he goes far beyond this. It is necessary for him to explain how it comes to pass that such masses are left behind as that at Montalto, at the entrance of the Valley of Aosta, or those upon which the castles of Sion stand. The valleys of Aosta and of the Rhone, he says, have been excavated by glaciers, yet here are these obstinate crags stand-
