length of time in the valley, and no doubt with a higher rate of motion, than it was upon the bed of the Lake of Geneva.
I have often wondered, considering the extent to which Professors Ramsay and Tyndall lean upon soft places, that they, or some of their adherents, have not thought it worth while to point out examples, upon a small and upon a large scale, of soft rocks which have been eroded by glaciers to a greater extent than harder rocks in their immediate vicinity. If Professor Ramsay is correct in supposing that glaciers wear away soft rocks with much greater rapidity than hard ones, it ought to be a very easy thing to produce examples. Yet, as far as I know, not one of the principal writers upon the subject has ever attempted to prove that glacier-erosion proceeds at an accelerated rate upon soft rocks, and is retarded by hard ones. It has been repeatedly asserted, or assumed, that such is the case, but proofs have been very rarely advanced.
Whilst this is the case, it has been continually remarked by writers upon glacier-action (who have not, however, attached any particular importance to the fact), that quartz-veins are cut down, by the passage of ice over them, to the level of the rocks in which they are found. Quartz, one of the very hardest of commonly diffused minerals, is unable to resist the grinding of glacier. Its hardness does not prevent its being polished down to the same extent as the much less resistant rocks which enclose it. If it suffered less than its surroundings, it would, of course, protrude. It does not, because it is eroded equally with the much softer rock. No distinction is made by the glacier, and the presence of the quartz is not sensible to the touch from any elevation or depression.
If glacier-eroded rocks containing veins of quartz are exposed to the influences of sun, frost, and water, it is not long before the quartz begins to assert its superior resistancy. If it is in gneiss, the gneiss in contact with it speedily suffers. Minute cracks radiate from the junction of the two substances over the surface of the weaker material. Water enters the tiny fissures, and, ex
