My dear Lyell

Your point seems to me a very good one.¹ When reading Croll, I thought of the perfectly preserved glaciation of the rocks in Scotland & N. Wales, & of the preservation of the turf-covered shelves of Glen Roy & of all turf-covered surfaces; but I vaguely supposed that Croll would say that the wear & tear was confined chiefly to the valleys & ravines & was much less than average when rocks hard.— But when the wear & tear is put into figures, as you have done, it seems scarcely possible that the water channels could supply detritus enough.—²

It w^d be most interesting to visit Glen Roy for this sole purpose, & estimate how large an area, consisted of naked rock or naked detritus. Certainly my impression is strong that the shelves themselves are now, except when crossed by a rill, just as they were when they bordered the lake or water.—³ One is led to suspect that Crolls & Geikies estimate of the amount of surface degradation, is much too high, even supposing that the amount of loose matter now carried away by the rivers is correct.—⁴ Somehow I cannot persuade myself that the Glacial period was much more recent than hitherto supposed. I sh^d like to hear what Croll w^d say to your line of argument—⁵

I am sorry to hear about the Amazonian shells.—⁶

My dear Lyell | Yours most sincerely | Ch. Darwin