[[p. 431]] My friend, Mr. Thomas Belt, F.G.S., has kindly sent me the following extract from a paper by Prof. Joseph Le Conte, in the American Journal of Science for 1874 (p. 366), in which an explanation is given of the above-named formation (Nature, vol. xv. p. 274) and of similar mounds farther north. It will be seen that Prof. Le Conte refers them wholly to "surface-erosion," but it is not clear whether he means "pluvial" or "aerial" erosion, or the two combined. More explanation seems required to account for the removal of the eroded matter over a surface thirty miles wide without producing any continuous ravines or other water channels:--

     "Prairie Mounds.--The irregularly ramifying grassy glades or prairies already described as existing at the southern extremity of Puget Sound are studded over as thickly as possible with mounds about three to four feet high and thirty or forty feet in diameter at base." . . .

     "The whole country between the Dalles and the upper bridge of Des Chutes Rivers, a distance of about thirty miles, is literally covered with these mounds." . . . "The true key to their formation is given here, as it was not at Mound Prairie, by the great variety of forms, sizes, and degrees of regularity which they assume. They vary in size from scarcely detectable pimples to mounds five feet high and forty feet in diameter at base, and in form from circular through elliptic and long-elliptic to ordinary hill-side erosion-furrows and ridges." . . .

     [[p. 432]] "No one, I think, can ride over those thirty miles and observe closely without being convinced that these mounds and wholly the result of surface-erosion, acting under peculiar conditions. The conditions are a treeless country and a drift soil consisting of two layers, a fine and more moveable one above and coarser and less movable one below." . . . "The necessary condition, I believe, is the greater movableness of the surface soil compared with the sub-soil." . . . "Surface erosion cuts through the finer superficial layer into the pebble layer beneath, leaving, however, portions of the superficial layer as mounds."

     "Similar less conspicuous mounds, under the name of 'Hog-wallows,' are well known to exist over wide areas in middle and southern California."

     The words in italics are so in the original.

Alfred R. Wallace