200 The American Geologist. March, 189-s 
Now there are certain points in this explanation which do not quite 
accord with the results of experimental work on ripple-marks, nor with 
the facts observed elsewhere in the field in either ripple-marked sand- 
stones or recent detrital material. Ripple-mark cannot be reproduced 
in the laboratory in material consisting of mixed pebbles and sand; this 
is true of marks formed by single currents in one direction, as well as 
of regular ripple-mark formed by oscillation. ThiB I have proven by ex- 
periment, and it is confirmed by observations of Messrs. Darwin* and 
De Candolle.t In recent stream beds and on sea beaches, ripple-mark 
is found only on surfaces of homogeneous sand; in pebbly areas the 
ripple-mark cannot exist, as the larger fragments interfere with the 
growth of the vortexes, to which, as Prof. Darwin has shown, ripple- 
mark is due. Fossil ripple-mark is almost invariably limited to sandy 
beds; it occasionally occurs in arenaceous clays and fine-grained lime- 
stones, but never in conglomerate. This statement is based on an ex- 
tended examination of the literature, and on personal field observation. 
Furthermore, ripple-mark formed by a continuous current in one di- 
rection is irregular, migratory, and continually changing in form, and 
has been so distinguished by experimenters,^ from regular ripple-mark 
formed by harmonic oscillations of water in contact with a sandy bot- 
tom. In sandstone strata, continuous series of rippled layers, crest over 
crest and trough over trough are very rare. Scrope§ noted this as early 
as 1830, observing that the upper and lower surfaces of a single ripple- 
marked slab do not correspond, when the lower surface retains the cast 
of the marks below. 
In the section described in Minnesota, assuming even that a current 
could form regular marks in mixed material, it seems improbable that 
the velocity and rate of deposition of the stream remained absolutely 
uniform while fifteen or twenty feet of stratified sediment was added 
to its bottom; and in any case the continual shifting of the marks 
would be more than likely to produce great irregularties in the sup- 
posed false bedding. Finally, could ripple-mark exist at all in a current 
of sufficient velocity to "sweep along pebbles the size of an egg"? A 
current of less than one half of this velocity is quoted by Lyell as 
sufficient to "tear up fine gravel.'' In such case a current flowing two 
miles an hour would certainly tear up fine sand. 
It is of course impossible to do more than suggest an explanation 
wmich will replace that based on the presence of ripple-mark. The 
occurrence as described, however, is strikingly similar to the fore-Eet 
beds of the ideal sand-plain figured by Prof. Davis.* It is possible that 
♦George Darwin, Proc. Roy. Soc, vol. xxxvi, No. 228, Nov. 22, 1883. I. "On the 
formation of ripple-mark." 
TCasimir de Candolle, Arch, des Sci. Phys. et Nat., Geneve, vol. rx, No. 3, 1883. 
''Rides formees, etc." 
iDarwin, De Candolle (v. e.), also A. R. Hunt. Proc. Roy. Soc, vol. xxxiv, April 20, 
1882. ''On the formation of ripple-mark." 
SPoulett Scrope, Proc. Geol. Soc, London, vol. I, No. 21, March 2, 1831. 
Lyell, Principles of Geology, 1889, p. 342. 
r \ Bulletin Geol. Soc. America, vol. i. p. 197. 
