Correspondence. 201 
the inclined layers may actually be the true bedding, deposited oblique- 
ly in the manner characteristic of fore-set beds. They would naturally 
contain much coarse material near the head of the sand-plain, and be- 
come finer-grained towards its front; the suggestion that the supposed 
ripple-marked layers "when not obscured, might be easily mistaken for 
a minor cross-bedding," accords with this hypothesis, for cross-bedding 
is a frequent accessory of fore-set deposits. 
Harvard University. January 30, 1894. T. A. Jaggar, Jr. 
Oscillation and single current ripple-marks. Through the 
kindness of the editors, the writer is enabled to reply to the foregoing 
communication in this number. The comments in the preceding letter 
are very suggestive, and doubly valuable on account of the acknowl- 
edged familiarity of the contributor with his subject. In defence of 
my position, I desire primarily to emphasize certain features which 
were only casually touched upon before, it being considered that this 
brief treatment was sufficient for the importance of the subject in hand. 
The cut from which the section for description was taken is, as before 
stated, "in a level plain of sand and gravel.'' and not "in a glacial sand- 
plain," that is, a delta plateau. There appeared to the writer no evi- 
dence that it was a delta formation. The dimensions given, it must 
also be pointed out, are those of the cut alone. The "level plain" in 
which it is made is part of an immense sheet of stratified drift which 
covers the surrounding country, and which usually shows a remarkable 
lack of topographical peculiarities. 
The material in this cut varies in coarseness from a fine sand to a 
type made up mainly of pebbles, with all gradations between the two. 
The transition from one phase to another is vertical, so that thick hori- 
zontal beds of coarser and finer material alternate. These beds, where 
they are individualized by a more sudden change of material than 
usual, may be stated as being from three to ten feet in thickness. Ordi- 
narily, the change from one to another is not striking, but occasionally 
the difference of average texture isgreat. Within these larger horizon- 
tal divisions, marking the coarser alternations of the true bedding, 
there may be a minor diversity in the size of material at different hori- 
zons, or, more often, the material is so nearly uniform that the grada- 
tions are hardly discernible. In the finer sand, there are successive 
continuous horizontal laminations which are as beautifully marked and 
as distinct as in any stratified sand deposit whatever; but instead of 
being perfectly straight, they are undulating and follow the familiar 
outline which is peculiar to the ripple- mark formed by a strong and 
constant current. In the material of mixed sand and pebbles, these 
lines of successive lamination are fainter, but they may be distinguished 
and traced continuously in almost every case. In the material which 
is made up mainly of pebbles the lamination becomes still fainter, and 
often entirely disappears. Between the cleanly marked lamination of 
the sand and the nearly obliterated stratification of the pebbles there 
is, of course, every possible gradation. 
