Covvespondence. 203 
traverses the first two, must have been formed under the same condi- 
tions. The constant relation of certain layers of the steeply inclined 
banding to certain parts of the ripple-marked lamination, the regular 
•and continued alternation of the coarse and loose with fine and firm 
material, the thickness of these layers being always the same, — these 
seem to the writer to be sufficient evidence of the correctness of the 
explanation given in the January Geologist. If, as suggested, the 
•layers are the true fore-set beds of a sand plain, the explanation of the 
continued recurrence, over a horizontal distance of many yards, of simi- 
lar layers, separated always one from another by a constant distance of 
several inches, will be difficult. But the presence of the true bedding 
does not admit such an hypothesis. 
In reconciling this conclusion with the observations of previous ob- 
servers upon the subject of ripple-marks it is necessary to insist upon 
the distinction between those ridges formed by the oscillation of water 
upon a sandy bottom and denominated "regular'' by Darwin and oth- 
ers, and those which are formed under a single current, called "ir- 
regular" by the same investigators. To the former class belong nearly 
all the examples which are found on sea-beaches and shallow lake bot- 
toms, and which, on account of their being by far the commoner, have 
been chiefly studied. It is here that the restrictions urged in Mr. Jag- 
gar's communication are applicable. The places where these are formed 
not being seats of rapid deposition, there is at the beginning of the 
period of formation of the ripple-mark the same material as at the end. 
Further the oscillations are relatively slight, and the resulting vortexes 
•comparatively weak, so that, although they are quite sufficient to move 
sand, they fail at coarser material or pebbles, and as a result the 
growth of the vortex is broken and the ridge is not completed. Thus it 
happens that the oscillation ripple-mark is never found in pebbly beds; 
but only where all the materials are fine enough to be moved easily. All 
preglacial fossil ripple-marks which the writer has seen come under 
this head. Owing to the instability of the oscillating currents and the 
consequent incessant shifting of the vortexes, it is by the merest chance 
that successive ripple-marked surfaces coincide; and, in view of the 
length of time which generally separates two layers, such rare coinci- 
dences must be regarded as accidental. 
To the second class belong those ripple-marks with which we have to 
deal. Because the limit of the strength of the single current is indefi- 
nite, the strength of the resulting vortexes may also become indefinitely 
great. So these ripple-marks are characteristically developed on a 
larger scale than the former class, — they are higher and the distance 
from crest to crest is greater.* Under the pressure of the current they 
move forward, but with so little change of form and relative position 
that the movement will ordinarily be overlooked, unless a mark is set. 
The variety of single-current ripple-marks formed by wind on dry sand 
has been chiefly described by geologists.t This structure is figured by 
*L)aua, Manual of Geology, second ed., p. 072 ; third ed„ i>. 684. 
tLyell, Principles of Geology, p. 842. Prestwich, Geology, vol. I, p. 148 Gelkie, 
Text-book of Geology, p. 385 1 1*9:*). 
