Correspondence. 205 
case a current flowing two miles an hour would certainly tear up fine 
sand." In single-current ripple-marks, unless both the current and 
the resulting vortex are strong enough to move the sand, it seems clear 
that the sand will not be moved, and no ripple-mark will be formed. 
It will appear quite clear, moreover, from a consideration of the cause 
of their formation, that the more exposed portions of these ridges do 
not bear the pressure of the bottom velocity of the stream, for they lie 
in a comparatively quiet zone, where the force of the direct current is 
to a great extent counteracted by the current of the vortex. 
At Cloquet, Carlton county, Minnesota, the writer, while in company 
with Mr. Warren Upham, observed a cut in a bed of glacial sand which 
had exactly the same features as those of the most sandy portions of 
the cut near Iron Junction. The following is an extract from notes 
made Oct. 21, 1893: "In the horizontal layers of one of the deposits near 
the town, on the west side of the river, a ripple-marked structure ex- 
actly like that south of Iron Junction was seen. The material, how- 
ever, was of homogeneous line sand, which did not so accentuate the 
steeply-inclined line of growth following the apexes of the ripple-marks." 
Very near this cut the St. Louis river flows, and the direction of the 
current indicated by the ripple-marked sand is nearly that of the pres- 
ent stream. 
Mr. Upham has shown me a photograph, taken by Prof. W. O. Crosby 
at Mt. Hope, a few miles southwest of Boston, which exhibits a similar 
ripple-marked structureina bed of stratified sand. The ripple-marked 
portion of the section photographed is about 30 feet long and 5 to 6 feet 
high. The separate layers in the beddiog are nearly horizontal in their 
general course, but are ripple-marked along their whole extent. They 
are always conformable, crest for crest, and trough for trough, and the 
thickness through which this extends is five to six feet. In this case 
the steeply inclined lines connecting corresponding portions of the suc- 
cessive ripple-marks, instead of being straight, follow a gentle curve, 
which at the bottom begins with an angle of about forty degrees from 
the horizontal and gradually increases this angle till its course is nearly 
perpendicular. This curve and the constantly diminishing size of the 
ridges toward the top of the section point to a gradual slackening of the 
current. In the photograph about seventy-five distinct ripple-marked 
layers can be counted. 
The writer has observed in various places well-formed ripple-mark in 
the bed of streams. The case of Embarass river in St. Louis county, 
Minnesota, is important. This river flows from Esquagama lake, the 
lowest in the series of the Embarass lakes, and empties into the St. 
Louis. It thus has a length of less than ten miles, but carries a consid- 
erable body of water. In September of 1893 this river was canoed by 
the writer; and it was observed that the bed of sand and pebbles was 
almost continuously strongly ripple-marked. At this season the water 
was so low that the structure could be well observed. Here the writer 
first saw ripple-marking in places where pebbles were present, and 
noticed that the pebbles almost always came to rest in the troughs. 
