Striation of Rocks by River Ice.—Todd. 399 
As if to leave no doubt, a long deep scratch, about four feet 
in length and about as high above the ledge, was made doubtless 
by the same agent, viz., river-ice, on the nearly vertical face of a 
large boulder. This was in the same direction as the striz on 
the ledge below. 
The condition obtaining here and not in some of the other 
localities I had examined was the occurrence of numerous siliceous 
boulders at the river margin, a little up-stream from the ledge 
showing abrasion. 
After this demonstration of the efficiency of river-ice,I had no 
difficulty in ascribing the cases previously noted to the same cause. 
I would refer some,reported by others, to probably the same cause, 
viz.: Some reported to me by Prof.J.W. Spencer, as found at low 
water mark, at St. Louis, also some, reported to me some years 
since, by Prof. 8. T. Trowbridge, from the vicinity of Glasgow, 
Mo. And less confidently those formerly seen and published by 
Dr. C. A. White, occurring at low water, at Omaha, Neb. 
Nor is the story complete, without adding an interesting case 
most recently observed, at Wellington, Mo. There, on a ledge 
near low water, not far from the depot, are a very few markings 
on a rough horizontal ledge of limestone. The patches striated 
were in each spot less than three inches across, but the markings 
in each were parallel to one another. In the first spot the direc- 
tion was 8. 45° E., on a surface dipping 4-5° to E. In the diree- 
tion of the striae and 14 inches away was another patch, level, 
and striated 8. 61° E.; and following that direction 18 inches, a 
patch sloping upward and striated 8. 77° E. was found.  Possi- 
bly this was all done by one floating block. 
The foregoing observations, seem to me to warrant the follow- 
ing conclusions: 
1. Planation and striation are sometimes the work of river- 
ice, armed with erratics. 
2. The localities most favorable for this phenomenon seem to 
be, on the outside of a bend, or near a strong current, near low 
water mark, and below a point where siliceous erratices lie near the 
water level. 
3. The dynamical conditions necessary are probably a sudden 
breaking up of the ice, before it is rotted by thawing, and with a 
flood to wield it. This, however, is largely conjectural. The proper 
conditions do not often occur in our present western streams. 
