20 IOWA ACADEMY OF SCIENCES. 



is 4-6 degrees E. N. E. The principal seams of the rock are N. 10='-12° E. The 

 surface, which was quite generally planed and striated, was 10 feet wide, on an 

 average, and 60-75 feet long. The direction of most of the strife wasS. 10° -11° W., 

 and of a few, S. 18o W. The striated surface reached from the water level up to 

 two or three feet above. A small patch toward the southern end of the area was 

 scratched in a direction, S. 56° E. The striae were, if anything, more strictly 

 parallel than in most glacial strife. They were short, being rarely more than three 

 inches long. This was mainly due, it would seem, to the much-cracked and nod- 

 ular character of the rock. One other peculiarity of the stone affected the form of 

 the markings. Scattered through it were numerous black grains like iron oxide. 

 These usually headed the narrow ridges between the strife. The strife were mostly 

 fine, rarely more than an eighth of an inch across. As if to leave no doubt con- 

 cerning the cause, a long, deep, horizontal scratch, about four feet long and as high 

 above the ledge just described, was found on the nearly vertical face of a large 

 sandstone boulder. This was in the same general direction as the strife below. 

 The reasons for referring these phenomena to river-ice are briefly, as follows: 



1. Their recency, as indicated by their appearance and their location where 

 water and weather would obliterate them in a short tune. 



2. Their parallelism with the present channel of the river. 



3. Their occurrence outside of the recognized limit of glacial action. 



Other localities where similar phenomena have been found which are reasonably 

 referred to the same origin, are as follows : 



Running Water, S. D., a little above landing, S. 73 E., "Chalkstone," few feet 

 above low-water. 



Sioux Falls, S. D., a few rods east of Cascade Mills, N. 57 W., Red Quartzite, 

 few feet above low-water. 



Wellington, Mo., a few rods N. W. of depot, S. 45, 61 and 73 E., Limestone, 

 few feet above low-water. 



Grand Tower. III., 3 miles up R. R. from depot, S. 10 and 18 W., Limestone, 

 few feet above low-water. 



Cape Girardeau, Mo., at landing, S. 10 to 35 E., Limestone, few feet above low- 

 water. 



All these directions are magnetic. 



Besides the«e, we would provisionally refer to the same cause strise reported by 

 Dr. C. A. White as found near low water at Omaha, Neb. [Geol. Iowa, Vol. I. 

 p. 95]; some reported by Prof. S. T. Trowbridge, from the vicinity of Glasgow, 

 Mo., and some reported by Prof. J. W. Spencer, as occurring at St. Louis, at low 

 water mark. 



It seems not unreasonafcle to suppose that this same influence was even more 

 efficient when the rivers were flowing at higher levels, with stronger currents and 

 when erratics were more abundant and ice cakes larger and more abundant, as 

 must have often been the case during the Glacial epoch. 



It is no doubt true that ledges are often exposed long to the ice action of rivers 

 without being striated. The conditions producing the effect may not yet be fully 

 understood, but the following seem to be some of them. 



1. The localities most favorable, seem to be on the outside of a bend, or near a 

 strong current, near low water mark, and below a point where silicious eiratics 

 are abundant at the water level. 



2. The dynamical conditions necessary are probably a sudden breaking up of 

 the ice before it is rotted by thawing, while it still adheres firmly to the shore, and 

 when there is a flood to wield it. 



