24 ON THE FRESH- WATER GLACIAL DRIFT 



furrows. One-half of the courses on my list are between south 20 and south 30 

 west. In the vicuiity of the Aztec, the Ohio, and the Adventure mines in the 

 Ontonagon district there are however some exceptions to the general southwesterly 

 motion. I omit these from the general average, because there are disturbing causes 

 in the configuration of the ground that are evidently local. In one case at the Ohio 

 mine the course is south 30° east, but this is in a gorge of the mountain through 

 which the glacier ice was forced as in a channel nearly at right angles to the gene- 

 ral movement. Here the vertical walls of the gorge, as in many other places, are 

 worn smooth and striated in the same manner as the floor. These striae are either 

 horizontal or inclined, so as to correspond with the bottom of the gorge. Where 

 the red clay has been penetrated, in the adit of the Adventure mine to the rock waU 

 of the ravine, the scourings and abrasion of the trap remain perfectly fresh, showing 

 that the surface had never been exposed to the weather. 



The gorges here are caused by fractures and dislocations of the trap formation, 

 which constitutes a long, high, bold and narrow range, 800 to 1000 feet above 

 Lake Superior. These sharp summits and cliffs are moutonne like those through- 

 out the trap range of Point Keweenaw. 



It is so likewise with the rocks of the iron region of Marquette which are very 

 much worn and scoured down, from the rounded Islets at lake level, to the hard 

 quartz and iron summits 1000 feet above. Messrs. Foster and Whitney observed, 

 near Teal lake, the same markings on vertical faces of the rocks which I have 

 noticed on the waters of the Ontonagon and Bad rivers. The low rocky islands 

 of Rainy lake and of Vermilion lake, are artificially rounded by drift action. 



In the New England district ice markings have been noticed at 3000 feet eleva- 

 tion. The glacial mass may have been in that quarter thick enough to rise to the 

 highest summit, but no doubt decreased in thickness towards the southwest. In 

 Ohio the highest land which has well defined polished surfaces and strite is 1300 

 to 1400 feet above the ocean level, and the same may be observed on the trap 

 ranges of Point Keweenaw having about an equal elevation of 1400 to 1600 feet. 



A less thickness of ice and neve would therefore suffice at the west to cover the 

 greatest elevations of the country. The southern limit of distinct glacial etchings 

 corresponds very well Avith the line of dots representing the lowest or southern edge 

 of the large boulders of northern rocks. Water-worn fragments and pebbles of 

 these rocks are seen farther south, but not in such numbers or of such size but 

 that they may have been transported by water alone. The modifications of the 

 drift and other superficial materials below this line are doubtless due to this agent. 



Encroachments of the Water tipon the Land. 



The clay bluffs of aU the lakes are easily worn away by the action of springs, 

 rains, winds, and waves. Being more or less marly, and having a large proportion 

 of sand in a finely divided state, water readily dissolves this deposit, and the shore 

 line rapidly gains upon the land. 



There is in the drift clay little tenacity or power of support. Where it is 

 attacked at the base and partially undermined, the bank settles down suddenly in 



