204 APPENDIX. 



as regards the accumulation of such deposits, the cause of 

 which is still open to controversy, it is now very generally 

 abandoned, and the designation of erratic deposits, among 

 others, adopted in its stead. I have, therefore, used the 

 latter expression, as comprehending a vast deposit of sand, 

 gravel, pebbles, clay (arranged in zones, and occupying 

 almost always the bottoms), and masses of rocks transported 

 to a distance from their original position, usually called erra- 

 tic blocks. This deposit always occurs between the vegeta- 

 ble soil and the rocky strata of all ages that constitute the 

 geological basis of each section of country. To the north 

 and to the south of the western portion of Lake Superior, as 

 far as the upper half of St. Peter's River, it overlies prima- 

 ry rocks ; south of the St. Peter's, to the west and east of 

 the Missisippi, it covers silurian rocks ; whilst, on the Upper 

 Missouri, it rests upon a cretaceous formation, everywhere 

 mixing itself with the detritus of the rocks in place. The 

 thickness of this deposit is very variable ; sometimes only 

 quite superficial, and, when of a more important character, 

 from 150 to 160 feet in depth. It is met with, indifferently, 

 with its erratic fragments, on the summits of hills, on the 

 upland plateaux, over the plains, and in the valleys. It has 

 contributed towards levelling the original irregularities of the 

 soil, by filling up hollows ; or varied them, by transporting 

 over the country new materials ; whilst the subsequent action 

 of water and weather has further brought about its charac- 

 teristic features. 



" The erratic blocks of this deposit are not generally 

 rounded, but they are still boulders. Those which have the 

 rounded appearance, most usual to boulders, among a great 

 many that I had an opportunit}^ of examining, owe it to their 

 exposure to the atmospheric agents which have worn them 

 down. This is made evident by examining those portions of 



