144 Wisconsin Academy of Sciences^ Arts^ and Letters. 



that it must have been dry land since the Silurian period. In 

 the immense ages succeeding the time of the Silurian oceans, 

 the rocks being exposed to the destructive atmospheric influ- 

 ences must have been cut up by the ravines and valleys en- 

 croaching on each other in endless confusiou. The pre-glacial 

 erosions of even the hard Azoic rocks which formed the dry 

 land of the Silurian period can still be distinguished from those 

 made since. When the glaciers came they planed down the 

 whole region of the upper Mississippi river, removing silurian 

 strata 500 feet in thickness over hundreds ot' miles. Tiae south- 

 western limit of the glacial drift action is the Missouri river 

 from the 48th down to the 43d parallel of latitude. From the 

 Missouri river to the Rocky Mountains, over a space varying 

 from 300 to 500 miles in width, there is no drift. The motion 

 of the glacial mass must have been along the line of least 

 resistance ; and towards this limiting line, the glacial 

 scratchings in the northwest show that the glacial motion was 

 southwest. There, then, on that limit, a river must have been 

 formed to carry away the melting water from the glacier, and 

 this limit was the Missouri river, and that was the river so 

 formed. As the glaciers began to retire to the northeast, as 

 long as the general slope of the plain was towards the glacial 

 mass, successive rivers were marked out by it along the west- 

 ern face ; and all have a parallism and are close to each other, 

 and have short tributaries or parallel branches, if any. There 

 are, besides minor streams, the James, Big Sioux, Des Moines, 

 Iowa and Cedar rivers ; and finally the Minnesota and Missis- 

 sippi the last of the parallel rivers. After the lowest line of 

 the continental valley was passed, the glacier would retire, so 

 that the melting water would run directly from it, and thus we 

 see the origin of the tributaries of the Mississippi on the east 

 side. 



This direction corresponds with that of the pre-glacial rivers, 

 and it is probable that many of them were washed out and re- 

 gained their old beds ; such as the St. Croix, Chippewa and 



