172 GENEKAL GEOLOGY. 



a large part of the northern portion of the United States and 

 of Canada, and reaches a known thickness in Iowa of abont 

 three hundred feet, it nevertheless forms rather an incon- 

 spicuous feature in the geology of Iowa. It is exposed only 

 in a small portion of the northeastern part of the State, and 

 has been brought to view there by the erosion of the river 

 valleys. Consequently, it is only to be seen in the bases of 

 the bluffs, and steep valley sides which border the rivers 

 there, while in the same steep valley sides the Lower Magne- 

 sian limestone, St. Peter's sandstone, and Trenton limestone, 

 are all frequently seen to overlie it in their regular order. In 

 this position it is to be seen along the bluffs of the Missis- 

 sippi from the northern boundary of the State as far south 

 as the town of Guttenburg, along those of the Upper Iowa 

 river for the distance of about twenty miles from its mouth, 

 and in the bottoms of a few other small valleys whose 

 streams empty into the Mississippi within Allamakee county. 



The base of this formation does not appear anywhere in 

 Iowa, consequently its full thickness here is not known, 

 nor is it certainly known what it rests upon. It is probable 

 that it rests upon the Sioux Quartzite, but of this we have 

 no direct proof. 



The rock is everywhere soft; usually a very friable sand- 

 stone, but sometimes containing some clayey material and 

 approaching in character a sandy shale. 



Economic Value. The Potsdam sandstone in Iowa is 

 nearly valueless for any economic purpose. It is almost 

 nowhere of sufficient hardness to serve even the commonest 

 purposes of masonry. It is sometimes, however, so incohe- 

 rent as to be serviceable as building sand. 



Fossils. No fossils have yet been discovered in this 

 formation in Iowa, but they are found in it quite abundantly 

 in some parts of Wisconsin. 



LOWER MAGNESIAN LIMESTONE. 



Area and General Character. This formation has but very 

 little greater geographical extent in Iowa than the Potsdam 



