THE 



JOURNAL OF GEOLOGY 



OCTOBER-NOVEMBER, 1906 



NOTES ON THE GEOLOGICAL SECTION OF IOWA 



SAMUEL CALVIN 

 Iowa City, la. 



The columnar section is not drawn to scale. The approximate 

 average thickness is indicated in the appropriate column. 



The Sioux quartzite occupies only a few acres in the northwest 

 corner of the state, and in this locality it is Cretaceous sediments 

 which are found, in place, abutting against it. That it is pre- Cam- 

 brian in age admits of no doubt, and that it is the equivalent of the 

 Baraboo quartzite of Wisconsin is equally certain. The 25 feet 

 exposed in Iowa is only a small part of the thickness of this formation. 



The Cambrian sandstones are exposed by the erosion of the river 

 valleys in the northeastern part of the state. The basal contact 

 with the Algonkian quartzites is not seen in Iowa; a thickness of 

 fully 700 feet of this formation lies below the level of the Mississippi 

 River at Lansing and New Albin. This sandstone has been referred 

 to the Upper Cambrian, or Potsdam series, in the geological reports 

 of Iowa and Wisconsin. The special formation name, St. Croix, 

 and the names apphed to the smaller divisions, have been adopted 

 from the reports on the geology of Minnesota. 



Lower Magnesian. — This term is retained tentatively for the 

 sediments lying between the top of the Cambrian, the Jordan sand- 

 stone, and the very consistent geologic unit which has been univer- 

 sally recognized under the name of the St. Peter sandstone. The 

 New Richmond sandstone, when present, divides the Lower Mag- 

 Vol. XIV, No. 7 571 



