MICHIGAN ACADEMY OP SCIENCE. 103 



exception that, on the south side of Iron Hill, the ore comes down so as to 

 partly overlap the St. Peter sandstone." The origin of these deposits is 

 explained by Calvin as follows: "Ever since the process of rock decay 

 began over Allamakee county the geest has been highly ferruginous and 

 capable of serving as a source of supply to agents concerned in the accumula- 

 tion of iron ore. During the greater part of the time since the Carboniferous 

 period the surface of the country has been covered with a rank growth of 

 Vegetation, mostly aboreal, which, falling and decaying in countless successive 

 generations, furnished a second essential for ore concentration. An im- 

 perfectly drained area, or marsh of sufficient extent, constant as to position 

 for a considerable period of time, and receiving the ground water from many 

 square miles of adjacent territory, that rises in very gentle slopes, is the only 

 requisite condition remaining." 



Thus, the deposits in Allamakee county, Iowa, originated, according to 

 Calvin, under physiographic conditions which were identical with those 

 which are believed to have prevailed in the vicinity of Spring Valley when 

 the Gilman and Cady deposits were formed. In both areas the ores were 

 formed prior to an uplift which probably affected both areas at the same time 

 and which terminated the formation of ores in both areas. In other words, 

 the deposits of Allamakee county, Iowa, and those of Pierce coujity, Wisconsin, 

 are ■probably conternporaneous or nearly so and are to be connected ivith a period 

 of planation, or possibly peneplanation, which affected the entire region, prior 

 to the uplift, within the limits of the driftless area and an indefinite extent of 

 country surrounding it. 



Ann Arbor, April 1, 1909. 



