346 ARTHUR C. TROWBRIDGE 



The oldest rocks of the district are granite, diorite, and rhyolite, 

 which lie around the borders of the quartzite syncline and seem to 

 form the basement upon which the quartzite was deposited. Next 

 above the igneous rocks lies the Baraboo quartzite, which in turn 

 is overlain conformably by the Seeley slate and the Freedom forma- 

 tion. The Baraboo, Seeley, and Freedom formations are all in- 

 volved in the folded structure of the district, although the Seeley 

 and Freedom beds do not outcrop. The total thickness of the 

 Proterozoic formations is about 6,000 feet, the quartzite alone 

 measuring in the North and South ranges about 5,000 feet. 



There is a great unconformity between the Proterozoic and 

 Paleozoic groups of rock. After the Proterozoic formations were 

 deposited and folded, the region was eroded in one or more cycles 

 until the surface in this district had a relief of at least 1,300 feet, and 

 upon this rugged surface the Cambrian rocks were deposited. 



The Cambrian system includes the Potsdam sandstone, between 

 600 and 700 feet thick where thickest, the Mendota limestone, 3-1 1 

 feet thick in the Devils Lake district, and the Madison sandstone, 

 80-90 feet thick. This is a conformable series and the strata are 

 essentially horizontal. Away from the ranges the top of the 

 Madison sandstone is about 1,020 A.T., but the standsone laps up 

 on the ranges to altitudes of 1,200 feet or higher. 



Only the Prairie du Chien and St. Peter formations of the Ordo- 

 vician system now appear in the vicinity of Devils Lake, although 

 it is nearly certain that younger formations were deposited here 

 originally and have been eroded away. 



The Prairie du Chien formation overlies the Madison sandstone 

 conformably, but outcrops in only a few localities within the 

 boundaries of the Devils Lake district. It is hard, cherty dolomite. 

 Its average thickness in the Mississippi Valley is about 200 feet, 

 although in the immediate vicinity of Devils Lake its greatest 

 thickness is 20 feet. 



An unconformity is known to exist between the St. Peter and 

 Prairie du Chien formations in Iowa, Illinois, Wisconsin, and 

 Minnesota, and is well represented in the Devils Lake district. 

 If the total thickness of 200 feet of Prairie du Chien dolomite was 



