280 



THE GEOLOGY OF MINNESOTA. 



[Geological structure 



Viburnum Lentago, L. Sheep-berry. 



Rhus typhina, L. Staghorn sumach. Bare. 



Celastrus scandens, L. Climbing bitter-sweet. 



Trees in ike order of abundance at Lime City. Mr. L. G. Odell estimates the trees about Lime 

 City in the following order of abundance. Black oak, basswood, sugar maple (largely cut off), 

 aspen, white ash, ironwood, white elm, white oak, red elm, rock elm, black ash, butternut, bitter- 

 nut, hackberry, white pine, red cedar, box-elder, cotton wood, black cherry, water beech, black wal- 

 nut, bur oak (on the outskirts of the timber), juneberry, black haw, stag (or spotted) alder, juniper 

 (a shrub that hangs over the bluffs, growing in clumps), balsam fir, whitewood (also called yellow 

 poplar, formerly cut into lumber, uow nearly gone, probably the Populus gmndidentata), leather- 

 wood (on the tops of the bluffs), and doubtfully the American yew. 



THE GEOLOGICAL STRUCTURE. 



The rocks of the county belong to the Devonian, and to the Silurian 

 and Cambrian ages. The Cretaceous also appears in Sumner township, in 

 the extreme northwestern corner of the county. They occur as arranged 

 in the following order, with their approximate thicknesses. 



1. Cretaceous, thickness unknown, perhaps twenty-five feet, lying 

 unconformably over the older rocks. 



2. Upper Devonian, Hamilton, ) 



3. Lower Devonian, Corniferous ? ^ 



4. Niagara, of the Upper Silurian, 



5. Maquoketa, of the Lower Silurian, 



6. Galena, of the Lower Silurian, 



7. Trenton, of the Lower Silurian, 



100 ft ? 



i Hudson River, 

 i and Utica slate ? 



8. St. Peter, 



9. Shakopee, 



10. Jordan, 



11. St. Lawrence, 



12. St. Croix, 



Cambrian, 





- 100-150 ft. 

 75-100 ft.* 

 75-100 ft. 



160 ft. 

 122 ft. 

 75ft. 



- 25-40 ft. 



200 ft. 

 - Exposed 375 ft. 



With the exception of the Cretaceous these formations have a strike 

 across the county northwest and southeast. They have a gentle dip, at least 

 theoretically, toward the southwest, though no general dip is perceptible. 

 The oldest rock in the county is the St. Croix sandstone, which appears in 

 the northeastern corner of the county. The latest, except the Cretaceous, 

 is the Devonian, in the southwestern part of the county. The areas of out- 

 crop are shown by the colored map of the county (plate 10) accompanying 

 this report. The boundary between the Trenton and the St. Peter is the 



'Geology of Witeontin. 1862. Vol. I., p. 181. 



