APPLICATION OF PRINCIPLE OF TRANSGRESSIVE OVERLAP 58l 



Lower Magnesian. Feet 

 Saint Croix series. 



6. Madison sandstone 35 



5. Mendota limestone (including shale and sandstone) 60 



4. Sandstone (calcareous) 155 



3. Bluish shale (calcareous) 80 



2. Sandstone (slightly calcareous) 160 



1. Sandstone (very coarse, non-calcareous) 28 ° 



Total 



770 



Northward the lower members disappear by overlap of the higher. 

 The Mendota bed (number 5) is the fifth Trilobite bed of Owen, with 

 Dicellocephalus minnesotensis, D. pepinensis, Lingula aurora, and L. 

 mosia. 



The section at the Saint Croix Dalles has been studied in great detail 

 by Berkey.* He recognized the following subdivisions : 



Magnesian 

 series (Hall 

 and Sarde- 

 son). 



Basal sand 

 stone series 



" Shakopee dolomite. 



New Richmond sandstone. 



Oneota dolomite. 



Jordan sandstone. 



Saint Lawrence dolomites and shales. j 



f 3. Franconia sandstone ( 100 feet) The Saint Croix 



f Obolella politai Greensands , formation 



2. Dresbach | zone. I and shales. fWiTmh«m 



shales(150 -{ u , • • f Calcareous and 



(modified «{ feet). \ ZfoTmis zone \ P^iferous 



from Nor- [ nxjormis zone. ^ ghales< 



1. The lowest formation of this series is not exposed in the Dalles 

 area, but it includes the lowest sandstone beds and possibly also 

 the " Hinckley sandstone" (0 to 1,000 feet). 



ton). 



(Winchell). 



The Jordan sandstone of this section is correlated by Winchell with 

 the Madison sandstone of Wisconsin. At the Dalles of the Saint Croix 

 it contains a considerable fauna, listed by Berkey, f including Dicello- 

 cephalus osceola. The Saint Lawrence shales are correlated with the 

 Mendota beds of Wisconsin (fifth trilobite bed of Owen), and include, 

 besides a considerable fauna, J Dicellocephalus minnesotensis and D. pep- 

 inensis Owen. The Franconia, or third trilobite bed of Owen, contains 

 a rich trilobite fauna, § and so does the Dresbach, though this fauna is 

 quite distinct from that of the overlying bed. The species here agree 

 more closely with those of the Potsdam of New York, with which Hall 

 and Sardeson correlate this and the Franconia sandstones. The thick- 

 ness of this basal series at Minneapolis is nearly 1,550 feet. || 



* American Geologist, vol. xx, p. 377. 



t Ibid., vol. 21, p. 270. 



t Berkey : Loc. clt., p. 271. 



§ Berkey : Loc. cit, p. 272. 



II Hall and Sardeson : Bull. Geol. Soc. Am., vol. 3, p. 338. 



