CAMBKI AN CONGLOMERATES. 337 



Upham* have observed the conglomeratic character of these deposits 

 along- Snake river. 



Finally, at Taylors Falls in two or three places, one of which is near 

 the crossing of the Saint Paul and Duluth railway at the entrance to the 

 village and the carriage road running southward from the public school 

 building, lies a continuous belt of very coarse conglomerate. The length 

 of the exposure is twenty rods or more, and it is covered toward the 

 southwest by the drift material pushed over the edge of the river gorge 

 from the northwest, The same kind of a conglomerate, together with 

 great cracked cliffs of diabase, whose crevices are filled with fossiliferous 

 material, is to be seen on the Wisconsin side of the river at Saint Croix 

 falls. Another bed is in the banks of the river between Taylors Falls and 

 Osceola, Wisconsin. This conglomerate is made up of pebbles of diabase 

 like the rock constituting the high, massive cliffs which along both sides 

 of the river here form the picturesque " dalles " of the Saint Croix. They 

 are dark colored; frequent fine examples of concentric weathering are 

 seen, a peculiarity very common among the diabases of the lake Superior 

 region. Some of the pebbles are very small, while others are of tons' 

 weight, They are cemented together by a shaly magnesian sandsone 

 carrying numerous cavities lined with crystals of dolomite, alternating 

 with compact portions well filled with shells of Lingulepis pinnaeformis, 

 Owen; Obolella polita, Hall, etc. A typical locality of this conglomerate 

 is represented by plate 11, figure 2, which is a photomechanical repro- 

 duction of the photograph taken from the western side of the carriage 

 road entering Taylors Falls from the south. 



Localities of the Potsdam. — In addition to the places just enumerated, 

 this sandstone can lie seen in strongly marked exposures along Saint 

 Croix river from Taylors Falls to Marine, and in many localities in 

 Winona. Houston and Fillmore counties along the bluffs of the Missis- 

 sippi and its tributaries, particularly Root river and Rollingstone creek. 

 In places tins eroded formation produces the most conspicuous feature 

 of the bluffs along the streams named. In the Minnesota river valley it 

 docs not appear as a surface formation, but it is reached in several 

 wells. 



Structural ('huniftcr*. — The Potsdam sandstone appears to have been 

 laid down in a greal basin whose present rim is at the surface or beneath 

 the glacial drift from Watonwan county, in southern Minnesota, north- 

 easterly into Kanabec county, and from Chengwatona across the Saint 

 Croix at the Kettle river rapids into Wisconsin, where il rests againsl the 

 Huronian quartzites in Barron county and the gneisses and -ehists of 

 Archean and Algonkian age, past Chippewa Falls, Black River balls. 



♦ The Geology of Minnesota, Final Report, vol. ii. L888, p. 021. 



