250 Deep Well at Minneopa^ Minn. — Hall, 



selves. In short, these pebbles are fragments not of 



quartz but of quartzite 215 ft. 800 ft. 



The records for the remaining distance are regarded 

 as valueless, but the record of the Mankato well 

 (this volume Bull. 1, p. 143) leads to the belief that 

 the rocks penetrated must have been silicious sedi- 

 ments 200 ft. 1,000 ft. 



However, authentic records have been preserved for the first 

 800 feet of this well; and that depth is sufficient to prove its im- 

 portance to geologists, for one more fact is presented in evidence 

 of the correctness of the position for which the Wisconsin geolo- 

 gists have contended, namely, that the great red quartzite forma- 

 tion of the northwest belongs to an earlier geologic age than the 

 white and friable sandstones of the Upper Mississippi valley. Only 

 16 miles from this well the Courtland exposures of red quartzite 

 can be seen and they show a rock identical in chemical and physi- 

 cal characters with the material out of which these pebbles were 

 worn. These quartzite exposures with their southwesterly exten- 

 sion through Watonwan and Cottonwood counties into South Dakota 

 are the only belt of this kind of rock known in Southern Minnesota. 

 W^e conclude that the conglomerates penetrated in sinking this well 

 must have been formed from the erosion of these quartzite beds. 



But in the case of the sandstones the way to a conclusion is not 

 so clear. For all the well record can show to the contrary, there may 

 be a great unconformity between the quartzite conglomerates and 

 the overlying sandstones and shales. Such unconformity, did it ex- 

 exist, would afford a place here in Southern Minnesota for the Kewee- 

 nawan formation between the Cambrain complex of sandstones, 

 shales and dolomites and the quartzites and would offer strong, 

 presumptive evidence that the quartzites are of Huronian age. 

 An unconformity here, it must be admitted, is far from proven. 



Whether we here call them Huronian quartzites or not, we 

 know that the time necessary for their formation, their thorougk 

 vitrification and their subsequent erosion must have been enor- 

 mous. Therefore the Sioux quartzite, as the formation of thi& 

 lithologic character stretching from Courtland to the southwest 

 into South Dakota has been called by Dr. White, belongs to an ear- 

 lier and entirely distinct horizon from that of the so-called Potsdam 

 or Saint Croix formation of the Upper Mississippi valley, and the 

 time-gap between the two is one of great extent. 



October 2, 1888. 



