I 22 



Cincinnati Society of Natural History. 



THK BASE. 



The base of the formation has been but seldom observed. 

 Professor Winehell has recorded one observation upon it. 

 This was in Nicollet County, Minnesota, and he says about 

 two feet of white sandstone with a thin strip of shale overlay 

 the Magnesian limestone below. It frequently retains its 

 arenaceous character to its contact with the underlying rock/ ;: 

 Professor Hall has also noticed the junction, and says that 

 sometimes there is a finely laminated clay with stripes of 

 green, f and the sandstone is often similarly colored. The 

 irregular surface of the Lower Magnesian limestone is fre- 

 quently the cause of great variations in its thickness, as it fills 

 up hollows and covers ridges which exist there. In Missouri 

 this feature is especially remarkable. Mention is made of one 

 locality where the formation thickens so rapidly as to present 

 the appearance of a dyke cutting off the sandstone both 

 above and below. + In another place it forms a bed 240 feet 

 long and sixty feet deep, filling a cavity in the limestone of 

 this extent. 1 1 



In 1877, Professor Chamberlin refers to the St. Peter's sand- 

 stone in eastern Wisconsin. £ He notes the unequal deposition 

 of the sandstone on the Lower Magnesian limestone, stating 

 that, instead of being of uniform thickness, as has been often 

 asserted, it ranges from a thickness of 212 feet down to a 

 single layer of sand grains. It sometimes varies from 100 

 feet to zero in the course of a quarter of a mile. Instances 

 are known of its entire disappearance, and the consequent 

 resting of the Trenton limestone on the upper surface of the 

 Lower Magnesian, while in the near vicinity a thickness of 

 from fifty-four to 100 feet has been noted. 



THE TOP. 



Its junction with the overlying limestone has been more 

 frequently observed. Sometimes the transition is abrupt, but 



"•'Second annual report of the Geological and Natural History Survey of Minne- 

 sota, for 1874, 1875, P- J 3 2 - 



fReport on the Geology of Wisconsin, Vol. I, 1862, p. 29. 



JFirst and Second Annual Reports of Geological Survey of Missouri, 1855, p. 119. 

 HReports on the Geological Survey of Missouri, 1855-1871, 1873, pp. 142-145. 

 {•Geology of Wisconsin. .Survey of 1873-1877. Vol. 2, pp. 285-290. 



