BASAL SANDSTONES. 



35 



Worthen regarded this sandstone as exposed on the Mis- 

 sissippi river at Oap-au-Gris, above the mouth of the Missouri, 

 the representative of the Saint Peter sandstone of northeast- 

 ern Iowa and the adjoining parts of Illinois, Wisconsin and 

 Minnesota ; but the correctness of this correlation is very 

 doubtful. 



Until quite recently, little additional information has sup- 

 plemented Swallow's observations of forty years ago. Eecent 

 investigations in the field go to show that much confusion has 

 existed concerning the diiferent limestones and sandstones, 

 and that, for instance, the "second" sandstone of one locality 

 is not the " second " sandstone of other places. Furthermore, 

 there appear to be good grounds for believing that there is a 

 decided physical break between the " saccharoidal " sandstone 

 and the " second " Magnesian limestone, though thus far the 

 line of unconformity has been noticed at but few points. 



rig. 1. Contact of Cambrian acd Porphyry. Fredericitown . 



Although personal study of the field relations of the dif- 

 ferent members of the "Magnesian limestone series " has not 

 been as extensive as is to be desired, a careful comparison of 

 both these notes and other references has given the impression 

 at least that that part of the " series " below the " first " sand- 

 stone is to be regarded as one great limestone, or series of 

 limestone beds, and that the sandstones are merely local facie s, 

 forming lens-shaped masses of limited extent. 



In the Iron Mountain region the base of the series has 

 been described as made up of grits, shales and limestones rest- 

 ing upon the crystallines. Exposures showing the line of 

 unconformity are numerous. They indicate plainly that the 

 conglomerates and sandstones are the immediate shore 

 deposits, quite limited in extent and varying with every crys- 

 talline elevation. One of these sections along the Little Saint 



