216 SIXTEENTH REPORT ON THE CABINET OF NAT. HISTORY. 



In assigning a position to the sandstone of the south shore of 

 Lake Superior, to the south and east of Keweena point, from the 

 evidence before us, and in the absence of any fossils which may 

 aid the decision, we are forced to conclude that this formation 

 is a greatly augmented development of the St.Peters sandstone ; 

 or, that the Lower Magnesian limestone (''Calciferous sandrock'') 

 has thinned out, so as to leave the St.Peters sandstone and the 

 Potsdam below (as developed in the Mississippi valley) to go on 

 as one mass to the northward. 



This latter inference would be sustained, in some measure, by 

 the facts observed in Missouri, where we have nearly nine hundred 

 feet of the three Lower Magnesian limestones, which, in Southern, 

 Central, and Southwestern Wisconsin, are represented by rarely 

 more than two hundred or two hundred and fifty feet of similar 

 rock. At anything like this ratio of thinning, the Lower Magnesian 

 limestone would have disappeared long before reaching the pa- 

 rallel of the south shore of Lake Superior, or it might continue 

 to occur in isolated lenticular masses. 



It is scarcely possible to suppose that the lower sandstone of 

 the Upper Mississippi valley has not, at some time or in some 

 form, extended as far as Lake Superior ; but it is far from being 

 proved that the sandstone now so largely developed on the south 

 shore is that sandstone, as we have shown. If this sandstone 

 consist of both that above and that below the Calciferous, or of 

 the St.Peters and the Potsdam proper, then at some point we 

 should expect to find a change of character, or nonconformity 

 between the beds, to indicate the lapse of time in the deposition 

 of the Lower Magnesian limestone of more southern localities ; 

 and this view is sustained by the observed want of conformity 

 between the sandstone and Magnesian limestone near Dead river 

 just cited. 



Admitting the deposition of the lower sandstone of the Mis- 

 sissippi valley to have been continued in its higher stage into the 

 region of Lake Superior, it seems remarkable that the rock now 



surface of the dolomite, and dips at a moderate angle to the southwest. The dolomite 

 is cut by what appears to be a vertical dyke, which, instead of intersecting the sand- 

 stone, abuts againt the bottom of it (See Geology of Canada, 1863, pp. 83 &, 84). 



In the Mississippi valley, the Upper sandstone is apparently conformable to the 

 Magnesian limestone on which it reposes; but its lower beds sometimes consist of a 

 fine argillaceous sediment, indicating a lapse of time before the arenaceous deposit 

 began; while in other places, the superincumbent sand has penetrated into fissures in 

 the rock below, and I have never observed any beds of passage between the two for- 

 mations* 



