186 Contributions to Palceontology. 



beneath the sandstone of the south shore of Lake Superior, 

 be the true Lower Magnesian limestone or " Calciferous 

 sandrock." 



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 develop- 

 ment 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 Pots- 

 dam below (as developed in the Mississippi valley) to go 

 on as one mass to the northward. 



This latter inference would be sustained, in some mea- 

 sure, 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 hun- 

 dred or two hundred and fifty feet of similar rock. 

 At anything like this ratio of thinning, the Lower Mag- 

 nesian limestone would have disappeared long before 

 reaching the parallel 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 



fills up the inequalities in the surface of the dolomite, and dips at a mode- 

 rate angle to the southwest. The dolomite is cut by what appears to be a 

 vertical dyke, which, instead of intersecting the sandstone, abuts against 

 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 some- 

 times consist of a fine argillaceous sediment, indicating a lapse of time 

 before the arenaceous deposit began; while in other places, the superin- 

 cumbent sand has penetrated into fissures in the rock below, and I have 

 never observed any beds of passage between the two formations. 



