190 AGE OF THE RED SANDSTONES 



trap, in place. So, also, on Snake and Kettle Rivers, ascending to the northwest, 

 the change is gradual, from the buff and white quartzose sandstones prevailing on 

 the St. Croix, below the mouth of Snake, to the reddish-brown, argillaceous, ferru- 

 ginous variety. 



Now, it is very certain, that (if they still occupy, undisturbed, their original 

 position) these red sandstones, so closely resembling, in aspect and connexion, the 

 formation of Lake Superior, must rise from beneath the quartzose sandstone of 

 F. 1, and must constitute, in fact, one of the inferior beds of that formation. And 

 so of the red sandstones in further ascending the St. Croix. As a general rule, 

 they retain their red tint, their argillaceous, ferruginous character, and their south- 

 easterly dip, as far as any rocks can be traced, in place, up that stream, and until 

 they disappear, under the drift, some ten miles below Upper St. Croix Lake, which 

 is the source of the river. 



Here, it is true, an interval of some width occurs. From the point where these 

 rocks are lost, under drift, on the Upper St. Croix, across to where other red sand- 

 stones, similar in appearance, in their associated rocks and in their dip, are found 

 in place on the Bois Brule of Lake Superior, it is, in a northerly direction, about 

 twenty-three miles ; the intervening space being a region of heavy drift and 

 erratics, in which no rocks whatever can be reached, in place. 



Here is a close approximation to proof, by superposition, that the red sandstones 

 of Lake Superior underlie the Lingula and Orbicula beds of the Upper Mississippi 

 Yalle}^, represented on Tab. 1, B. The proof would be complete, if we could be 

 assured, that the sandstones of the Upper St. Croix, and of Snake and Kettle 

 Rivers, with their general southeasterly dip, have preserved the inclination of their 

 original deposition ; and, further, that in the drift-covered interval of twenty-three 

 miles, now devoid of visible rocks, there is continuity of strata and persistence of 

 general dip. 



It is an imaginable case, however, and one which may be advanced by those 

 who set down the sandstones of Lake Superior as of Upper Silurian or Post-Silurian 

 date, that, at some remote distance of time, there might have existed two indepen- 

 dent geological basins, of which the margins came together in the vicinity of the 

 confluence of Snake River and the St. Croix ; that, in the southern of these two 

 basins, stretching down the Mississippi Valley, there might have been deposited 

 sandstone of Lower Silurian date ; while, in the northern basin, at a subsequent 

 era, the red sandstone, with its subordinate slates and conglomerates, might have 

 supervened, say at the Old Red or New Red Period. 



"£ ° a> 



_i; — O 



THEORETICAL SECTION OF TWO ANCIENT BASINS, ONE OF QUARTZOSE SANDSTONE, THE OTHER OF RED ARGILLACEOUS 

 SANDSTONE, IN THEIR ORIGINAL UN DISTURB ED CONDITION. 



