192 



AGE OF THE RED SANDSTONES 



between the heads of the Brule and St. Croix, is but 120 feet; and though the eastern 

 extension of the ridge in question, between the Manidowish and head-waters of 

 Montreal River, reaches an elevation of about 1,150 feet, its entire elevation above 

 the Lake level, where it separates the St. Croix and the Brule, scarcely exceeds 650 

 feet ; and, as it extends westward, it sinks to less than 500, probably to 400 feet. 



To all this is to be added the highly important fact, substantiated by Dr. Nor- 

 wood's observations and my own, of the general prevalence throughout all the sand- 

 stones (ferruginous, as well as light-coloured, of Wisconsin and Minnesota, whether 

 west or south of Lake Superior,*) of a southeasterly clip, — a phenomenon wholly at 

 variance with the supposition that a basin once existed, stretching, with a northern 

 dip, from its southern margin on the Upper St. Croix ; a phenomenon, in fact, 

 which can be reasonably explained only by regarding the great Plutonic chain, 

 which lies north of the Lake, and runs nearly parallel with its north shore, from 

 northeast to southwest, as the main axis of dislocation, whence the sandstones in 

 question stretch, with a long, gradual, southeasterly slope, not arrested, in its ge- 

 neral inclination, by the low water-shed between the Lake and the Mississippi, but 

 passing on, and reaching down the valley of that river, until it disappears beneath 

 the Lower Magnesian Limestone of Southern Wisconsin. With this view coincides 

 the fact, that the dip in question is considerably greater north of the Lake than 

 south of it. From its south shore, across to the Mississippi Valley, the dip of the 

 strata, when undisturbed by subordinate igneous intrusions, does not, probably, ave- 

 rage more than six or seven degrees ; and, at many localities, it approaches a level. 



I conceive, then, that the natural and reasonable inference, in ascending the St. 

 Croix and meeting red sandstone beds, with a dip corresponding to that of the adja- 

 cent white and buff sandstone, is, that the red sandstone in question is a lower 

 member of F. 1, and that the white and buff layers do actually rest conformably 

 upon it. 



This reasonable inference is further confirmed by the fact, that, on several locali- 

 ties, on various other tributaries of the Upper Mississippi, phenomena somewhat 

 similar to those noticed on the St. Croix have been observed. On the St. Peter's, 

 near its confluence with the Waraju ; on the Wisconsin, eight miles above its Dalles ; 

 on the Barraboo, near Devil's Lake ; on the Cedar Branch of the Chippewa, near its 

 head, — the lowest beds of sandstone, found usually in proximity to the low ranges 



* With the exception of the high, northern dip of the sandstone, at the mouth of Montreal River (see 

 Colonel Whittlesey's Section, No. 4, and also the wood-cut at the close of this chapter), tilted by a trap 

 range, which cuts across that river, only a few hundred yards above its junction with the Lake, there has 

 not been observed, on the southern shore, from Montreal River to Fond du Lac, a single bed of sandstone 

 or its associated rocks, of which the dip, when it could be detected, was not southeasterly; not one which 

 dipped northerly towards the Lake ; though many writers have assumed this latter to be the true state of 

 the case, and have thence been led into sundry false conclusions, touching the formations of this region 

 of country. 



On the several forks of Bad River (see Colonel Whittlesey's Sections), on the Brule, and other Wis- 

 consin streams that empty into the Lake, there are, crossing them at certain points, trap intrusions, which 

 tilt the strata, often at a high angle, to the north, for a limited distance; but as soon as their immediate 

 influence ceases (which rarely reaches two or three miles, usually much less), the strata conform to their 

 usual place of deposition, to the southeast. 



