330 



NOTES UPON THE GEOLOGY OF SOME PORTrONS OF MINNESOTA, 



stone. Beneath this is the Potsdam sandstone, which is exposed at a few places. At one 

 point on the southwest side of the river, there is still a little of the St. Peter's sandstone 

 visible above the Lower Magncsian limestone. The valley of the river is occupied by the 

 Lower Magncsian limestone as far as Mankato at the bend of the Minnesota, beyond 

 which I have not seen the formation.* 



Taking the road from St. Peter to New Ulm, we ascend from the plateau over a slope, 

 composed of northern drift with numerous boulders of crystalline rocks, to the level of the 

 high prairie, which continues with moderate undulations and a few deeper ravines, nearly 

 to the Minnesota River. Approaching within about three miles of the river along this 

 road, there rises to the surface a reddish-brown crystalline rock, having a granitic aspect, 

 but which when critically examined is found to consist mainly of quartz, or of quartz and 

 feldspar, without the perceptible admixture of other minerals. 



The rock is cut vertically by numerous joints, the walls of which are sometimes sepa- 

 rated or worn into wide, deep fissures. There arc no distinct lines of stratification, and 

 from its appearance on the prairie it would be difficult to indicate either stratification, dip, 

 or direction, to the mass. It continues with scarcely an interruption to the cast bank of 

 the Minnesota below New Ulm, at the Redstone Ferry. Here, under a more careful ex- 

 amination, the true character of the rock is disclosed, and the ■ top of the hill is found to 

 be essentially a metamorphosed quartz rock, or conglomerate, succeeded below by a more 

 compact quartz rock ; with some of the beds sienitic. This graduates below into a com- 

 pact purple, and finally reddish quartz rock, in distinct layers, which soon become alter- 

 nated with shaly seams. 



A section made from the river margin about a quarter of a mile below the Ferry, gives 

 the strata shown on the opposite page in a descending order, from a point about 125 feet 

 above the river. 



We have therefore an almost continuous exposure of the strata for more than one hun- 

 dred feet in thickness. There is a distinct dip to the northeast,t which in the lower beds 

 amounts to twenty or thirty degrees, while the general trend of the outcrop is to the west- 

 ward or northwestward. 



We have here an opportunity for observing, in the most satisfactory manner, this inter- 

 esting fact, — that while the lower beds of the formation are scarcely or not at all meta- 

 morphosed (the lamination and alternations of soft shaly matter being as clearly defined 



* Dr. D. D. Owen, in his Geological Map, has colored Potsdam sandstone as far as the mouth of the Rig Cot- 

 tonwood River; but this was probably the work of his assistants, who explored the Minnesota River; and my 

 own investigations do not confirm this view. 



I" The dip, by the magnetic needle, is northeast by north; but making the allowance for variation, the true dip 

 is about northeast. 



