498 



LOCAL SECTIONS 



a height of from eight to fifteen feet. The strata present an undulated appearance 

 along the course of this exposure, the result probably of lateral pressure. About 

 one mile above Gray Cloud Island, on the left bank of the river, the Lower Magne- 

 sian Limestone again appears,"and one of the layers, five feet above the water-level, 

 is densely crowded with fossil univalves, closely allied to the genus Ophileta of the 

 Calciferous Sandrock of the New York system. The fossils were all casts, and none 

 of them could be procured in a well-preserved condition. These beds form con- 

 tinuous escarpments to Gray Cloud Island, with an elevation varying from ten to 

 fifty feet. At Gray Cloud Island, the uppermost layers are quarried for burning 

 into lime. They are highly magnesian, containing, according to Dr. Norwood's 

 analysis, 40 - 7 per cent, of the carbonate of that earth, and do not make as good lime 

 as that obtained by burning the fossiliferous limestone at the mouth of the St. Peter's, 

 and bed No. 3 of the section at the Falls of St. Anthony. 



After passing this exposure, no more rocks are to be seen on the Mississippi for 

 the distance of about ten miles. The river bottoms are well timbered with oak, 

 maple, elm, and basswood. A short distance back from the river, the land is roll- 

 ing prairie, and the soil is usually good and well adapted for cultivation. 



About three miles above the mouth of Lake St. Croix, on the north bank of the 

 river, is a blnff two hundred and five feet high. It consists, at the base, of the Lower 

 Sandstone, F. 1,/, which has a thickness of thirty feet; on this is superimposed the 

 Lower Magnesian Limestone, F. 2, with a thickness of a hundred and seventy-five 

 feet. This locality is the most northerly point on the Mississippi, where the Lower 

 Sandstone is seen above the waters of that stream. 



Two miles above the mouth of the St. Croix, the Lower Magnesian Limestone 

 rises to the height of two hundred and thirty feet above the bed of the Mississippi. 

 Near the water-level, the strata contain OpMleta. After passing this point, no 

 rocks are seen in place for the distance of a mile and a half, after which the Lower 

 Magnesian Limestone again appears, and continues with but little interruption to 

 the confluence of the St. Croix. 



Eleven miles below this is " Old Man's Prairie," situated on the east bank of the 

 Mississippi. The Lower Magnesian Limestone is the prevailing rock to this place. 

 It is seen projecting in perpendicular ledges from different portions of the slopes of 

 the bluffs. These bluffs often rise to the height of three hundred and twenty feet 

 above the bed of the Mississippi. The rock exposures usually occur near their 

 summits, while the lower portion of the hills are generally less abrupt, and mostly 

 covered with soil, vegetation, and loose fragments of rock, which have fallen from 

 the strata above, and which conceal the underlying strata. Judging from the 

 character of the slopes, it is highly probable that the Lower Sandstone forms the 



Silica, 



Carbonate of lime, 



Alumina by muriatic solution, 



69-40 

 2-00 



1- 50 



2- 00 

 1-22 

 0-18 



3- 40 



" sulphuric solution, 



Magnesia, 

 Alkalies, 

 Peroxide of iron, 



