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 bluff two hundred and five feet high. It consists, at the base, of the Lower 
Sandstone, F. 1, 4, 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 Ophilefa. 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. 
Kleven 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, : ; ; : : : ‘ 69-40 
Carbonate of lime ; ; i : : ‘ 9-00 
Alumina by muriatiec solution, . : : : 1°50 
«¢ sulphuric solution, . : : : : 2-00 
Magnesia, : . , : ; j 1-22 
Alkalies, : poe es 0-18 
Peroxide of iron, : : : : 3-40 
