ON THE UPPER MISSISSIPPI. 



497 



Feet. 



4. Ash -colon red, argillaceous, hydraulic limestone, in thin layers, some- 



times with a conchoidal fracture. It effervesces slightly with acids, 

 and disintegrates rapidly when exposed to the weather.* No fossils 

 were observed in it. Thickness, ..... 5 



5. Grayish, buff-coloured, highly magnesian limestone, with numerous casts 



of fossils, of which the most characteristic are Pleurotomaria umbili- 

 cata, P. subconica, Murchisonia tricar inata, Ci/rtoceras macrostomum, 

 Trocholites ammon ius, Leptsena deltoidea, lllsenus ovatus, and several 

 species of Orthoceratitcs. 



Three miles below Fort Snelling, on the right bank, the white sandstone is at an 

 elevation of sixty-eight feet above the water-level of the Mississippi, and supports 

 twelve feet of fossiliferous limestone. From one to two miles below this place, 

 twenty-nine feet of sandstone is exposed, with a capping of eleven feet of fossiliferous 

 limestone ; and a quarter of a mile still lower down, it has an elevation of only 

 twenty-four feet, with a capping of eleven feet of limestone. Between the last two 

 points, at the junction of the sandstone and limestone, is the remarkable accumu- 

 lation of trappose and granitic boulders, mentioned in Dr. Owen's Report. 



About half a mile above St. Paul's, near the entrance of a small cave, the sand- 

 stone has an elevation of only fourteen feet above the river-level, and on it rests 

 eleven feet of shell limestone. 



At St. Paul's, the strata again rise ; here the cliffs are from seventy to eighty feet 

 high, of which the lower sixty-five feet consists of white sandstone, the remainder 

 being shell-limestone. 



About one mile below this point the hills recede from the river. At Red Rock, 

 they are half a mile back, and have an elevation of eighty feet, and can be seen 

 running parallel with the river, and at about the same distance removed from it, 

 for about two miles. 



Immediately on the river, at Red Rock, the Lower Magnesian Limestone occurs 

 in thin layers, some of which have an oolitic structure. 



This is the first ■ i)lace at which this formation was observed to occur in our 

 descent of the Upper Mississippi. 



The soil on the plain between the river and the bluffs is a rich dark vegetable 

 mould.f The same formation continues for about one mile below Red Rock, with 



* The following is the composition of this rock : 



Carbonate of lime, ...... 42-60 



" " magnesia, . . . . . .28-46 



Insoluble matter, . . . . . .14-16 



Alumina, oxide of iron, and manganese, .... 7-83 



Water, . . . . . . . .5-33 



Loss, ........ 1-60 



100-00 



f An analysis of one hundred grains of a rich black soil from Pied Rock resulted as follows : 



Passed through the fine sieve, . . . . .55 parts in 100. 



Moisture, . . . . . . . 5-10 



Soluble organic matter, ..... 8-80 



Insoluble organic matter, ..... 5-80 



63 



