488 



DESCRIPTION OP THE COUNTRY BORDERING 



of erratics, sand, and gravel, usually without any distinct bedding, resting on strati- 

 fied ash-coloured and yellowish arenaceous marl. Even where the hills ranged from 

 one to two hundred and fifty feet in height, no ledges of rock could be discovered. 



This being the boundary between the outcrop of rocks of Lower Silurian date 

 and the region of drift, such as Dr. Owen encountered towards the heads of the Iowa 

 and Des Moines Rivers, we proceeded no farther up the valley of this stream. It 

 is worthy of note that we here first observed specimens of a kind of coaly lignite, 

 which will be mentioned more particularly hereafter ; these only occurred, however, 

 in loose fragments, swept out by the current of the river from the adjacent drift 

 deposits. 



We now proceeded to examine the Blue Earth River. The first good exposure 

 of rocks is at the celebrated Blue Earth Bluff, mentioned by Featherstonhaugh and 

 other explorers. It is composed of sandstone, F. 1, at the base, capped with mag- 

 nesian limestone, F. 2, with about two feet of thin layers of greenish, blue, and 

 yellow marl, interstratified at the junction of the two formations. This earth the 

 Indians collect, and esteem it highly as a paint. Its appearance and composition is 

 similar to that of the green earth under the fucoidal layers of F. 1, at Marine Mills, 

 which Dr. Owen found derived its colour chiefly from silicate of iron. On the Blue 

 Earth River, it occupies rather a higher stratigraphical position than on the St. 

 Croix, being at the very top of F. 1.* The section of the different beds at this 

 locality is as follows : 



Feet. 



1. White and brown sandstone, F. 1, . . . . 50 to 60 



2. Blue, green, and yellow marl, ..... 2 



3. Magnesian limestone, F. 2, . . . . 35 



4. Erratics and nodules of iron ore, ..... 2 



5. Other drift deposits, ...... 86 



The magnesian limestone contains here a Lingula, like that obtained at White 

 Rock. About half a mile above this section is the last exposure of magnesian lime- 

 stone, witnessed on the Blue Earth. Here the sandstone, F. 1, is sixty feet thick ; 

 the magnesian limestone (F. 2), fifteen feet. The sandstone continues to be visible 

 some miles further south. One mile above the Blue Earth Bluff, it is exposed, 

 seventy feet above the river ; five miles above, sixty feet, and continues for about 

 a mile, when it gradually decreases in elevation, and finally disappears altogether, 

 about a mile above the mouth of the Watonwan. The only organic remains found 

 in F. 1 in this region of country were imperfect specimens of Enomphalus Minneso- 

 tensis, obtained at a locality four miles below the mouth of the Watonwan.f 



Where the rocks of Lower Silurian date are finally lost beneath the drift, the 

 height of the table-land is one hundred and fifty feet. 



* A deposit of similar character, but too sandy to be employed as a pigment, was observed also at 

 the White Earth Bluff on the St. Peter's. 



'(• On the Blue Earth and Psah Rivers, some fine Unios of the following species were noticed. U. 

 rectus, crassus. fragilis, tenuissimus, bullatus, plicatus, triyonus, alatus, siliquoides, parvus. Also Ano- 

 donta plana, imbeciles ; Alasmodonta truncata, ambigua, rugosa and complanata ; Paludina decisa ; 

 Physa heterostropha, and L)/mnea megasoma. 



