488 DESCRIPTION OF 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 
Harth 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 : 
1. White and brown sandstone, F. 1, eine ae ; - 50 to 60 
2. Blue, green, and yellow marl, e : ¢ . 2 
3. Magnesian limestone, F. 2, : ‘ 4 i ; 35 
4. Erratics and nodules of iron ore, . d é ‘ : 2 
5. Other drift deposits, : F Boo 4s : i 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. i in this region of country were imperfect specimens of Huomphalus Minneso- 
éensis, obtained at a locality four miles below the mouth of the Watonwan.+ 
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. 
7 Qn the Blue Earth and Psah Rivers, some fine Unios of the following species were noticed. U. 
rectus, crassus, fragilis, tenuissimus, bullatus, plicatus, trigonus, alatus, siliquoides, parvus. Also Ano- 
donta plana, imbeciles; Alasmodonta truncata, ambigua, rugosa and complanata; Paludina decisa ; 
Physa heterostropha, and Lymnea megasoma. 
