ON THE UPPER MISSISSIPPI. 



501 



Upper Mississippi, as every layer, from near the base of the coarse Lingula grit and 

 Obolus grit, up to the base of the Lower Magnesian Limestone, admits of being traced 

 out as follows : 



Feet. 



1. Coarse Lingula and Obolus grit, especially fossiliferous, towards the top, . 104 



2. Green sand and soft green sandstone, with Trilobites, ... 6 



3. Soft thin-bedded, green, micaceous sandstone, . . . .14 



4. Brown siliceo-calcareous rock, ...... 4 



5. Thin layers of green sandstone, alternating with green earth, . . 36 



6. Micaceous sandstone with Trilobites. D. Miniscaensi's, ... 2 



7. Alternations of green and ferruginous sandstone, . . . .20 



8. Micaceous sandstone, with D. Miniscacnsis, .... 2 



9. Loose green sand and soft green sandstone, . . . .15 



10. Green and red sandstone, charged with silicate of iron, ... 5 



11. Six inches of bluish-green earth. 



12. Thin-bedded green and yellow sandstone, . . . .35 



13. Band of concretionary red and yellow sandstone, with silicate of iron dis- 



seminated. ....... 3 



14. Green, red, and yellow, compact sandstone, with thin dolomitic layers pass- 



ing downwards into siliceo-calcareous rock, . . .40 



15. Yellow and ash-coloured argillo-calcareous layers, with D. Minnesotensix, . 9 



16. Alternations of thin-bedded, light brown, and ash-coloured sandstone, . 6 



17. Thick beds of soft yellowish sandstone, &c, as at the Great Slide, . 50 



18. Mammillary and botryoidal layers, ..... 2 



19. Quartzose sandstone, with intercalations of magnesian limestone, . . 80 



433 



The top of bed No. 1 is almost made up of Lingulas and Obolus, and five feet 

 above the fossiliferous layers, the gritstone contains the same species of Trilobite 

 that occurs in the third Trilobite bed, but the rock was so friable that it was almost 

 impossible to preserve specimens. The upper and lower divisions of the third Trilo- 

 bite bed are here separated by twenty feet of green sandstone, alternating with 

 green earth ; the species in both members are, however, the same, and there appear 

 to be at least three different species. Associated with them is a nearly circular 

 species of Orthvs, and a small Lingula. The Trilobite beds are mostly easily 

 cleavable gritstones, of a grayish-green colour, until exposed to the weather, when 

 they assume a pale buff hue, and become harder. 



Five miles below Mountain Island, a fine-grained, yellow sandstone, charged with 

 Obolus Appolinus, contains also a minute species of Trilobite ; the head has a thick- 

 ness of about four feet, and underlies the coarse Lingula grit, numbered 1 in the 

 last section. These strata have a slight dip here to the northwest, and can bo 

 traced for a mile down stream ; and near the termination of the exposure the forked- 

 tailed Trilobite was observed associated with the above-mentioned fossils, but it is 

 difficult to procure good specimens here, owing to the soft nature of the rock. This 

 latter Trilobite has not been observed in any of the higher Trilobite beds. The 

 Obolus Appolinus, which occurs here in great profusion, associated with these Trilo- 

 bites, has even the nacre preserved with much of its original lustre and colour. 

 These layers also furnished two species of Lingulas, one of which, a large oval 



