504 



LOCAL SECTIONS 



and fifty feet above the water-level, alternating with sandstone, F. 1, /, and covered 

 by one hundred and eighty-seven feet of magnesian limestone. 



Similar bluffs extend to the mouth of Yellow River, the junction between F. 1 

 and 2 declining gradually about one hundred feet in a distance of about three 

 miles. At this place the magnesian limestone is traversed, near its summit, by 

 seams of chert, and contains Euomplialus and another Gasteropod, which appears to 

 be a species of Natica. 



About two miles above Prairie du Chien, I found, towards the head of a ravine, 

 the Upper Sandstone, F. 2, c, exposed in heavy beds, with a thickness of fifty-five 

 feet, and still higher up the shell-beds, F. 3, A, containing many of the same species 

 of Mollusca as at St. Anthony. The most characteristic are : Illcenus ovatus, Isote- 

 lus gigas, an undetermined species of Phacops, Leptcena alternata, L. sericea, Ortliis 

 testudinaria, Belleroplion bilobatus, Chcetetes lycoperdon, Pleurotomaria lenticular is, 

 Ambonycliia undata, and a Modiolopsis. 



Not far from Prairie du Chien I found the succession and thickness of the beds, 

 as far as exposed, nearly the same as on Section No. 1, A. For about forty feet 

 above the river, no rocks can be seen. Then we have : 



Fret. 



1. An escarpment of one hundred and twenty feet of magnesian limestone, 



with slope of forty feet, where no rocks are exposed, . . . 160 



2. White sandstone, F. 2, c, . . . . . . 50 to GO 



3. Buff magnesian limestone, with few fossils, chiefly Or/his and columns 



of Crinoidea,* . . . . . . . 20 



4. Beds of shell limestone, about, ..... 100 



This last bed afforded the following species : Illcenus crassicauda, I. ovatus, Iso- 

 telus megistos (J), I. gigas, Ceraurus pleurexanthemus, Phacops callicephalus, Pleuro- 

 tomaria subconica, P. umbilicata, P. lenticularis, Subulites elongata, Murchisonia 

 tricarinata, M. bellicincta, Belleroplion bilobatus, Orthoceras junceum, Cyrtoceras 

 macrostomum, Trocliolites ammonius, Ortliis testudinaria, O. tricenaria, Leptcena alter- 

 nata, L. sericea, Terebratida capax, T. modesta, Ambonycliia undata, besides undeter- 

 mined species of Avicida, Modiolopsis, and Orbicula. 



b. Below the Mouth of the Wisconsin. — I was directed, at the close of the season's 

 operations in 1849, to continue my detailed sections on the Mississippi, below the 

 mouth of the Wisconsin River, as far as its confluence with Red Cedar, for the 

 purpose chiefly of ascertaining whether any limestones of carboniferous date could 

 be detected immediately south and west of the boundary line of the Upper Mag- 

 nesian Limestone, or adjacent to the limestone of Cedar and Iowa Rivers. 



The results of these investigations are here submitted. 



As the Upper Magnesian Limestone of the Dubuque and Mineral Point Dis- 

 tricts, together with its subordinate shell-beds, have been described in Dr. Owen's 

 Report of 1839, I will merely mention here a few localities where we found some 

 well-defined characteristic fossils, on our descent of the Mississippi. At Cassville, 

 in the shell-beds, F. 3, A, Leptcena alternata, L. planumbona, Calymene senaria. 

 Fifteen miles above Dubuque, Cyrtoceras macrostomum, and the pygidium of a Tri- 



* This bed seems to be absent at the Falls of St. Anthony. 



