144 



Geology of Upper Illinois. 



Rockwell is situated on section fourteen, Avhich is next to the 

 section against which the canal terminates. The width of the 

 Illinois valley varies in this vicinity from one to two miles. Its 

 bluffs, which are generally of naked rock, and nearly perpendicu- 

 lar, are between one hundred and one hundred and twenty feet in 

 height. Their course and position with respect to the channel of 

 the river, may be seen upon the accompanying map. The bot- 

 tom of the river in the vicinity of Utica is solid rock, with which 

 also the bottom-lands are underlaid at very partial depths. In- 

 deed, the strata often attain the surface over considerable breadths 

 of the meadows between Rockwell and Ottawa. The course of 

 the canal, which is also traced on the map, is directly at the foot 

 of the northern bluff. 



sian limestone in Wisconsin, Upper Illinois, and Missouri, struck me with sur- 

 prise. I observed it, in addition to the country already noticed between Chicago 

 and Ottawa, as the prevailing formation about the northern extremity of Michi- 

 gan, the islands about Michillimacinac, the mouth of Green bay, as well as near 

 Navarino, at the head of the bay. In the last mentioned region, it abounded in a 

 species of Producta, which I take to be undescribed, and shall therefore denomi- 

 nate the incurvata. Specific character. Scini-circtdav : lunge nearly straight and 

 the length of the shell ; with fine longitudinal stria ; flattish ; edge crenated ; shal- 

 loio valve concave, basal margiji incurved ; mvscular impressions and hinge-pro- 

 cess very distinct. (Figs. 1 and 2.) The space between the valves is very small 

 in this species. 



Fig. 1. Fig. 2. 



Fig. 3. 



Fig. 3, represents a second species of the 

 same genus, which is also probably new. The 

 large valve is deep and ver}' gibbous. Its strise 

 are distinct, and resemble the ribs of the Pec- 

 tens. 



Along with these species occurs a polypifera, 

 apparently belonging to the genus Flustra. 



