l86 NATURAL HISTORY BULLETIN. 



is undulating and irregular. If subsidence, followed by quiet 

 deposition, should take place in any such region, the new 

 strata would conform to the irregularities of the bed; they 

 would stand at a high angle on the flanks of the ridges; 

 they would bend down into the intervening troughs, and in 

 general they would exhibit a number of sweeping undulations 

 that would be simply the visible expression of the irregularity 

 of the surface upon which the new beds were laid down. The 

 Le Claire stage was a time of forming shoals and bars, with 

 intervening channels, by the heaping up of obliquely bedded 

 masses of calcareous material. The Anamosa stage that fol- 

 lowed represents a time of relatively quiet deposition when 

 evenly laminated beds were laid down, so as to conform to the 

 very irregular surface which the previous conditions had de- 

 veloped. 



Professor Hall accurately describes some of the variations in 

 the inclination and direction of dip in the Le Claire lime-stone 

 as seen at Le Claire,^ but he assumes that the inclination of 

 the beds is due to folding and uplift subsequent to their deposi- 

 tion. On this assumption the Le Claire limestone would have 

 a thickness of more than six hundred feet, whereas the 

 maximum thickness does not exceed eighty feet, and the 

 average over the whole area is very much less. Professor A. 

 H. Worthen^ studied this limestone at Port Byron, Illinois, 

 and Le Claire, Iowa, and describes it as "presenting no regu- 

 lar lines of bedding or stratification, but showing lines of false 

 bedding or cleavage." In White's report on the geology of 

 lowa^ the oblique bedding seems to have been taken as evi- 

 dence that a line of disturbance crossed the Mississippi river 

 at Le Claire with a direction nearly parallel to the Wapsipin- 

 con valley. This apparent disturbance was last recognized 

 about three miles west of Anamosa, The angle of dip it is 

 said has reached in some places twenty-eight degrees with 



^ Rept. on the Geo. Surv. of the State of Iowa, Hall and Whitney. Vol. 

 I.,Part I., pp. 73, 74. 185S. 



* Geo. Surv. of III. Vol. I., p. 130. 1866. 



* Report on Geol. Surv. of the State of Iowa. Charles A. White, Vol. I., 

 p. 133. 1870. 



