l88 NATURAL HISTORY BULLETIN. 



bedding is real and not a mere deceptive appearance due to 

 cleavage of a mass of sediment that was originally built up 

 regularly and evenly on a horizontal base. As in other groups 

 of strata there are faunal and lithological variations when the 

 beds are compared one with another. These varying character- 

 istics do not intersect the beds in horizontal planes as they 

 would if the present bedding were due to cleavage of a mass 

 that had risen vertically at a uniform rate, but they follow the 

 individual layers in their oblique course from top to bottom of 

 the exposure. The facts confirm the statement that the beds 

 were deposited one by one in the position in which we now 

 find them. 



On the west side of the Mississippi south of Le Claire, 

 the usual oblique bedding is shown in remarkable perfec- 

 tion in the bank of the river, below the level of the plain 

 on which the lower part of the town is built. (Plate I., 

 Fig. 2.) The individual beds, as in all the characteristic 

 exposures of this formation, range from eight to twelve inches 

 in thickness. Above the level of the beds exposed in the 

 river bank there is another series of Le Claire beds that de- 

 part somewhat from the ordinary type. Near the base of 

 this second series the layers are thick and the rock is a light 

 gray, porous, soft, non-crystalline dolomite. These grade up 

 into thinner and compacter beds, but the lithological characters 

 are never quite the same as those of the more typical beds 

 at a lower level. The soft, porous, gray, colored beds con- 

 tain casts of Dinoholus conradi Hall. The same species ranges 

 up into the harder beds, but the characteristic fossils above 

 the soft, porous layers are casts of small individuals of Atrypa 

 retictdarh and a small smooth-surfaced Spirifer. The layers 

 become quite thin in the upper part of the Le Claire. They 

 show many anomalies of dip, but, so far as observed, they do 

 not as a rule stand at as high angles as do the harder and 

 more perfectl}' crystalline beds of the lower series. The 

 existence, however, of tumultuous seas is no less clearl}^ indi- 

 cated in this horizon than in the lower beds that pitch at 

 greater angles. In the town of Le Claire, on the west side of 



