THE SWEETLAND CREEK BEDS 75 



high. In such places the lamination appears indistinct, and the 

 shales are oxidized and leached. After the deposition of the 

 Sweetland Creek beds they were raised and subjected to erosion 

 and sculpturing, which no doubt removed the greater part of 

 them. Only remnants are left. Then, again, the land was sub- 

 merged, and the topography just sculptured was covered over 

 by the variable shore deposits of the Coal Measures. 



It has also been shown that there is an unconformity with 

 the underlying Cedar Valley limestone. But this unconformity 

 indicates altogether different conditions. The upper formation 

 is, in this case, not a shore deposit. The basal member of the 

 Sweetland Creek beds is a thin layer of argillaceous dolomite 

 containing no littoral detritus, and it is unusally uniformly devel- 

 oped, though only two or three inches thick. It is a sediment 

 made in the sea at such a slow rate that the teeth of dying 

 fishes accumulated rapidly enough to make at one place as much 

 as one fourth of its bulk. This layer follows the small ine- 

 qualities in the surface of the lower rock like a mantle. None 

 of these are very high or deep. On a distance of a few rods 

 none appear to exceed two feet in vertical extent. Near the 

 Geneva school the basal tooth-bearing layer appears to occupy 

 a place eieht feet lower than the highest ledge in an abandoned 

 quarry close by. The surface of the limestone is, however, 

 plainly eroded and apparently to some extent oxidized. In the 

 east bank of Sweetland Creek the highest ledges of the lime- 

 stone run out to the south, and the overlying formation comes 

 down over their beveled edges. An unconformity of this kind 

 is most likely caused by subaqueous erosion, due to marine cur- 

 rents, followed by renewed sedimentation in the same sea. Such 

 events may have been accompanied by an approach of the shore 

 line. This is, perhaps, indicated by the presence of faint traces 

 of vegetation in the later member in this case. But at the very 

 beginning of the second accumulation the shore was not near 

 enough to leave a trace of anything coarser than clay. Even 

 this was scarce at first, when calcareous sediments predominated. 

 The persistence of each thin layer over distances of several 



