58 IOWA ACADEMY OF SCIENCKS. 



although perfect in form and detail. Also they are usually very small, not exceed- 

 ing one-fourth the size of individuals of the satne species found in the associated 

 limestone layers. So White's statement that the conditions seemed to have been 

 unfavorable for the support of life, is true, at least in part. But I think the flint 

 beds contain much more evidence of life than he has given them credit for. There 

 is no doubt, however, that the flint beds of both the upper and lower divisions 

 mark eras of change in fossil forms. It would be strange if they did not, consider- 

 ing that the minimum thickness of either is fully ten feet. But they do not fur- 

 nish the most distinct lines of change inasmuch as some of those in the limestone 

 takes place in passing from one stratum to the one directly superimposed. 



In the upper division I have found direct evidence of disturbance and erosion 

 at one of these lines of change in fossil forms. Everywhere in the vicinity of Bur- 

 lington, where the upper division is found, there occurs, well down toward the 

 base, a stratum of heavy bedded white limestone. It is about six feet in thickness 

 and generally underlying it there is either a thin layer of blue clay or friable, yel- 

 low, sandy limerock. Immediately overlying it there is uniformly found a bed of 

 tough blue shale. I had frequently noticed the upper surface of the limestone 

 layer as exhibitmg a water-worn appearance and so was not surprised when I found 

 direct evidence of erosion. This discovery was made in the Cascade quarry in the 

 south part of the city limits of Burlington. In this quarry nearly the whole depth 

 of the Upper Burlington limestone is worked. The massive white layer spoken of 

 is here between 5 and 6 feet in thickness and furnishes a goodly part of the rock 

 taken out. 



The Cascade ravine is about half a mile in length and enters the Mississippi 

 river at right angles. These quarries are situated about a quarter of a mile back 

 from the river and on both sides of the ravine. It was in the one on the south 

 side that the discovery was made. This quarry is on both sides of a short, but 

 deep, lateral ravine, the bottom of which is several feet lower than the stratum of 

 white limestone. In working off the corner between the main and lateral ravines, 

 the white limestone layer was found to be much eroded. The erosion is lateral 

 rather than surface and occurs on the side toward the lateral ravine. The layer of 

 blue shale is deposited directly upon the eroded surface and conforms to all the 

 inequalities, some of which are quite abrupt. One bench of the eroded surface 

 amounts to fully two feet and yet the blue shale covers this without a break. The 

 blue shale itself is capped by well-bedded limestone. 



This is direct evidence of erosion in the early part of the deposition of the 

 upper Burlington limestone. An interesting fact is developed that the present 

 drainage system was probably fixed at that early date. The position of the eroded 

 surface of the white limestone layer, and the inclination of the directly superim- 

 posed beds all indicate that the lateral ravine had its beginning at a time at least 

 as early as that. Of course the principal ravine must have existed to furnish an 

 outlet. Along the banks of the principal ravine I have never seen the white lime- 

 stone layer exposed, but have no doubt it would exhibit the same erosion as found 

 along the lateral ravine. All the superimposed strata exhibit a decidert inclina- 

 tion towards the ravine, which would tend to confirm the theory. 



In conclusion, I would state that there seems to be no doubt whatever that the 

 deposition of the Burlington limestones was not continuous. I expect to see other 

 evidence of this fact discovered in the near future. 



