160 DAVENPORT ACADEMY OF NATURAL SCIENCES. 



ently no signs of any cnlinary operations having been carried on. As to the 

 superficial heaps at the head of Rock Island, already alluded to, a differ- 

 ent theory might be suggested, but we will consider farther on whether 

 that is necessary or tenable. I conclude, then, that the shell beds of our 

 vicinity are natural deposits, but the question is, 



HOW FOR3IED? 



It has been mentioned that in most, if not all cases our shell- 

 beds are situated where the bank is in a position more or less opposed to 

 the current of the river, and also where it is sloping to the river and 

 rocky. In the breaking up of the ice, the floating cakes are often pushed 

 up these slopes, and piled upon the bank, carrying with them, of course, 

 whatever loose material may be lying in the way. 



At the head of the island, the whole extent, where are found the 

 largest shell deposits in this region, stands square across the path of the 

 current of floating ice, and is a very long, rocky slope. Doubtless the 

 shells were much more abundant in this part of the country formerly 

 than now, as they are growing scarcer CA'ery year, and in the neighbor- 

 hood of Moline they are still more plentiful than in any other locality I 

 have seen. At the time when the coffer dam was built there some eight 

 years since, and a considerable extent of the river bed exposed, many 

 wagon loads of shells could have readily been shoveled up immediately 

 opposite that town, and no doubt there must have been immense quanti- 

 ties there in former times. 



Under these circumstances it must follow, as a matter of course, that 

 quantities of shells would be pushed up the bank by the moving ice at its 

 breaking up in the spring, and would not unfreqnently be carried to the 

 top of the bank, and must be left there by the melting ice. This opera- 

 tion, repeated from year to year, or. at least, many times, would neces- 

 sarily result in beds of shells of greater or less thickness, according to 

 the circumstances, such as we find them. Of course, it would be only at 

 longer intervals that the debris in the river at and near its edge, would 

 be carried up to the fullest extent, when the river was high, the ice strong 

 and the break-up sudden. 



I have stated that at prominent or projecting points of the river bank, 

 shell-beds are usually found at the up-stream side, provided the bank is 

 firm and sloping. In short, it appears that the form and position of the 

 river bank relatively to the motion of the floating ice, is the key to the 

 whole matter of these wide-spread shell deposits. Where the ice would 

 run up most readi'y and certainly, we find most shells at the top. 



With this principle to guide us, we can, by observing the contour of 

 the river banks from the distance of a mile or two, point out the places, 

 with almost unerring certainty, where the accumulations of shells will 

 be found. It may be asked why this operation should cease. I do not 

 know that it has ceased everywhere, but, as before remarked, shell life in 

 the river (as well as on the land), is constantly diminished by the progress 

 of civilization. A city soon almost utterly destroys it in its own vicinity, 

 and for some distance down the stream. As the prairie grass and many 



