PEATT ON SHELL-BEDS. 157 



back from the present edge of the bank, though it is impossible to say 

 just how much of it may have been carried away by the encroachments 

 of the river. It should also be observed that the river bank in front of 

 these beds is usually neither muddy, nor abrupt, or overhanging, but is 

 rocky and sloping toward the water. 



PARTICULARS. 



A bed of shells, about two-thirds of a meter in thickness, at the foot 

 of Mississippi avenue, at East Davenport, now removed or rendered in- 

 accessible by the late improvements there, was some one and one-half 

 meters above high water, but this is the only instance of the kind ob- 

 served, and it is at a point in the rapids where the river bed would 

 probably wear down in a century or two, considerably. 



CT v;r:^- 



FiG. IG. Section of Shell Bed at the head of Rock Island. 0, Bed of limestone rock. 6, Shell 

 bed. c, c, General surface, d. Shell heap superposed upon the general surface. 



At the head of the Island (Rock Island), where are found the most ex- 

 tensive accumulations in this region, we find, at several places along the 

 edge of the bank, an additional deposit of shells heaped up above the 

 general shell bed, which is itself very heavy at the same point. One of 

 these heaps is still over two meters high above the regular continuous 

 bed, its contents being similar in every respect. These superficial depos- 

 its ^ope off or thin out inland rather rapidly, extending back but a short 

 distance from the present edge of the bank, and the face of the bank is 

 vertical here down a meter or two to the solid limestone rock, being 

 broken down and washed away by the high waters of every season, thus 

 always presenting a good vertical section of the strata. 



Usually, and notably in .the case of the shell deposits along the river a 

 short distance above Moline, these deposits are found at intervals, and 

 situated on the up-stream side of the projecting points of land, where, 

 by the sinuosities of the shore, the curves are exposed more directly 

 to the action of the current when the water is high. 



CONTENTS. 



These beds are composed of shells of the same species now living here. 

 Xo species now extinct have ever been reported as found among them? 

 and they are of the usual size, and as far as has been determined, about 

 in the same proportions of the several species as are now found close by, 

 with this exception, however, that the very small, as well as the very 

 thin species are seldom recognized, as, for example, the Unto parvus, 



[Proc. D. A. N. S. Vol. II.] 22 [Apbil, 187S.] 



