PKATT ON SHELL-BEDS. 161 



indigenous plants disappear from a region which is pastured and trodden 

 bj' domestic animals ; as the beaver, the deer and the buffalo, the forest 

 and the cataract, languish and vanish at the approach of the white man, 

 one of whose chief characteristics is vandalism, so it is with the seem- 

 ingly not very sensitive mollusk. The disturbance of the balance of the 

 conditions in which it flourislies, dwarfs, deforms and destroys it. We 

 must now visit unsettled districts to secure numerous good specimens of 

 aquatic shells. 



Again, it is well known that the river bed is constantly wearing deeper, 

 that the great Mississippi is gradually letting itself down into the earth, 

 and especially is this the case on the rapids, and the labors of man in 

 making a channel for the improvement of navigation hasten this depres- 

 sion. So every few years new " high water" and '' low water" marks must 

 be adopted, and always lower than of old, and the waters will never more 

 reach altitudes which formerly were frequently attained. These two 

 facts— the decrease of molluscan life and the lowering of the river-bed- 

 will doubtless fully account for the cessation of the formation of shell- 

 beds here, and, of course, no very long period would be requisite to cover 

 them with a light layer of soil. 



SHELL HEAPS. 



In regard to the shell heaps above the general surface at the head of 

 the island, I believe they may be rationally accounted for in the same 

 way. After a shell bed of such extent as we find there was formed, three 

 or four feet in thickness in some parts, the constantly encroaching 

 waters, breaking down the bank containing it, and washing away the 

 light soil, deposited, as it still does, great quantities of these old shells 

 upon the denuded rocky slope, which is there but a meter or two below 

 the sod. Then, upon the rush of a mass of strong and thick ice, great 

 quantities of these must be carried up and superposed, forming great 

 heaps or a ridge along that shore. 



I have already remarked that beside the largest heap there still remain 

 portions of several others, or perhaps of a continuous ridge along the 

 portion of the bank which is nearly at right angles to the general direc- 

 tion of the stream. Much of this has already evidently been washed 

 down, and ere long, unless the authorities protect the bank or destroy it 

 themselves, the river will complete its work of removing its own shell 

 defences along that coast. 



MOUND AT THE LOWER END OF THE ISLAND. 



In this connection we ought not to overlook a bed of shells formerly 

 existing near the foot of Rock Island, at the bottom of which the skull 

 which we have designated the " shell-bed skull," was found by Mr. Tif- 

 fany in the fall of 1871. He described it as follows* : *■' . . . at a 

 depth of three feet from the top is a deposit of shells, mostly Unios, but 

 including Melantho subsolida, and two or more species of Helix. 

 This shell bed at this exposure varies from six to sixteen inches in 



*Proc D. A. N. S., Vol. 1, p. 42, Plate XXVI, fig 1 ; Plate XXI, fig. 26. 



