84 LIFE OF THE PLEISTOCENE 



clearly indicating the route by which this race reached the Niagara Falls 

 locality. 



The presence of Amnicola letsoni in these deposits is also of interest. This 

 little species was first recorded from the gravel deposits on Goat Island, Niagara 

 Falls, 46 in strata laid down by the waters of Lake Algonquin. The Toleston 

 stratum is earlier, contemporaneous in fact with early Lake Algonquin, and 

 it is probable that the species reached the Goat Island locality by way of the 

 Chicago outlet. Letsoni was at first thot to be extinct, but has since been 

 found "recent, tho dead, washed up on the shore of Lake Erie 47 in Monroe 

 County, Michigan. I have also a single dead specimen from the drift of the 

 Raisin River at Dundee, Michigan." Letsoni, like Pleurobema coccineum 

 magnalacustris, may be a species approaching extinction. It was apparently 

 abundant during the Toleston stage of Lake Chicago and persisted at Niagara 

 Falls until a comparatively recent date. 



It is a matter of great interest that these two distinctive species originally 

 discovered (in a fossil condition) at Niagara Falls, should also be found in these 

 earlier deposits of glacial Lake Chicago, clearly showing that the migration 

 into the englaciated territory was by way of the Chicago outlet. The Fort 

 Wayne outlet of Lake Maumee may also have been a factor in repopulating 

 the Lake Erie region, as suggested by Walker, 48 but it must be remembered 

 that this outlet persisted for but a comparatively short period (until the forma- 

 tion of Lake Whittlesey) while the Chicago outlet persisted until almost the 

 close of the lake stages (until late in the Nipissing stage). The evidence 

 afforded by the material under discussion favors a migration for the most part 

 by way of the Chicago outlet. 



A comparison of the molluscan faunas of the Niagara Falls and the Chicago 

 outlet regions shows that of the 33 species found in the Toleston deposits, 

 only 10 are found in the Goat Island gravels, 21 of the latter being different 

 species. The gastropods are more nearly related to the subsequent stage (of 

 Lake Algonquin). 



No insect remains have been seen from these deposits, but borers have 

 been reported from the fossil oaks at Evanston. 49 



ii. Toleston Deposits in or near the Outlet 



In the Sag outlet at a point where the Calumet-Sag channel crosses 92nd 

 Avenue, the fauna listed below lies just above the boulder pavement of the- till. 

 In this deposit the absence of a fauna referable to the previous low water stage 

 is noteworthy. If such strata were laid down they were destroyed by the 



« Letson, Bull. Buffalo Soc. Nat. Hist., p. 241, fig. 165. 



47 Bryant Walker in letter. 



48 Nautilus, XXVII, p. 58 and ante. 



49 Higley and Raddin, page XV. 



