i.^6 ROCHESTER ACADEMY OF SCIENCE. 



ancient depression. These central fillings terminate, however, to-day 

 at the Float Bridge to be replaced immediately by the open waters- 

 of the Bay. The protruding point of the silt mesa just east of the 

 bridge, in the mid line of the valley, is not unlike the protruding 

 marshy delta of the present river at its foot ; moreover, the excava- 

 tions for sand in its northern face reveal a sloping stratification con- 

 forming to the surface slope of the hill itself. These facts seem 

 to warrant the inference that this central filling of silts, or sub- 

 aqueous Iroquois delta of the Irondequoit river, did not extend north 

 of this point but ended here with the usual lobate front. This con- 

 clusion is the same as that derived just above from the study of the 

 lateral deposits on the north. The contrast between the filled and 

 unfilled areas is marked and the dividing line between them is abrupt. 



On their landward side, the margins of the silt plains blend, as 

 might be expected, with wave eroded benches in the till, and in 

 proportion as the beaches weaken toward the upper ( south ) end of 

 the embayment the limits of the plains grow more and more obscure, 

 partly through subsequent rainwash. Thus both criteria fail to- 

 gether, the Iroquois plains merging upward with those of Dawson 

 in the region between Fairport and Bushnell's Basin, south of our 

 maj). 



Iro-Gcncsce subaqueous delta. Similar in origin to the silt 

 plains, and consisting like them of the same detrital material, is the 

 great subaqueous Iroquois delta of the Genesee river, which con- 

 stitutes a large .share of the town of Irondequoit. The embouchure 

 of the (ienesee into Lake Iroquois lay about a mile to the west of 

 our map limits at the end of Norton Street. But its silts cover all 

 the district east and north as far as Lake Ontario and the Bay ; nor 

 are they, ai)i)arently, terminated by the latter, for, if our interi:)reta- 

 tion is correct, they also give shape to the remarkable angular plat- 

 form that narrows the bay 'on the east shore to less than half its 

 normal width from opposite the Newport House northward to near 

 'its mouth. While this great mesa, with its excessively steep bluffs 

 and its surmounting sand dunes, may have as its core a gravelly 

 esker-fan of early Iroquois age (see PI. V and compare Figure S, A) 

 yet the mass as a whole bears every evidence of being a now sun- 

 dered continuation of the Genesee silts, extending east as far as the- 



