140 ROCHESTER ACADEMY OF SCIENCE. 



way" section, both on the west side of the present valley, and in the 

 summits of two tiny sugarloaves that margin one of these benches. 

 This terrace extends practically continuously from Atlantic Avenue 

 to beyond the Rich's Dugway, at the latter point being especially 

 well developed but containing some kame-like gravels that need ex- 

 planation. The second bench may be traced from the narrow portal 

 just above Zarges ]\Iill south to the Penfield road,- halfway to which 

 it rises above the 340 contour. This section retains an abandoned 

 channel of the stream on its inner margin. These two stretches 

 totaling over two miles in length and having a maximum width of 

 about forty rods apparently bespeak a distinct pause in the erosion 

 of the modern valley, such as one would most naturally link with 

 the base-level furnished by the Emmons waters. That this interpre- 

 tation may be too simple is suggested, however, by the absence of 

 any Emmons delta at the Float Bridge. See map D ( Plate VTl) in 

 which they have nevertheless been given this interpretation. 



Soundings in Irondcqitoit Bay. Some very significant facts 

 regarding the vuiderwater topography of the present bay are revealed 

 (see Figure j) by the U. S. Lake Survey large scale manuscript 

 map of 1875, of which Professor Fairchild possesses a blueprint. 

 1 he scale is 1 : 10,000, or 6.336 inches per mile. In the mile section 

 (D of Fig. 3) from the Float Bridge to Cilen Haven there are 90 

 soundings whose total average is not quite five feet, nearly a third 

 (28) of them being two feet or less while over two-thirds are imder 

 one fathom. The four deepest soundings recorded are respectively 

 11, 12, 14, and 16 feet, but this is exclusive of 14 soundings in the 

 delta channel of the Irondecptoit river ranging from six to eighteen 

 feet with an average of nearly twelve feet. It also excludes 15 

 others in an abandoned (or artificial?) channel close to the west 

 shore which average eight and a third feet. Apart from these the 

 area under one fathom is localized on the east side farthest from 

 the river, where the filling would naturally be deficient. 



Northward of a concave line the soundings deejien abruptly and 

 in the middle broad section ( C ) of the Bay there are, exclusive of 

 the marginal platform due to recent wavework, 125 soundings, 104' 

 of which fall between the limits 35 and 40 feet inclusive. Only 

 eight exceed 45 feet (maxinuim 54) and these are all in the extreme 



