Geology of southwestern New York. — Harris. 173 
Hence, rate of clip of lower surface of Marcellus group = 35 — ft. 
per mile, or, slightly more than that of the upper surface of the 
Genesee slate. 
It should be borne in mind that the " dips " given above are but 
general averages along a line from Black Rock to Jamestown, or 
about S. 18° 10' W. , and do not, therefore, represent the true 
rates of dip in the various formations referred to. This fact, 
however, has no particular bearing on the results just obtained ; 
but there is another consideration that must be taken into account 
here, and that is local dip. * 
In the Colburn well at Fredonia, the Corniferous limestone was 
found at a depth of 315 feet* below tide ; in the Jamestown well, 
as has already been remarked, the same formation was met with 
at a depth of 2775 feet below the surface, or 1450 feet below tide ; 
the distance between the two localities being 24 miles, it necessa- 
rily follows that the average rate of dip of the Corniferous lime- 
stone is here no less than 47 feet per mile. This is greater by 4 
feet per mile than the average dip given for the area, Black Bock 
— Jamestown — Erie, evidencing, accordingly, a local rise in this 
limestone formation in the vicinity of Fredonia. This would 
probably slightly modify the general inclinations of the various 
formations given on plate mi, and would tend to lessen the rate of 
dip from Black Rock to a point in the section not far from "B," 
and to cause an increase from the same point to Jamestown. 
(B). Thickness. — With the above-given data, it now becomes 
possible to determine the total thickness of the Marcellus, Hamil- 
ton, and Genesee, as well as that of the Portage, with greater 
precision than has been done heretofore. Making a slight allow- 
ance for the influence of the Fredonia rise, the dip from"D" 
(the point at which the upper surface of the Corniferous limestone 
descends to the level of lake Erie) to " C " (where the base of the 
Portage dips beneath the lake) is about 32 feet per mile. Since 
the distance is 15 miles, the total thickness of the Marcellus, 
Hamilton, and Genesee, is 15x32 ft. or 480 ft. 
Again, the dip along line "b"-"b," from "C" to "B ' is about 
31^ ft. per mile. The distance between these two points is 16 
miles; "B, " moreover, is about 577t feet above "C. " The thick- 
*Ibidem. 
fDetermined by the writer while under the auspices el' the Palaeozoic 
Division of the U. S. Geological Survey. 
