174 The American Geologist. March, isol 
ness of the Portage group at "B," is therefore 31£ ft. x 16 -f- 
577. = 1081 ft., or, approximately, 1080 feet. 
Hence, contrasting these figures with those derived from the 
Jamestown well section we have : 
Erie, or Northern 
Chautauqua county. Jamestown well. 
(1) 1080 feet Portage 1315 feet. 
( Genesee 
(2) 480 feet - Hamilton \ 510 feet 
( Marcellus 
Professor Hall has estimated the * l aggregate thickness" of the 
Portage group in northern Chautauqua county as " little less than 
1400 feet."* This result he evidently obtained by supposing that 
the dip along the shore of lake Erie from near the mouth of 
Eighteen Mile creek to Chautauqua creek is 25 feet per mile. 
Calculations based upon the depths already given at which the 
Corniferous limestone was struck in the Fredonia and Erie wells, 
together with its altitude at Black Rock, show conclusively that 
the true rate is but 1G feet per mile. Making this change, the 
"aggregate thickness" becomes 1040 feet, — a fairly accurate 
result. 
The Genesee slate is said by Hall to be 23 feet and 7 inches 
thickf as it crops out on the shore of lake Erie, near the mouth 
of Eighteen Mile creek. Add to this, 300 feet, for the combined 
thicknesses of the Hamilton and Marcellus as given by professor 
Williams in "The Petroleum Age, "J and the total for the three 
groups becomes 323 feet and 7 inches; or, about 150 feet less 
than the estimate given above (see "2"). || 
III. Addenda. 
POPE HOLLOW SECTION. 
In order to further the correlation of the conglomerate and 
inter-conglomerate beds in southern Chautauqua county, N. Y. , 
with those of Warren county, Pa. , the writer takes this opportu- 
nity to make known the following details regarding a section 
*Geology of New York. Part IV. 1843, pp. 238-239. 
•f-Geol. N. Y. Pt. iv, 1843, p. 221. 
JOp. cit. vol. vi, June, 1887, p. 1642. 
|| According to the MS. notes of Mr. Prosser, the aggregate thickness 
of these groups in the East Aurora well section, amounts to 520 feet at 
least. This is not surprising, since according to Hall, both Genesee 
and Hamilton thicken rapidly going eastward from lake Erie. 
