A high-resolution d¹³C_carb stratigraphic study recently performed on strata of the Clinton and Lockport Groups in the Niagara River Power Project S-1 core (Lewiston, New York) resulted in the first-ever recognition in those strata of the most widely known and best-documented positive d¹³C excursion of the Silurian, the Early Wenlock Ireviken Excursion.  A sharp increase in the d¹³C_carb values (+2.96‰ to +5.55‰) near the Rockway Dolomite/Irondequoit Limestone contact marks the base of the Ireviken Excursion in the upper part of the Clinton Group, for it is known to have begun shortly following the extinction of Pterospathodus amorphognathoides amorphognathoides (just above the Llandovery/Wenlock boundary), which has its highest representation in western New York in the uppermost Rockway or basal Irondequoit.  Apart from a slight decease in values in the lower Rochester Shale, d¹³C_carb values are consistently around +5.00‰ in the Irondequoit Limestone, Rochester Shale, and DeCew and Gasport Dolomites.  d¹³C_carb values gradually return to baseline (approximately +2.50‰) in the lower part of the Goat Island Dolomite (Niagara Falls Member), marking the end of the Ireviken Excursion.  The Ireviken Excursion is known to end within the middle Sheinwoodian Kockelella walliseri Zonal Group, a zone based on the range of the nominative species.  That species has its highest representation in the lower part of the Ancaster Member of the Goat Island in western New York, just slightly above the level at which d¹³C_carb values return to baseline.  Since the Ireviken Excursion began in the earliest Sheinwoodian and ended well before the end of the Sheinwoodian, all of the strata containing a record of that excursion in western New York must also be Sheinwoodian (Early Wenlock) in age.  A steady decline in d¹³C_carb values of nearly 5.00‰ from baseline is recorded in the Ancaster and Vinemount Members of the Goat Island through the lower Eramosa Dolomite.  A steady decline in d¹³C_carb values continuing well after the Ireviken Excursion has never previously been documented, and remains enigmatic until further study.