The discovery of a crinoidal limestone near Northfield has necessitated a drastic revision of the stratigraphy in central Vermont east of the Green Mountain axis. The “Cambrian-Ordovician” boundary of earlier papers proves to be a horizon probably within the Ordovician system; it marks the base of the Northfield slate, which is tentatively correlated with the basal beds of the Tomifobia formation (Middle Ordovician?) in Quebec. Two new formations are defined for central Vermont—the Shaw Mountain formation, which contains the crinoidal limestone and lies immediately beneath the Northfield slate, and the Cram Hill formation, which is tentatively correlated with the upper part of the Magog slates of Quebec and thus is probably of Middle Ordovician age. The Northfield slate of Richardson is redefined with respect to the “Memphremagog slate” of earlier usage.
The earlier interpretation of Beekmantown age for the Northfield slate is held untenable because of its stratigraphic relations to the Shaw Mountain formation and because the supposed graptolites upon which the earlier correlation was made have now proved to be inorganic features.
The Irasburg conglomerate is an intraformational bed within the Waits River formation (Middle Ordovician or later?) and as such has no important regional significance.