The nepheline syenites lying east and northeast of Bancroft in the townships of Dungannon and Monteagle are the most extensive of the southeast Ontario nepheline syenite-syenite gneiss belt. They include the Bancroft nepheline syenite (the East Road occurrence of Currie, 1976a, p. 186), the Great Bend nepheline syenite (Bronson Landing area of Currie, op.cit., Fig. 79), the York River area (which is described separately - 031-00-095) and the Bresnahan Lake nepheline syenites (Monteagle corundum belt of Currie, op. cit., Fig. 79). There are several smaller occurrences in Carlow Township. The nepheline syenites lie within the Haliburton- Hastings Highland gneiss complex which is associated with marbles, amphibolite, syenite, granite and gabbro. The general strike is northeast-southwest, with steep dips to the south and east. The Bancroft nepheline syenites (Hewitt, 1961, p. 66) are gneissic and lie conformably with the associated syenitic gneisses and amphibolites; they are not found cutting any other rock type. Nepheline-bearing pegmatites cut and replace the nepheline syenite gneisses, and the gneisses are intruded and replaced by syenites and granites. The nepheline syenite gneisses consist essentially of nepheline, albite/oligoclase, biotite and magnetite, and accessory calcite, hastingsite, microcline, corundum, scapolite, muscovite, apatite, zircon, sodalite, cancrinite and tourmaline. Several pegmatites have become well-known collecting localities including the Lily Robertson Corundum Deposit from which corundum crystals up to 7 cm across have been obtained, and the Robbins Nepheline Pegmatite with nepheline and albite crystals up to 90 cm across. The Princess Sodalite Quarry produces deep blue sodalite which is widely represented in mineral collections. The Great Bend nepheline syenite area (Hewitt, 1961, p. 69) occurs in the hills on either side of the 'Great Bend' of the York River in north Dungannon Township. The rocks are foliated and gneissic nepheline syenites of nepheline (10-40%) and sodic plagioclase with accessory biotite, hornblende, magnetite, corundum, garnet, scapolite and apatite. Corundum is widespread in this area and worked from time-to-time. The rock type 'dungannonite', a corundum and nepheline-bearing diorite, was described from this vicinity. The Bresnahan Lake nepheline syenite varies from a narrow belt of nepheline-poor plagioclase gneisses near the southern boundary of Monteagle Township, to a zone of nepheline gneiss with nepheline varying from 20-50% and biotite and hornblende-rich facies (Hewitt, 1961, p. 81). Small scattered occurrences of nepheline syenite gneiss occur in two belts in Carlow Township, the more southerly group forming bands in marble, the more northerly one being engulfed and replaced by leucosyenite (Hewitt, 1961, p. 82). Although there is wide variation in detail, the nepheline syenites are essentially similar to those described above.
CURRIE, K.L. 1976a. The alkaline rocks of Canada. Bulletin, Geological Survey of Canada, 239: 1-228.
HEWITT, D.F. 1957. Haliburton-Bancroft area. Ontario Department of Mines, Map No. 1957b.
HEWITT, D.F. 1961. Nepheline syenite deposits of southern Ontario. Annual Report, Ontario Department of Mines, 69(8): 1-194.
HEWITT, D.F. and JAMES, W. 1956. Geology of Dungannon and Mayo Townships. Annual Report, Ontario Department of Mines, 64(8): 1-65.