A regionally metamorphosed complex of agpaitic nepheline-bearing rocks is poorly exposed along the eastern shore and hills beside Sheffield Lake. The dominant rocks are mafic gneisses composed of nepheline, sodic hedenbergite to aegirine-augite, arfvedsonite and katophorite, K-feldspar and eudialyte, which are intercalated with more leucocratic rocks in which nepheline, albite and K- feldspar predominate. Calcite and fluorite are ubiquitous accessories and locally assume major proportions. These rocks contain pods and lenses of pegmatite composed of exceptionally coarse eudialyte, feldspar, nepheline, agrellite, for which this is the type locality (Gittins et al., 1976), a wide variety of woehlerite group minerals, vlasovite (Gittins and Gasparrini, 1973), britholite, gittinsite ( also the type area, Ansell et al., 1980), and others. Alkali amphibolites also occur although their field relationships are not known, while non-agpaitic nepheline syenite occurs on islands at the south end of Sheffield Lake, but its relationship to the agpaitic rocks is also obscure. A chemical analysis of a eudialyte-rich rock together with REE data are given by Fryer and Edgar (1977).
Lyall (1959) has mapped a further mass of syenite on an island in Lac Tortue and along the eastern shore of that lake, but the degree of alkalinity of these rocks is not known.