The Francon Quarry, St-Michel district, Montreal is in the lower and middle parts of the Trenton limestone (Ordovician), and contains a silico-carbonatite sill associated with which is a very extensive suite of rare minerals. The sill is 2 m thick and extends over a known area of about 1.1x0.45 km. There are chilled contacts 4-5 cm thick and the rock is highly vesicular with individual cavities up to several cm in diameter. The vesicles are most abundant in the central and upper zones and are generally elongated parallel to the contacts. The chilled selvages are essentially microcrystalline intergrowths of dawsonite and feldspar with dawsonite pseudomorphs, probably after analcime. The central portion of the sill contains similar dawsonite pseudomorphs in a matrix of K-feldspar laths, dawsonite and other carbonates. Locally dolomite and siderite may be the principal carbonates; calcite is confined to patches and vesicles. In one area of the sill aegirine-augite needles and phenocrysts of analcime occur, the latter probably precursors of those pseudomorphed by dawsonite. Another lower sill about 1.3 m thick and apparently similar has been encountered in quarrying operations (Jambor et al., 1976, p. 361). Over 60 mineral species have been identified from the sill, principally in vesicles, and four species are unique to this locality (Sabina, 1979).