The Crowsnest Formation consists of a sequence of trachytic analcime-bearing agglomerates, tuffs, rare flows, and sedimentary rocks derived from them, lying conformably on mudstones and sandstones of the Lower Cretaceous Blairmore Group, and overlain disconformably by Upper Cretaceous shales of the Alberta Group. The sequence is up to 500 m thick. Trachytic dykes and plugs also occur, particularly in the southern part of the area. The lower part of the formation is predominantly trachytic tuff with abundant sanidine phenocrysts, garnet and pyroxene; the upper part is agglomeratic with sanidine, analcime, garnet and rock fragments. Sanidine phenocrysts, up to 5 cm long, are rhythmically zoned, range from Or58-91, and are less potassic than the matrix feldspar; there is a little albite. Analcime phenocrysts are widespread and form large (up to 3 cm) trapezohedral, red, green, orange or brown crystals. They are zoned and chemically stoichiometric analcime with only minor K, Ca and Fe. Analcime also occurs in the matrix. Aegirine-augite forms phenocrysts up to 3 mm, which are sometimes zoned, and strongly zoned melanite forms phenocrysts up to 7.5 mm in diameter. Accessories include andesine-oligoclase, nosean, sphene, magnetite, pyrite, calcite and apatite. Depending on the proportions of the minerals present, the rocks vary from trachyte, through analcime phonolite to blairmorite, the last term having been first given to the analcime-phyric rocks of this area. A primary or replacement origin for the euhedral analcime crystals has been a matter of some debate (Pearce, 1970; Ferguson and Edgar, 1978, and references cited therein). Dykes and intrusive bodies up to 3 km in diameter consist of aegirine and aegirine- augite trachytes and syenites. A study of the mineral chemistry of zoned melanites has been made by Dingwell and Brearley (1985).