Emplaced in Cambro-Ordovician sedimentary rocks, the Shefford Complex has a hornfelsed aureole which is poorly exposed. Around a dioritic and a gabbroic core are concentric intrusions of more salic rocks which show inward dipping foliation and some banding, possibly about two centres. The core of the complex is formed of diorites and gabbros which have gradational relationships and include poikilitic gabbros and diorites, monzodiorite and quartz monzonite. Diorites are more abundant than gabbros and both consist of andesine or labradorite, kaersutite and minor augite and olivine, and with increase in alkali feldspar and biotite they grade into monzonites. Small areas of monzodiorite and nepheline diorite outcrop within pulaskite and are cut by pulaskite dykes. They consist of andesine zoned to oligoclase, kaersutite and ferrohastingsite, and accessory nepheline, cancrinite, sodalite, sphene, up to 10% in some rocks, apatite and opaques. Pulaskites and micropulaskites grade into nepheline-rich varieties of foyaite and laurdalite, but the average rock type contains 90% perthite and less than 5% nepheline, with accessory ferrohastingsite, augite/aegirine-augite, apatite, sphene, zircon and opaques. Quartz-bearing syenites (nordmarkite) develop a larvikitic facies at the contacts with diorite and gabbro, which consists of alkali feldspar, less than 2% quartz and aegirine-augite, ferrohastingsite, altered fayalite and biotite. The central part of the west substructure contains a heterogeneous assemblage of breccias and porphyries including fragments of latite and trachyte in a matrix of plagioclase porphyry, chemically identical to the gabbros and diorites. A varied suite of dyke rocks includes types similar to the principal rock types of the complex. Rock analyses, including REE and Sr isotopic data, as well as pyroxene and olivine data are available in Eby (1985a) and Pb isotopes in Eby (1985b). Frisch (1970, 1972) has given a detailed account of amphiboles, pyroxenes and iron knebelite from Shefford.