The 160 km2 Caledonian Fongen-Hyllingen complex is an extremely differentiated, layered, basic intrusion, synorogenically emplaced at 5–6 kb in the allochthonous Trondheim nappe complex, situated in the Trondheim region of Norway. A zone of gabbroic rocks without rythmic layering usually occurs along the margin and a supposed feeder to at least part of the complex is preserved. A wide variety of magmatic sedimentary structures are present in the c. 10,000 m thick sequence of rhythmically layered rocks which vary from olivine-picotite cumulates at the base to quartz-bearing ferrosyenites at the top. Mineral compositions, fractionation trends, and the composition of feeder rocks suggest a tholeiitic parent. Mineral compositions cover extreme ranges. Olivine varies from Fo86·2 to Fo0·2 with a hiatus between about Fo71 and Fo61. Plagioclase ranges from An79·5 to An1·5, albite coexisting with orthoclase microperthite in the final differentiates. Cumulus Ca-poor pyroxene (Wo2.4En66.8Fs30.8-Wo2·0En17·0Fs81·0) first shows sporadic inversion from pigeonite at the Fe-rich composition of Fs67 and the final Ca-poor pyroxenes are replaced by magmatic grunerite which reaches an Mg: Fe ratio of 12:88. Ca-rich pyroxenes (Wo44·7En43·8Fs11·5-Wo47·0En0Fs53·0) are highly calcic and have a slight Ca-minimum in the early stages, unrelated to the disappearance of Ca-poor pyroxene. Calcic amphibole, a constant intercumulus phase in most of the complex, becomes a cumulus phase in the later stages and varies from titanian-pargasite to ferro-edenite. Magnetite and ilmenite join the cumulate assemblage at Fo55 and ilmenite persists into the final quartz-bearing ferrosyenite where it shows replacement by sphene. Apatite, biotite, zircon, quartz, K-feldspar and allanite join the final extreme differentiates in the named sequence. The fractionation trend is, in many respects, transitional between those typical of the tholeiitic and calc-alkaline series, and is interpreted as reflecting crystallization under moderate, increasing PH2O. Cryptic layering shows several reversals to higher temperature assemblages with increasing stratigraphic height. Successive reversals are to irregular compositions and measured in terms of olivine composition, can be up to about 30 mole per cent Fo. The minimum stratigraphic thickness to include the entire fractionation range is reduced to about 2200 m after ‘removal’ of the compositional overlaps due to the reversals. Thus roughly three-quarters of the present cumulate stratigraphic sequence represents magma replenishment. A mechanism involving the mixing of fresh magma batches with the residual, differentiated magma from the previous influx, is envisaged. The periodic influx of fresh magma took place into a chamber which was probably closed to the exit of material.