The study of petrographic, chemical, and physical properties of 183 unweathered drill cores from oil wells which have penetrated the crystalline basement underlying the Western Canadian sedimentary basin supplements previous data from this area. For the investigated area of approximately 400 000 miles2 (1 000 000 km2), lying between the latitudes 49° and 62 °N, the available data suggest the following: (i) The existence of a broad, east northeast–west southwest striking mobile zone between Great Slave Lake and southern Alberta and central Saskatchewan (the Athabasca mobile zone) containing a high proportion of largely pre-Hudsonian crystalline rocks, which underwent strong postcrystalline deformation and metasomatic recrystallization (poly-metamorphism) during the Hudsonian orogeny, 1700–1900 m.y. ago. (ii) A close relationship between the degree of postcrystalline (syn-crystalline) deformation and the intensity of metasomatic recrystallization. (iii) The possibility of large-scale regional K-metasomatism and granitization in deformed crystalline zones leading to microclinization of acid and intermediate rocks,(iv) A high proportion of reworked older rocks and the probable non-eugeosynclinal character of the metamorphosed supracrustal rocks in the mobile zone, (v) The very strong influence of postcrystalline deformation and recrystallization upon isotopic age determinations, (vi) The wide coexistence of newly formed microcline with low-grade minerals like chlorite and epidote, challenging the validity of the use of microcline as a metamorphic facies index mineral.