K-Ar Measurements on Mineral Pairs - R. K. Wanless and J. A. Lowdon The ages of twenty-six biotite -muscovite pairs were compared. Of these, sixteen pairs agree; the remaining ten show differences ranging from 11.5 to 38 per cent. Third Report on Structural Provinces, Orogenies, and Time CIassification of Rock s of the Canadian Precambrian Shield C. H. Stockwell The reconnaissance study of the Canadian Shield was continued during 1962. and the additional potassium-argon dates obtained substantiate the divisions proposed in two previous reports (Stockwell, 1961, 1963-see "References"). The Grenville front has been extended eastward to the Atlantic Ocean. A new structural province, called the "Nain province", has been added. It lies mainly in Labrador north of the Grenville province. The Sickle Group has been place<il in the Archaean, the Nonacho in the Lower Proterozoic, and the Athabasca in the Middle Proterozoic. Additional evidence indicates that certain parts of the Churchill province consist of Archaean rocks that were first involved in the Kenoran orogeny and were then moderately affected during the Hudsonian. As found along the Grenville front and elsewhere, new evidence is presented in support of the hypothesis that argon lost from an older biotite may be added to a younger biotite. Ages of Regional Metamorphism of the Aldridge Formation near Kimberley, B. C. {Preliminary Report}-G. B. Leech Potassium-argon ages of muscovite and biotite from interbedded quartzite and argillite, showing typical regional metamorphism (greenschist facies). of the Aldridge Formation of the Purcell (Precambrian) sedimentary sequence were determined. These were found to span the Palaeozoic era. The ages are regarded as hybrids reflecting chiefly polymetamorphism, with relatively minor contributions by detrital mica. They are interpreted to mean that Precambrian regional metamorphism was relatively more important and Mesozoic regional metamorphism correspondingly Iess important than has commonly been supposed.