Information provided by previous workers shows a considerable diversification of lava types in the early Tertiary deposits of the Kettle River region of British Columbia. The aim of the present study is to establish, on the basis of mineralogical and chemical evidence, whether or not these lavas form a single magma series. A suitable spectrochemical technique was adapted for the determination of Sr, Ba, CaO, MgO, and FeO in rock samples and mineral extracts. A method based on the work of Shaw and Filby, using lanthanum as an internal standard for determination of Sr, Ba, and Ca, is extended to include the determination of MgO and FeO. Extension of the available chemical data was possible using a fused glass bead technique for refractive index determination of the lavas. In view of good correlation between refractive index and the most refractory constituents of the lavas CaO, MgO, and FeO, the refractive index serves as a good magmatic differentiation index. Examination of the refractive indices of three suites of samples, taken from widely separated points within the Kettle River region, shows that the lavas range from semi-basic to acid composition with intermediate types, andesites and trachyandesites, predominating. There is a marked paucity of basaltic rocks. The stratigraphic distribution of the lavas reveals no consistant trends toward either acid or basic compositions; neither is there any relation between stratigraphic position and lava types within the region concerned. In spite of close age and spatial association of the lavas, petrological evidence points to a three-fold division of these rocks (termed 'A', 'B' , and 'C' series). 'A' series is similar to the silica-enrichment 'calc-alkali' trend showing regular mineral and chemical progression from andesites to latite and rhyolite. 'B' series is characterized by a somewhat less well defined group of two-feldspar lavas, trachyandesites, and trachytes. An unusual undersaturated lava, termed 'analcite rhomb-porphyry' or 'shackanite' is thought to be genetically related to 'B' series but probably belongs to a poorly developed semi-basic alkali trend, 'C' series. A mechanism involving crystal fractionation of hornblende, or alternately, augite and plagioclase, is envisaged as the probable mode of origin for many of the rocks of 'A' series. On the other hand, Daly's view that the shackanite lavas were formed as a result of limestone assimilation is supported by field evidence as well as mineral and chemical data. The rocks of 'B' series, the trachyandesites and trachytes, are intermediate in composition to 'A' and 'C' series and have probably formed by process of crystal fractionation in consequence of normal cooling conditions and limestone assimilation. / Thesis / Master of Science (MSc)