An investigation of the petrography, mineralogy and chemistry of primitive basalts found in a E-W transect of southern Washington and northern Oregon provides constraints concerning the thermal structure and composition of the mantle wedge beneath the Cascadia subduction zone. The Cascades represents a warm, slowly subducting end-member of subduction zone types. Chemical and petrographical characteristics of basalts from a transect across southern Washington representing at least six variants of primitive melts can be recognized. Estimates of magma temperature and depth based on mineral and whole-rock chemistry provide estimations on the Cascadia subduction zone's thermal structure. Mineral chemistry and whole-rock trace element chemistry are further used to interpret the mantle chemistry and the processes involved in producing subduction zone basalts. The results can be explained by multiple sources found within the subduction zone mantle, melting at various depths and temperatures.