The Santiago Peak Volcanics represent an island arc system which existed adjacent to the North American continent during Jurassic to Early (?) Cretaceous time. They are typically composed of mildly metamorphosed volcanic, volcaniclastic and sedimentary rocks that crop out in a N-S trending belt from San Diego to Riverside, California. The objectives of this thesis were to outline and define the geochemical character of the Santiago Peak Volcanics, deduce the tectonic environment of this ancient island-arc system from the stratigraphic, geochemical and petrographic relationships, and to constrain the polarity and orientation of the paleo- subduction zone. XRF analyses of major and selected trace elements for the basaltic and andesitic flows show that the ancient island arc can be divided into a calc-alkaline series and a tholeiitic series. The calc-alkaline rocks are exposed in the northern portion of the volcanic belt and the tholeiitic rocks are found in the south. Geochemical plots, such as FeO*/MgO-SiO2, FeO*/MgO-FeO*, AFM and Zr-Ti-Sr diagrams, were used to distinguish between the two suites of rocks. These trends are similar to those in both the Japanese and Aleutian arc systems. The Japanese arc exhibits an across-arc change from tholeiitic rocks nearest the trench to calc-alkaline rocks toward the continent whereas the Aleutian arc shows a trend from tholeiitic to calc-alkaline nature along strike of the arc. Whether the variation in chemistry is an across-arc or an along-arc trend, the conclusion is the same. The subduction zone was located parallel to the rocks of tholeiitic character and must have dipped to the northeast under the present-day exposures of the Santiago Peak Volcanics.