Chemical analyses of cores from the Cajon Pass Deep Drillhole confirm vertical lithologic diversity of the basement rocks. They are metaluminous and calcalkaline; weak major element correlations with wide‐ranging SiO2 suggest they are not a simple cogenetic suite. Quartz contents correlate with the thermal gradient profile, supporting conduction as the principal thermal transport mechanism. Abundances of heat producing elements are variable and somewhat lower than average crustal values. Cross‐calibration with other measures of Th, U and K are underway. Ages of surface granitoids and core 17 are late Cretaceous; other plutonic rocks appear older but late Mesozoic. Initial lead isotopic signatures of the plutonic rocks change dramatically with depth in the hole.