The alpha proton x-ray spectrometer (APXS) on board the rover of the Mars Pathfinder mission measured the chemical composition of six soils and five rocks at the Ares Vallis landing site, The soil analyses show similarity to those determined by the Viking missions. The analyzed rocks were partially covered by dust but otherwise compositionally similar to each other. They are unexpectedly high in silica and potassium, but low in magnesium compared to martian soils and martian meteorites, The analyzed rocks are similar in composition to terrestrial andesites and close to the mean composition of Earth’s crust. Addition of a mafic component and reaction products of volcanic gases to the local rock material is necessary to explain the soil composition.