The calibrated Pathfinder alpha proton X-ray spectrometer (APXS) instrument is capable of measuring concentrations of all major and minor rock-forming elements ranging from carbon through zirconium in atomic number. Therefore it is capable of constraining the petrology of the measured samples. In particular, the Pathfinder data are the first in situ measurements of Martian rocks and may be compared with previous remote-sensing measurements as well as with the compositions of SNC meteorites. Pathfinder soil and rock sample abundances from the alpha, proton, and X-ray modes have been quantified. The abundances suggest that (1) the rocks are covered with various amounts of soil; (2) the soil-free rocks, on a volatile-free basis, have some element ratios similar to Mars meteorites, yet have different bulk chemistry indicative of more evolved rocks with higher silica abundances; (3) the carbon and nitrogen contents are below detection limits; and (4) the alpha mode oxygen reveals excess amounts of oxygen in some samples, which is indicative of sample-bound water (contained within minerals or glasses in samples).