  We present Far Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope
Imaging Spectrograph (STIS) observations of interstellar ultraviolet absorption
lines in the Galactic high-velocity cloud Complex C and the Intermediate
Velocity Arch (IV Arch) in direction of the quasar PG 1259+593 (l=120,b=+58
deg). Absorption lines from CII, NI, NII, OI, AlII, SiII, PII, SII, ArI, FeII,
and FeIII are used to study the atomic abundances in these two halo clouds at
V_LSR=-130 km/s (Complex C) and V_LSR=-55 km/s (IV Arch). The OI/HI ratio
provides the best measure of the overall metallicity in the diffuse
interstellar medium, because ionization effects do not alter the ratio, and
oxygen is at most only lightly depleted from the gas into dust grains. For
Complex C, we find an oxygen abundance of 0.093 (+0.125, -0.047) solar,
consistent with the idea that Complex C represents the infall of low
metallicity gas onto the Milky Way. In contrast, the oxygen abundance in the IV
Arch is 0.98 (+1.21,-0.46) solar, which indicates a Galactic origin. We report
the detection of an intermediate- velocity absorption component at +60 km/s
that is not seen in HI 21cm emission. The clouds along the PG 1259+593 sight
line have a variety of properties, proving that multiple processes are
responsible for the creation and circulation of intermediate- and high-velocity
gas in the Milky Way halo.
