  We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of
interstellar molecular hydrogen (H_2) in two Galactic high-velocity clouds
(HVCs). Molecular hydrogen absorption is detected in the Magellanic Stream
(abundance ~0.3 solar) toward the Seyfert galaxy Fairall 9 in the lowest three
rotational states (J=0-2) at v(LSR)=+190 km/s, yielding a total H_2 column
density of log N(H_2)=16.40(+0.26)(-0.53). In contrast, no H_2 absorption is
seen in the high-velocity cloud Complex C (abundance ~0.1 solar) toward the
quasar PG 1259+593 (log N(H_2)<13.96 at v(LSR)=-130 km/s) although both HVCs
have similar HI column densities on the order of log N(HI)~20. Weak H_2
absorption is detected in the Intermediate-Velocity Arch (IV Arch; abundance
\~1.0 solar) toward PG 1259+593 (log N(H_2)=14.10(+0.21)(-0.44) at v(LSR)=-55
km/s and log N(HI)=19.5). It thus appears that metal- and dust-poor halo clouds
like Complex C are not able to form and maintain widely distributed H_2,
whereas metal and dust-rich halo clouds like the IV Arch can maintain H_2 even
at low HI column densities.
