The Keck+Magellan Survey for Lyman Limit Absorption III: Sample
  Definition and Column Density Measurements
  We present an absorption-line survey of optically thick gas clouds -- Lyman
Limit Systems (LLSs) -- observed at high dispersion with spectrometers on the
Keck and Magellan telescopes. We measure column densities of neutral hydrogen
NHI and associated metal-line transitions for 157 LLSs at z=1.76-4.39
restricted to 10^17.3 < NHI < 10^20.3. An empirical analysis of ionic ratios
indicates an increasing ionization state of the gas with decreasing NHI and
that the majority of LLSs are highly ionized, confirming previous expectations.
The Si^+/H^0 ratio spans nearly four orders-of-magnitude, implying a large
dispersion in the gas metallicity. Fewer than 5% of these LLSs have no positive
detection of a metal transition; by z~3, nearly all gas that is dense enough to
exhibit a very high Lyman limit opacity has previously been polluted by heavy
elements. We add new measurements to the small subset of LLS (~5-10) that may
have super-solar abundances. High Si^+/Fe^+ ratios suggest an alpha-enhanced
medium whereas the Si^+/C^+ ratios do not exhibit the super-solar enhancement
inferred previously for the Lya forest.
