We present a comprehensive analysis of the damped Ly \alpha abundance database presented in the first paper of this series .
This database provides a homogeneous set of abundance measurements for many elements including Si , Cr , Ni , Zn , Fe , Al , S , Co , O , and Ar from 38 damped Ly \alpha systems with z _ { abs } > 1.5 .
With little exception , these damped Ly \alpha systems exhibit very similar relative abundances .
There is no significant correlation in X/Fe with [ Fe/H ] metallicity and the dispersion in X/Fe is small at all metallicity .

We search the database for trends indicative of dust depletion and in a few cases find strong evidence .
Specifically , we identify a correlation between [ Si/Ti ] and [ Zn/Fe ] which is unambiguous evidence for depletion .
Following ( 18 ) , we present [ X/Si ] abundances against [ Si/H ] + \log { N ( { HI } ) } and note trends of decreasing X/Si with increasing [ Si/H ] + \log { N ( { HI } ) } which argue for dust depletion .
Similarly , comparisons of [ Si/Fe ] and [ Si/Cr ] against [ Si/H ] indicate significant depletion at [ Si/H ] > -1 but suggest essentially dust-free damped systems at [ Si/H ] < -1.5 dex .

We present a discussion on the nucleosynthetic history of the damped Ly \alpha systems by focusing on abundance patterns which are minimally affected by dust depletion .
We find [ Si/Fe ] \to + 0.25 dex as [ Zn/Fe ] \to 0 and that the [ Si/Fe ] values exhibit a plateau of \approx + 0.3 dex at [ Si/H ] < -1.5 dex .
Together these trends indicate significant \alpha -enrichment in the damped Ly \alpha systems at low metallicity , an interpretation further supported by the observed O/Fe , S/Fe and Ar/Fe ratios .
Comparing the relative abundances of the Fe-peak elements , we identify important offsets from solar relative abundances for Cr , Ni , and Fe which suggest variations in nucleosynthesis along the Fe-peak .
Finally , the damped Ly \alpha systems exhibit a modest odd-even effect revealed by Si/Al , [ Si/Al ] \approx + 0.4 dex , which is significantly smaller than values observed in Galactic halo stars of comparable metallicity .
These observations present strong evidence that the damped Ly \alpha systems and Galactic halo had different enrichment histories .

To assess the impact of dust obscuration , we present estimates of the dust-to-gas ratios for the damped Ly \alpha sightlines and crudely calculate dust extinction corrections .
The distribution of extinction corrections suggests the effects of dust obscuration are minimal and that the population of ’ missing ’ damped systems has physical characteristics similar to the observed sample .

We update our investigation on the chemical evolution of the early universe in neutral gas .
The results are in good agreement with our previous work , but we emphasize two differences : ( 1 ) the unweighted and { N ( { HI } ) } -weighted [ Fe/H ] mean metallicities now have similar values at all epochs except z > 3.5 where small number statistics dominate the { N ( { HI } ) } -weighted mean ; ( 2 ) there is no evolution in the mean [ Fe/H ] metallicity from z = 1.7 \to 3.5 but possibly a marked drop at higher redshift .

We conclude with a general discussion on the physical nature of the damped Ly \alpha systems .
We stress the uniformity of the damped Ly \alpha chemical abundances which indicates that the protogalaxies identified with damped Ly \alpha systems have very similar enrichment histories , i.e .
a nearly constant relative contribution from Type Ia and Type II SN .
The damped Ly \alpha systems also show constant relative abundances within a given system which places strict constraints on the mixing timescales of the damped systems and may pose a great challenge to the protogalactic clump scenarios favored by hierarchical galaxy formation .